Voltage Divider Calculator
Calculate resistor divider output voltage, solve for missing resistor values, include load resistance, and check source current, branch current, power, tolerance, and practical design limits.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Select the unknown value. The required inputs update automatically.
Enter the known values
Use positive resistor values. The optional load resistance shows the real loaded output voltage.
Visual Check
See where R1, R2, Vout, and the optional load connect in the voltage divider.
Solution
Live result, design checks, warnings, and full solution steps.
Quick checks
- Check—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
Uses the standard two-resistor voltage divider relationship and optional parallel load model.
- Assumes ideal DC resistors unless tolerance is selected.
- Does not replace electrical design review, component datasheets, or code requirements.
On this page
Calculator Guide
How to Use the Voltage Divider Calculator
The Voltage Divider Calculator above helps calculate output voltage, solve for missing resistor values, include load resistance, and check current, power, tolerance, and output impedance. A voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input voltage.
Use this page to understand the formula behind the calculator, choose practical R1 and R2 values, interpret loaded versus unloaded output voltage, and avoid common circuit mistakes. The calculator is most useful for signal scaling, ADC inputs, reference voltages, battery monitoring, sensor interfaces, and electronics learning.
Quick Answer
The basic voltage divider formula is \(V_{out}=V_{in}\frac{R_2}{R_1+R_2}\). R1 is the top resistor from the input voltage to the output node, and R2 is the bottom resistor from the output node to ground. If a load is connected to the output, the load is in parallel with R2, so the actual output voltage is usually lower than the ideal no-load value.
Do not use a voltage divider as a power supply
A resistor divider is usually not appropriate for powering a load. If the connected device draws meaningful current, the output voltage will sag and the resistors may waste power or overheat. Use a regulator, buck converter, LDO, buffer, or dedicated power supply for loads that need stable current.
Voltage Divider Inputs and Outputs
The calculator uses the selected solve mode to decide which values are required. In the most common mode, you enter Vin, R1, and R2 to calculate Vout. In design mode, you can start with a target Vout and estimate practical resistor values.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Input Voltage, \(V_{in}\) | The source voltage applied across R1 and R2. | V, mV, kV |
| Input | Top Resistor, \(R_1\) | The resistor between the input voltage and the output node. | Ω, kΩ, MΩ |
| Input | Bottom Resistor, \(R_2\) | The resistor between the output node and ground. | Ω, kΩ, MΩ |
| Optional Input | Load Resistance, \(R_L\) | The resistance connected from Vout to ground. It changes the real output voltage. | Ω, kΩ, MΩ |
| Output | Output Voltage, \(V_{out}\) | The voltage at the divider midpoint relative to ground. | V, mV, kV |
| Output | Current and Power | Current through the divider and power dissipated in each resistor branch. | A, mA, W, mW |
| Output | Output Impedance | The Thevenin resistance seen looking back into the divider output. | Ω, kΩ, MΩ |
| Output | Recommended R1/R2 Pair | A practical resistor pair near the requested voltage ratio and current target. | Ω, kΩ, MΩ |
Voltage Divider Formula
The ideal voltage divider formula assumes that no meaningful load is connected to the output node. When a load is connected, the load resistance must be included because it sits in parallel with R2.
Ideal No-Load Voltage Divider Formula
Use this form when Vout is measured with a very high-impedance input, such as a meter or a circuit input that draws negligible current.
Loaded Voltage Divider Formula
Use this form when the circuit connected to Vout has a finite input resistance. The loaded result is usually lower than the ideal result.
Rearranged Formulas for Resistor Design
These forms are useful when you know the desired output voltage and want to solve for a missing resistor value in an unloaded divider.
Loaded Branch Current and Power
In loaded mode, current through R1 is source current. It splits at the output node into R2 current and load current, so each resistor branch needs its own current for power checks.
Practical insight
The ratio \(R_2/(R_1+R_2)\) sets the ideal voltage fraction. The absolute resistor sizes set the current draw, output impedance, loading sensitivity, resistor heating, and battery drain.
Voltage Divider Variables
Every variable in the voltage divider formula must use consistent units. Resistor units cancel in the ideal ratio if R1 and R2 use the same unit, but current and power checks require actual ohms.
| Symbol | Meaning | How to Enter It |
|---|---|---|
| \(V_{in}\) | Input voltage applied across the two-resistor divider. | Enter a positive DC or RMS signal voltage in volts, millivolts, or kilovolts. |
| \(V_{out}\) | Output voltage measured from the midpoint node to ground. | Use this as the result or target voltage, depending on solve mode. |
| \(R_1\) | Top resistor connected between Vin and Vout. | Enter a positive resistance value in Ω, kΩ, or MΩ. |
| \(R_2\) | Bottom resistor connected between Vout and ground. | Enter a positive resistance value in Ω, kΩ, or MΩ. |
| \(R_L\) | Load resistance connected from Vout to ground. | Use only when the output is connected to a circuit that draws current. |
| \(R_{eq}\) | Effective lower resistance when R2 and RL are in parallel. | The calculator derives this from \(R_2\parallel R_L\). |
| \(I_{R1}\) | Current through R1. In loaded mode, this is source current. | Used to calculate R1 power and source current draw. |
| \(I_{R2}\) | Current through the bottom resistor. | Used to calculate R2 power, especially in loaded mode. |
| \(I_L\) | Current drawn by the connected load. | Used only when load resistance is included. |
How to Use the Calculator
Start by choosing the value you want to solve for. Then enter the known values using matching units and review the quick checks before treating the result as a practical circuit design.
Choose the solve mode
Select Vout for a normal voltage divider calculation, or choose Vin, R1, R2, or resistor-pair mode if you are designing backward from a target output.
Enter the known values
Enter Vin, resistor values, target Vout, or target divider current depending on the selected mode. Use the unit selectors to avoid mixing Ω and kΩ.
Enable load resistance when needed
If anything connected to Vout draws current, include the load resistance. This is the difference between an ideal divider and a real loaded divider.
Review current, power, and warnings
Check divider current, output impedance, resistor power, loading error, and tolerance range before choosing real components.
How to Interpret Voltage Divider Results
A voltage divider result is only useful if it is electrically reasonable. The output voltage should be lower than the input voltage, the resistor power should be within rating, and the connected load should not pull the output too far from the ideal value.
| Result Pattern | What It Means | What to Do Next |
|---|---|---|
| \(V_{out}\) is between 0 and \(V_{in}\) | The result is physically possible for a passive two-resistor divider. | Check loading, current draw, and resistor power. |
| \(V_{out}\) is close to \(V_{in}\) | R2 is much larger than R1, so most voltage appears across R2. | Verify the intended ratio and ADC/input limit. |
| \(V_{out}\) is close to zero | R2 is much smaller than R1, so little voltage appears at the output. | Check whether R1 and R2 were swapped. |
| Loaded Vout is much lower than ideal Vout | The load resistance is too low compared with the divider output impedance. | Use lower divider resistances, a buffer, or a regulator. |
| Power is high | The resistor values may be too low, causing unnecessary current and heat. | Increase resistor values or use a higher wattage resistor. |
| \(V_{out} \ge V_{in}\) | A passive divider cannot boost voltage. | Use a boost converter or different circuit if higher voltage is required. |
What to do with the result
Use the calculated voltage to check whether the next circuit input is within its allowed range. Then confirm that the divider does not waste too much current, does not exceed resistor power ratings, and is not pulled down by the connected load.
What changes the result most?
The resistor ratio changes the ideal output voltage the most. The absolute resistor size changes current draw and output impedance. Load resistance changes the real measured output because it effectively reduces the lower leg of the divider.
Quick sanity check
For a normal passive divider, \(0<V_{out}<V_{in}\). If the result is negative, greater than Vin, or wildly different from your meter reading, check the units, resistor order, load resistance, and whether R1 and R2 were swapped.
Input Quality Checklist
Most wrong voltage divider results come from swapped resistors, hidden loading, or unit mistakes. Check these items before trusting the output.
R1 and R2 Placement
R1 is the top resistor from Vin to Vout. R2 is the bottom resistor from Vout to ground. Swapping them changes the output.
Unit Scale
Confirm whether each resistor is in Ω, kΩ, or MΩ. A 10 kΩ resistor is 10,000 Ω, not 10 Ω.
Load Resistance
If a circuit input, sensor, ADC, or module is connected to Vout, include its input resistance when accuracy matters.
Power Rating
Check resistor power, especially when using low resistor values or high input voltage.
Tolerance
Real resistors are not exact. A 5% resistor pair can shift the actual output voltage.
Input Limits
For ADCs and microcontrollers, make sure worst-case Vout stays below the maximum allowed input voltage.
Step-by-Step Worked Example
A common use case is reducing a 12 V signal to a lower voltage that can be read by an electronics input. The example below calculates the ideal no-load output voltage.
Formula
Substitution
Final Answer
Result
The ideal output voltage is about 4.86 V. This is reasonable because the output is less than the 12 V input and R2 is smaller than the total resistance.
Loaded example check
If a 10 kΩ load is connected to the output, R2 is no longer the only lower resistance. The effective lower resistance becomes \(R_2\parallel R_L\), so the actual output voltage drops below the ideal 4.86 V.
This loaded result is much lower because the 10 kΩ load is not large compared with the divider resistance. In this situation, a buffer or lower divider resistance may be needed if the output must stay near the ideal value.
Voltage Divider Circuit Layout
A two-resistor voltage divider is arranged as a simple series circuit: Vin connects to R1, R1 connects to the output node, R2 connects from the output node to ground, and Vout is measured across R2.
Text circuit layout
Vin → R1 → Vout node → R2 → Ground. If a load is connected, it usually connects from the Vout node to ground, which places the load in parallel with R2.
| Connection Point | Circuit Role | Calculation Impact |
|---|---|---|
| Vin to R1 | Applies the source voltage to the resistor string. | Higher Vin raises Vout in the same proportion. |
| R1 to Vout node | Creates the midpoint where output voltage is measured. | Larger R1 lowers Vout for the same R2. |
| Vout node to R2 | Sets the lower leg of the divider. | Larger R2 raises Vout for the same R1. |
| Vout node to load | Adds a parallel path to ground. | Lower load resistance pulls Vout down. |
Typical Values and Reference Ranges
There is no single correct resistor pair for every voltage divider. Practical values depend on current draw, loading, noise, ADC requirements, resistor availability, and power rating.
| Design Item | Typical Range or Check | Why It Matters |
|---|---|---|
| Divider current | Often µA to a few mA for signal dividers | Lower current saves power, but very high resistance is more sensitive to leakage and loading. |
| Common resistor values | 1 kΩ to 1 MΩ is common in many low-power signal circuits | The useful range depends on source impedance, load, ADC sampling, noise, and current budget. |
| Load resistance | Preferably much larger than the divider output impedance | Low load resistance causes the output voltage to sag. |
| Resistor tolerance | Common values include ±0.1%, ±1%, ±5%, and ±10% | Tighter tolerance gives a more predictable output voltage. |
| Resistor power | Keep calculated power well below the resistor rating | Thermal margin improves reliability and reduces drift. |
| Common Target | Example Use | Practical Note |
|---|---|---|
| 5V to 3.3V | Logic-level sensing or ADC scaling | Use only for signal input unless the load current is negligible. |
| 12V to 5V | Reading a higher DC signal with a 5 V input | Do not use as a 5 V power rail. |
| 12V to 3.3V | Microcontroller ADC or GPIO sensing | Check worst-case source voltage, not only nominal voltage. |
| 24V to 3.3V | Industrial signal monitoring | Check resistor power, transient protection, and input limits. |
| Battery monitor | ADC measurement of battery voltage | Size for the maximum charged voltage and balance current drain against output impedance. |
Design Ranges and Practical Checks
A mathematically correct divider may still be a poor circuit if it wastes too much power, loads down, overheats, or gives an unstable ADC reading. Use the checks below to move from math to practical design.
Low Resistor Values
Lower values make the divider less sensitive to load resistance, but they increase current draw and resistor power.
Moderate Values
Moderate kΩ-range resistors are often a good starting point for signal scaling, but ADC and load requirements should be checked.
High Resistor Values
Higher values reduce battery drain, but leakage current, noise, ADC sampling, and load resistance become more important.
Choosing practical resistor values
The ideal formula may produce values such as 14.142 kΩ or 6.731 kΩ, but real parts are usually selected from standard E-series values such as E12, E24, or E96. After choosing real resistor values, recalculate the actual Vout and percent error instead of assuming the ideal target is exact.
ADC and battery monitor design note
Microcontroller ADC inputs can be sensitive to source impedance and sampling time. For battery monitoring, size the divider for the maximum possible battery voltage, not just the nominal voltage. If readings are unstable, check the datasheet, reduce the divider resistance, increase acquisition time, or add an appropriately sized capacitor at the ADC input.
Unit Conversion Notes
Voltage divider math is ratio-based, but unit mistakes still matter. Use the same resistance unit for R1 and R2 when calculating the voltage ratio, and convert to ohms when calculating current and power.
| Quantity | Common Units | Conversion Reminder |
|---|---|---|
| Voltage | mV, V, kV | \(1\,V=1000\,mV\), \(1\,kV=1000\,V\) |
| Resistance | Ω, kΩ, MΩ | \(1\,k\Omega=1000\,\Omega\), \(1\,M\Omega=1,000,000\,\Omega\) |
| Current | µA, mA, A | \(1\,mA=0.001\,A\), \(1\,\mu A=0.000001\,A\) |
| Power | mW, W | \(1\,W=1000\,mW\) |
Most common unit trap
The most common mistake is entering 10 when the intended value is 10 kΩ, but the selected unit is Ω. That changes the current and power by a factor of 1,000 even if the divider ratio appears similar.
Voltage Divider vs. Related Circuit Methods
A voltage divider is simple, but it is not always the right tool. Compare it with nearby circuit methods before using it in a real design.
| Method | Best For | Main Limitation |
|---|---|---|
| Voltage divider | Signal scaling, ADC inputs, biasing, references, and sensing. | Output changes when the load draws current. |
| Voltage regulator or LDO | Providing a stable supply voltage to a load. | Requires a regulator component and must meet dropout, thermal, and current limits. |
| Buck converter | Efficiently stepping down voltage for loads that need current. | More complex and may require layout, switching noise, and component selection review. |
| Buffer or op-amp follower | Keeping a divider voltage stable while driving a higher load. | Requires power rails and an appropriate amplifier. |
| Current divider | Splitting current through parallel branches. | Solves a different problem than dividing voltage across series resistors. |
Common Voltage Divider Mistakes
The voltage divider formula is simple, but real-world use can still fail if the connected circuit is ignored.
Common Mistakes
- Using a voltage divider to power a load instead of only scaling a signal.
- Ignoring the load resistance connected to the output node.
- Swapping R1 and R2 and getting the opposite voltage ratio.
- Using resistor values that are so low they waste power or overheat.
- Using resistor values that are so high they become sensitive to leakage, noise, or ADC sampling.
- Forgetting resistor tolerance when the output voltage must be accurate.
- Sizing a battery monitor from nominal voltage instead of maximum charged voltage.
Better Practice
- Use the divider for signal scaling, sensing, biasing, and reference voltages.
- Include load resistance when the connected input is not extremely high impedance.
- Confirm R1 is above the output node and R2 is below it.
- Check current and resistor power before building the circuit.
- Use practical E-series resistor values and check the actual output error.
- Verify ADC input limits and datasheet source-impedance guidance.
- Design around worst-case source voltage, resistor tolerance, and load conditions.
Troubleshooting Unexpected Voltage Divider Results
If the calculated output does not match the measured output, the issue is usually loading, resistor order, tolerance, or unit selection.
| Problem | Likely Cause | Fix |
|---|---|---|
| Measured Vout is lower than calculated | The connected load is pulling the output down. | Include \(R_L\), lower the divider resistance, or add a buffer. |
| Vout is much higher or lower than expected | R1 and R2 may be swapped. | Confirm R1 is connected to Vin and R2 is connected to ground. |
| Resistors get warm | Divider current is too high or resistor power rating is too low. | Increase resistance values or use higher wattage resistors. |
| ADC reading is noisy or unstable | Output impedance may be too high for the ADC sampling behavior. | Check the ADC datasheet, lower resistor values, adjust sampling time, or add a capacitor. |
| Battery reading exceeds ADC limit | The divider was sized for nominal battery voltage instead of maximum charged voltage. | Recalculate using the highest expected source voltage plus reasonable safety margin. |
| Share or copy result seems right but circuit is wrong | The math may be correct, but the circuit use case is unsuitable. | Use a regulator or buffer when the divider must drive a load. |
Suspicious result cases
Be cautious if the output is nearly equal to Vin, nearly zero, above Vin, negative for a positive-input divider, or very different when a meter or circuit is connected. Those cases usually point to wrong resistor placement, hidden loading, or unit errors.
Assumptions, Sources, and Limitations
This calculator is intended for educational use, quick circuit estimates, and preliminary electronics design checks. It uses standard DC circuit relationships based on Ohm’s law, series resistance, and parallel resistance.
Ideal Resistor Assumption
The basic formula assumes ideal resistors unless tolerance is explicitly considered.
No-Load Assumption
The simple formula assumes no meaningful load current is drawn from the output node.
Loaded Divider Limit
The loaded model treats the connected load as a resistance from Vout to ground.
Final Design Note
For real products or safety-critical circuits, verify component ratings, tolerances, datasheets, temperature behavior, and circuit protection.
Calculation basis and external reference
The calculation basis is the standard two-resistor voltage divider relationship and Ohm’s law. For an additional educational explanation of the divider concept, see the All About Circuits voltage divider reference. This page does not replace datasheet review, electrical safety practices, or professional engineering judgment.
Glossary of Voltage Divider Terms
These terms help connect the calculator inputs, formulas, and circuit behavior.
Voltage Divider
A series resistor circuit that produces an output voltage equal to a fraction of the input voltage.
R1
The top resistor connected between the input voltage and the output node.
R2
The bottom resistor connected between the output node and ground.
Loaded Voltage Divider
A divider where a connected load draws current from the output node and changes the output voltage.
Output Impedance
The equivalent resistance seen looking back into the divider output, approximately \(R_1\parallel R_2\).
Divider Current
The current flowing through the resistor string in the no-load case.
Resistor Tolerance
The allowed variation between the marked resistor value and its actual resistance.
E-Series Resistors
Standard preferred resistor value series such as E12, E24, and E96.
Frequently Asked Questions
What does a voltage divider calculator calculate?
A voltage divider calculator finds the output voltage from an input voltage and two resistors. Depending on the solve mode, it can also help solve for Vin, R1, R2, loaded output voltage, divider current, output impedance, and resistor power.
What is the voltage divider formula?
The standard voltage divider formula is \(V_{out}=V_{in}\frac{R_2}{R_1+R_2}\), where R1 is the top resistor and R2 is the bottom resistor connected to ground.
Why is my measured voltage divider output lower than calculated?
The measured output is often lower because the connected load is in parallel with R2. That lowers the effective bottom resistance and reduces the actual output voltage.
Can a voltage divider power a load?
A resistor voltage divider is usually not a good power supply. It is best for signal scaling, sensing, biasing, and reference voltages. Loads that draw meaningful current should use a regulator, buck converter, LDO, buffer, or proper power supply.
How do I choose R1 and R2 for a voltage divider?
Choose the R1 to R2 ratio to set the output voltage, then choose the overall resistance level based on current draw, output impedance, loading, resistor power, tolerance, and available standard resistor values.
What resistor values work for 5V to 3.3V?
One common ideal ratio for 5V to 3.3V is about \(R_2/(R_1+R_2)=0.66\). Practical values such as 10 kΩ for R1 and 20 kΩ for R2 produce about 3.33 V with no load. This is for signal scaling, not powering a 3.3 V rail, and loading should still be checked.