Capacitance Calculator
Solve for capacitance, charge, or voltage for an ideal capacitor and see stored energy and RC time constant in one glance.
Calculation Steps
Practical Guide
Capacitance Calculator: From Inputs to Reliable Designs
This guide sits right under the capacitance calculator and walks you through how to choose inputs, interpret capacitance, charge, and voltage, and sanity-check energy and RC time-constant calculations so your design behaves the way you expect.
Quick Start: Using the Capacitance Calculator Safely
The calculator above implements the ideal capacitor relationships between capacitance \(C\), charge \(Q\), and voltage \(V\), along with stored energy and an optional RC time constant. Follow these steps to get numbers you can trust.
- 1 Pick what you want to solve for. Use the Solve For dropdown to choose \(C\), \(Q\), or \(V\). The calculator automatically hides the corresponding input and treats it as the output.
- 2 Set realistic units. For capacitance, most real parts are in the \(\mu\text{F}\), \(\text{nF}\), or \(\text{pF}\) range. Charge is usually in \(\mu\text{C}\) or \(\text{nC}\), and voltage is in \(\text{V}\) or \(\text{kV}\). Choose units that match your datasheet.
- 3 Enter the two known quantities. For example, to solve for capacitance, enter \(Q\) and \(V\). The calculator converts everything to SI units internally: \[ C = \frac{Q}{V}, \quad Q = C V, \quad V = \frac{Q}{C}. \]
- 4 Optionally set a reference resistance. If you care about charge/discharge speed, enter a series resistance \(R\) so the tool can compute the RC time constant \(\tau = R C\).
- 5 Read the result and quick stats. The result row shows the requested quantity in an auto-scaled unit, while the Quick Stats table shows normalized \(C\), \(Q\), stored energy \(\displaystyle E = \tfrac{1}{2} C V^2\), and \(\tau\).
- 6 Open the calculation steps if needed. Use Show Steps to see each equation with your numbers substituted. This is useful for reports and homework write-ups.
- 7 Share or archive the state. The share button encodes your inputs in the URL so you can send a link or save a PDF of the full page.
Tip: If the result looks wildly large or small, the first thing to check is whether you accidentally mixed units (e.g., entering microfarads but selecting F instead of \(\mu\text{F}\)).
Warning: The calculator assumes an ideal capacitor. It does not account for voltage derating, ESR, leakage current, or dielectric absorption. Always cross-check against the component datasheet.
Choosing Your Method: Solve for C, Q, or V
The same ideal capacitor equation \[ C = \frac{Q}{V} \] can be rearranged depending on your design question. The calculator exposes three main “methods” through the Solve For selector.
Method 1 — Solve for Capacitance \(C\)
Use this when you know how much charge or ripple you need to support at a given voltage.
- Great for sizing bulk capacitors on DC rails.
- Matches many datasheet curves that list charge or ripple at a given voltage.
- Directly leads into energy and RC time-constant checks.
- Requires a reasonable estimate of maximum \(Q\) or working voltage \(V\).
- Still needs a sanity check against package size, ESR, and cost.
Method 2 — Solve for Charge \(Q\)
Use this when capacitance and voltage are fixed, and you want to know how much charge can be delivered.
- Good for short-term hold-up or sample-and-hold circuits.
- Helps quantify how much charge is available during a transient.
- Does not by itself tell you whether the load will dip the voltage more than allowed.
- Still needs time-domain analysis for dynamic loads.
Method 3 — Solve for Voltage \(V\)
Use this when charge and capacitance are known and you want to estimate sag or overshoot.
- Direct way to estimate how far a node will droop after a known load event.
- Works nicely with piecewise charge-sharing models.
- Assumes all charge transfer is ideal and lossless.
- Doesn’t include inrush limiting, ESR, or wiring inductance.
Tip: On a new design, start by solving for \(C\) based on worst-case load and allowable ripple. Then use the same capacitance to check \(Q\), \(V\), and \(\tau = R C\) in a few key scenarios.
What Moves the Number: Key Levers Behind Capacitance
Even though the calculator works with lumped \(C\), it’s helpful to remember what actually drives the capacitance value in a real component. For a simple parallel-plate capacitor, \[ C = \varepsilon_r \varepsilon_0 \frac{A}{d}, \] where \(A\) is plate area, \(d\) is spacing, and \(\varepsilon_r\) is the relative dielectric constant.
Higher DC bias on many ceramic capacitors effectively reduces capacitance. A part labeled 10 \(\mu\text{F}\) at 6.3 V may be only 3–5 \(\mu\text{F}\) at its rated voltage.
High-\(k\) dielectrics (e.g., X5R, X7R) give large capacitance in small volumes but drift more with temperature and voltage. C0G/NP0 stay stable but have much lower capacitance per volume.
Larger packages generally offer bigger \(A\) and smaller \(d\), giving higher \(C\). However, mechanical stress and mounting can change effective capacitance over time.
Electrolytics and high-\(k\) ceramics can lose a significant fraction of their capacitance at temperature extremes. Size your capacitance at the worst-case operating temperature, not room temp.
A “10 \(\mu\text{F}\)” part with ±20% tolerance and aging may realistically sit around 7–8 \(\mu\text{F}\). Use the calculator to explore both nominal and worst-case values.
Capacitors in parallel add (\(C_{\text{eq}} = C_1 + C_2 + \dots\)), while capacitors in series reduce (\(1/C_{\text{eq}} = 1/C_1 + 1/C_2 + \dots\)). Banking ceramics in parallel is a practical way to hit higher \(C\).
Worked Examples: From Rail Decoupling to Hold-Up
Example 1 — Sizing a Bulk Capacitor for a 5 V Rail
- Goal: Limit droop on a 5 V rail to 0.5 V during a 2 ms load pulse.
- Load surge: 1.0 A additional current during the pulse.
- Assumed: Capacitor supplies most of the surge.
- Capacitance to find: \(C\).
In practice, you would implement this with multiple electrolytic or polymer capacitors in parallel, then re-run the calculator using the datasheet’s effective capacitance at 5 V and operating temperature to make sure the design still holds.
Example 2 — Estimating Hold-Up Time for a Microcontroller
- Microcontroller rail: 3.3 V, powered from a DC/DC converter.
- Capacitance on rail: 220 \(\mu\text{F}\) total (electrolytic + ceramics).
- Average current: 80 mA.
- Minimum safe voltage: 2.9 V.
- Question: How long can the rail stay above 2.9 V after power loss?
This simple model ignores ESR and converter behavior, but it gives a clear first-order answer. The calculator helps you experiment with different combinations of \(C\), \(I\), and \(\Delta V\) without re-deriving the algebra every time.
Common Layouts & Variations
Real designs rarely use a single capacitor in isolation. Below are common configurations, how they affect effective capacitance, and what to watch for when reading calculator outputs.
| Configuration | Typical Use | Pros | Watch-outs |
|---|---|---|---|
| Single electrolytic | Bulk energy storage on low-frequency DC rails. | High capacitance at low cost; good for hold-up. | High ESR/ESL; capacitance drops at low temperature and over life. |
| MLCC bank in parallel | High-frequency decoupling near IC power pins. | Low ESR/ESL; good high-frequency performance; flexible placement. | Effective \(C\) shrinks under DC bias and temperature; piezoelectric noise. |
| Series capacitors | AC coupling; sharing voltage across devices. | Higher effective voltage rating; blocks DC. | Effective capacitance decreases; balancing resistors may be required. |
| Film + electrolytic | Motor drives, inverters, and power factor correction. | Film handles ripple current; electrolytic provides bulk \(C\). | Layout sensitive; keep inductance low to avoid resonances. |
| Local + bulk decoupling | Distributed rails on complex PCBs. | Combines local high-frequency decoupling with global energy storage. | Need to coordinate values to avoid anti-resonances; simulate if critical. |
- Calculate effective \(C_{\text{eq}}\) for the configuration, then plug it into the calculator.
- Check energy \(E = \tfrac{1}{2} C V^2\) against worst-case transients.
- Verify voltage ratings with margin above the maximum applied voltage.
- Complement capacitance calculations with ESR and ripple-current checks.
- Place sensitive capacitors close to the load to avoid trace inductance.
- For safety-critical designs, validate with lab measurements, not just calculations.
Specs, Logistics & Sanity Checks
Once the capacitance calculator gives you a target \(C\), the next step is choosing parts that behave properly in the real world. Use the following panels as a checklist when moving from numbers to BOM lines.
Capacitance & Voltage Rating
- Select nominal \(C\) higher than the calculator’s value to cover tolerance and aging.
- Apply a safety margin on voltage, often 1.5–2× the maximum DC voltage in normal operation.
- For ceramics, check the capacitance vs. DC bias curve at your operating voltage.
ESR, Ripple & Thermal Limits
The calculator above focuses on \(C\), \(Q\), \(V\), and \(E\), but real capacitors also have equivalent series resistance (ESR) and a maximum ripple-current rating.
- Verify allowable ripple current at the switching frequency of your converter.
- Use multiple capacitors in parallel to share current and reduce ESR when needed.
- Confirm the part’s temperature rise at your worst-case ripple and ambient conditions.
Layout, Logistics & Test
- Group decoupling capacitors close to IC pins and connect with short, wide traces.
- Lock in footprint families early so alternatives are easy to source.
- Plan measurement points for verifying rail ripple and transient response in the lab.
A good practice is to run the capacitance calculator twice: once with your target values, and once with worst-case derated capacitance. If your design still meets hold-up and ripple requirements in the worst-case run, you have a much healthier margin.