Dew Point Calculator
Calculate dew point from air temperature and relative humidity, then optionally check whether a surface is cold enough for condensation.
Calculator is for informational purposes only. Terms and Conditions
This is an empirical saturation-vapor-pressure approximation over liquid water; temperatures are converted to °C for the equation.
Enter temperature and humidity
Air temperature and relative humidity are required. Open the optional condensation check only if you need to evaluate a surface.
Use the measured air temperature and relative humidity at the same location and time.
Dew point result
The dew point is the saturation temperature corresponding to the entered air temperature and relative humidity.
Result details
- Alternate unit—
- Air-to-dew-point spread—
- Condensation checkAdd surface temperature
Show calculation steps Review conversions, the Magnus-form equation, substitution, and checks
- Enter valid values to see the complete calculation.
Method, Sources, and Assumptions
Calculation basis, verified references, method limits, and the meaning of the optional condensation check.
Dew point is calculated from air temperature and relative humidity using the 17.502 and 240.97 coefficients documented by NOAA MADIS. The optional surface check compares surface temperature directly with the calculated dew point.
- The primary calculation assumes relative humidity is greater than 0% and no greater than 100%.
- NOAA MADIS documents this coefficient set for its RWIS dew-point calculation. The cited NOAA page does not define a universal validity range, so extreme conditions should be verified with a phase-appropriate saturation-vapor-pressure method.
- A surface at or below the calculated liquid-water dew point can support condensation when moist air contacts it; subfreezing surfaces may require frost-point analysis.
- Sensor accuracy, local temperature gradients, airflow, surface emissivity, and transient conditions can materially affect real condensation behavior.
Calculator guide
How the Dew Point Calculator Works
The Dew Point Calculator above calculates the temperature at which the entered air reaches saturation. Enter dry-bulb air temperature and relative humidity from the same air sample, and the tool returns dew point, the temperature-to-dew-point spread, and a surface-condensation threshold.
Dew point is especially useful for HVAC, building-envelope, glazing, duct, pipe, refrigeration, storage, and weather checks because it gives a direct temperature threshold for condensation potential. If a surface is at or below the dew point of the surrounding air, moisture can condense on that surface.
- Inputs
- Dry-bulb air temperature and relative humidity
- Main result
- Dew-point temperature in °F, °C, or K
- Best practical use
- Compare dew point with the coldest relevant surface temperature
How to Calculate Dew Point
The two inputs must represent the same air at approximately the same location and time.
-
Enter the air temperature
Use a dry-bulb air-temperature measurement, not the temperature of a wall, pipe, window, roof, duct, or other surface.
-
Enter relative humidity as a percentage
Enter 60 for 60% RH. The calculator converts the percentage internally before applying the logarithmic dew-point relationship.
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Select the desired answer units
The calculator can display dew point in Fahrenheit, Celsius, or Kelvin without changing the physical condition.
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Read the temperature–dew point spread
A smaller spread means the air is closer to saturation. At 100% RH, dew point equals air temperature and the spread is zero.
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Compare dew point with the coldest surface
If the surface is at or below dew point, condensation is possible. A larger positive surface-to-dew-point margin provides more protection against condensation under steady conditions.
Dew Point Inputs and Outputs
The live calculator intentionally uses only the two air measurements needed by the NOAA MADIS approximation and returns supporting checks that make the result easier to interpret.
- Air Temperature
- Dry-bulb temperature of the air sample in °F, °C, or K.
- Relative Humidity
- Current saturation percentage at the entered air temperature. Valid calculator input is greater than 0% and no more than 100%.
- Dew Point
- The temperature at which the same air would reach saturation if cooled without changing moisture content.
- Temperature–Dew Point Spread
- Air temperature minus dew point. National Weather Service terminology also calls this the dew point depression.
- Condensation Threshold
- The calculated dew-point temperature expressed as a surface-temperature threshold for screening condensation risk.
Dew Point Formula
The calculator uses the Magnus-type dew-point relationship documented by NOAA MADIS with constants 17.502 and 240.97°C.
Intermediate humidity term
\(T\) is air temperature in degrees Celsius and \(RH\) is relative humidity in percent.
Dew-point temperature
Reverse relative-humidity check
Substituting the calculated dew point and original air temperature should reproduce the original RH within rounding.
Worked Example: 75°F and 60% RH
Calculate the dew point for indoor air at 75°F and 60% relative humidity.
Convert temperature to Celsius
Calculate \(\alpha\)
Calculate dew point
Result
Dew point \(\approx60.18^\circ\mathrm{F}\)
The air-to-dew-point spread is approximately \(75-60.18=14.82^\circ\mathrm{F}\).
How to Check Surface Condensation
For HVAC and building-envelope work, the useful comparison is usually not “What is the dew point?” but “Is the coldest surface above or below the dew point?”
Surface margin above dew point
A positive value means the measured surface is above the calculated dew point. A value at or below zero indicates condensation potential.
| Surface Temperature | Margin Above Dew Point | Screening Interpretation |
|---|---|---|
| 70°F | +9.82°F | Well above threshold |
| 65°F | +4.82°F | Above threshold, but verify sensor uncertainty if critical |
| 60°F | −0.18°F | At/just below threshold; condensation possible |
| 55°F | −5.18°F | Below threshold; condensation potential is clear |
Critical surfaces are often colder than the room-average air temperature. Check uninsulated valves, pipe fittings, duct seams, glazing edges, metal framing, corners, roof/wall thermal bridges, and surfaces exposed to cold outdoor or process conditions.
Dew Point Reference Table
The table below provides quick calculated reference values using the same NOAA MADIS coefficient set as the calculator. Use the calculator for exact values and different temperature/humidity combinations.
| Air Temperature | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|
| 65°F | 40.1°F | 45.9°F | 50.8°F | 55.0°F |
| 70°F | 44.6°F | 50.5°F | 55.5°F | 59.8°F |
| 75°F | 49.1°F | 55.1°F | 60.2°F | 64.6°F |
| 80°F | 53.5°F | 59.7°F | 64.9°F | 69.3°F |
Dew Point vs Relative Humidity
Relative humidity and dew point both describe moisture, but they answer different questions.
Relative humidity
RH tells you how close the air is to saturation at its current temperature. The same moisture content can have a higher RH when cooled and a lower RH when warmed.
Dew point
Dew point gives the saturation temperature associated with the air’s current water-vapor content. It remains nearly unchanged during simple sensible heating or cooling until moisture is added or removed.
The National Weather Service describes dew point as a measure of atmospheric moisture and notes that a higher dew point indicates more moisture in the air. This makes dew point useful when comparing moisture conditions at different air temperatures.
How to Interpret the Result
The dew point is the saturation threshold for the entered air—not the temperature of the air itself and not a guarantee of visible water.
Dew point equals air temperature at saturation
At 100% RH, the dew point and air temperature are equal.
Smaller spread means closer to saturation
The NWS calls air temperature minus dew point the dew point depression. A smaller value indicates less cooling is required to reach saturation.
Sensor accuracy matters near the threshold
If a surface is only one or two degrees above the calculated dew point, temperature and humidity sensor uncertainty may be large enough to change the engineering conclusion.
Units and Measurement Checks
The equation is straightforward; measurement quality is often the dominant source of uncertainty in real condensation checks.
Match temperature and humidity location
Do not combine indoor RH with outdoor air temperature or a hallway temperature with humidity measured inside a closed room.
Use air temperature, not surface temperature
The dry-bulb input describes the air. Surface temperature is compared with the result afterward.
Absolute temperature conversion
\(T_C=(T_F-32)5/9\) and \(T_C=T_K-273.15\).
Temperature differences convert differently
A 10°C temperature difference equals an 18°F difference; do not add or subtract 32 when converting a temperature spread.
Allow sensors to stabilize
A humidity probe moved between environments may require time to equilibrate before temperature and RH represent the same air condition.
Do not confuse display precision with accuracy
More significant figures cannot compensate for an RH sensor that is uncertain by several percentage points.
Dew Point vs Frost Point
Dew point usually refers to saturation with respect to liquid water. At subfreezing conditions, direct deposition of water vapor as ice is better described with frost point or saturation with respect to ice.
Dew point
Useful for general atmospheric and condensation screening, especially when liquid water is the expected condensate.
Frost point
More appropriate when a cold surface is below freezing and direct ice deposition or frost formation is the controlling concern.
Common Dew Point Mistakes
Most incorrect conclusions come from mismatched measurements or comparing dew point with the wrong surface condition.
Entering 0.60 instead of 60%
The calculator expects RH as a percent value.
Using a surface temperature as the air input
Air temperature and surface temperature play different roles in the condensation check.
Measuring RH far from the cold surface
Local moisture and temperature conditions near a cold pipe, window, or duct can differ from a thermostat reading across the room.
Assuming all surfaces have one temperature
Metal fasteners, glazing edges, corners, duct seams, valves, and thermal bridges can be colder than adjacent surfaces.
Ignoring changing conditions
A surface that is safe at noon can fall below dew point overnight or during a different operating cycle.
Using liquid-water dew point for critical frost design
Below freezing, ice-specific saturation behavior may control.
Assumptions and Engineering Limits
The calculator is an empirical atmospheric approximation for dew-point and condensation screening. It is not a complete psychrometric or hygrothermal model.
NOAA MADIS coefficient set
The calculator uses the documented constants 17.502 and 240.97°C and does not apply a separate atmospheric-pressure correction.
Same-air-sample assumption
Temperature and RH should represent the same air mass. Stratification, infiltration, supply jets, and localized moisture sources can violate that assumption.
Constant-moisture saturation concept
The dew-point definition assumes air is cooled to saturation without changing moisture content; NWS also states the definition assuming constant air pressure and moisture content.
No surface heat-transfer model
The calculator does not predict the actual surface temperature of a wall, duct, pipe, window, or insulation system.
No condensation-rate calculation
It does not calculate condensate mass, latent load, drying, corrosion, mold growth, or material moisture accumulation.
Critical design requires more data
Use calibrated measurements, appropriate design conditions, detailed psychrometrics, heat-transfer analysis, and manufacturer or code requirements when consequences are significant.
Sources and Calculation Basis
The formula, constants, physical interpretation, and dew-point-spread terminology were checked against current NOAA and National Weather Service references.
- NOAA MADIS — Road Weather Information System Quality Control Notes — documents the dew-point calculation from air temperature and relative humidity using constants 17.502 and 240.97°C.
- National Weather Service — Dew Point Glossary — defines dew point as the temperature to which air must be cooled to reach saturation while pressure and moisture content remain constant and defines dew point depression as air temperature minus dew point.
The 75°F and 60% RH worked example was independently recalculated with the NOAA MADIS coefficients and returns approximately 60.18°F dew point and a 14.82°F dew-point spread.
Dew Point Calculator FAQ
These questions cover the most common dew-point and condensation interpretations.
How do I calculate dew point from temperature and humidity?
Enter dry-bulb air temperature and relative humidity from the same air sample. The calculator applies the NOAA MADIS Magnus-type relationship and returns the saturation temperature in °F, °C, or K.
Can dew point be higher than air temperature?
Not for a physically consistent air sample at 100% RH or less. Dew point equals air temperature at saturation and is lower when RH is below 100%.
What dew point causes condensation?
Condensation becomes possible when a surface is cooled to the dew point of the surrounding air or below it. The exact amount and visibility of condensate depend on local heat transfer, airflow, moisture supply, and time.
What is the difference between dew point and relative humidity?
Relative humidity tells how close the air is to saturation at its current temperature. Dew point is the saturation temperature associated with the air’s current moisture content.
What does a small temperature-to-dew-point spread mean?
It means the air is close to saturation and requires only a small temperature drop to reach 100% RH. The NWS calls this difference the dew point depression.
Should I use dew point or frost point below freezing?
Use an ice-specific frost-point method when direct frost or vapor deposition onto ice is the controlling concern. The calculator’s liquid-water Magnus approximation is best treated as a general screening method in those conditions.
Is dew point a better moisture indicator than relative humidity?
They describe different aspects of moisture. Dew point is often easier to compare across changing temperatures because RH itself changes when air is heated or cooled even without adding or removing water vapor.