Calculator Inputs
Example Data Table
These sample values use Celsius and standard pressure.
| Air Temp | Relative Humidity | Approx Dew Point | Feel |
|---|---|---|---|
| 20 °C | 40% | 6.0 °C | Dry |
| 25 °C | 60% | 16.7 °C | Mild |
| 30 °C | 75% | 25.1 °C | Oppressive |
| 10 °C | 90% | 8.4 °C | Damp |
Formula Used
This calculator uses the Magnus approximation for saturation vapor pressure.
For water, it uses a = 17.625 and b = 243.04.
For ice, it uses a = 22.587 and b = 273.86.
Dew point is calculated with:
γ = ln(RH / 100) + (aT / (b + T)),
then Td = bγ / (a - γ).
Relative humidity from dew point is calculated with:
RH = 100 × e(Td) / es(T).
Actual vapor pressure, absolute humidity, mixing ratio, specific humidity,
humidex, wet bulb estimate, and enthalpy are also shown.
How to Use This Calculator
- Select the calculation type.
- Enter the air temperature and its unit.
- Enter relative humidity when finding dew point.
- Enter dew point when finding relative humidity.
- Add pressure for advanced moisture values.
- Add surface temperature to check condensation risk.
- Choose output units and decimal places.
- Press Calculate and download CSV or PDF results.
Dew Point and Relative Humidity Guide
Why Dew Point Matters
Dew point shows the temperature where air becomes saturated. It is a direct measure of moisture in the air. Relative humidity changes when temperature changes. Dew point stays more stable when moisture stays constant. That makes it useful for comfort, storage, weather, and ventilation.
Better Moisture Decisions
A high dew point means the air contains more water vapor. A low dew point means the air is drier. Warm air can hold more moisture than cool air. So the same water vapor can create different humidity readings. This calculator helps separate those two ideas.
Indoor Comfort and Condensation
Dew point also helps predict condensation. If a wall, pipe, window, or duct is colder than the dew point, moisture can collect on that surface. This can cause fogging, corrosion, mold, or product damage. The optional surface input gives a quick risk message.
Advanced Air Values
The calculator also estimates vapor pressure deficit. This value helps growers, drying rooms, and climate systems. Absolute humidity shows grams of water per cubic meter. Mixing ratio shows water vapor per kilogram of dry air. Enthalpy estimates heat content in moist air. These values are helpful for HVAC checks and environmental reports.
Practical Uses
Use the tool for homes, laboratories, warehouses, greenhouses, classrooms, server rooms, weather stations, and field inspections. Enter measured values from a hygrometer or weather sensor. Then compare the result with comfort levels and condensation risk. Download the results when you need a record.
FAQs
1. What is dew point?
Dew point is the temperature where air becomes saturated with water vapor. At that point, condensation can start if surfaces are cold enough.
2. What is relative humidity?
Relative humidity is the percentage of moisture in air compared with the maximum moisture air can hold at the current temperature.
3. Which value is better for comfort?
Dew point is often better for comfort because it shows actual moisture. Relative humidity can change quickly when temperature changes.
4. Can dew point be higher than air temperature?
Normally dew point should not exceed air temperature. If it does, the inputs may suggest supersaturation or measurement error.
5. What pressure should I use?
Use 1013.25 hPa for standard sea level conditions. Use local pressure for more accurate mixing ratio and enthalpy results.
6. What does vapor pressure deficit mean?
Vapor pressure deficit shows the drying power of air. Higher values mean air can pull more moisture from surfaces, plants, or materials.
7. How does the surface temperature option help?
It compares surface temperature with dew point. If the surface is colder than the dew point, condensation is likely.
8. Are the results exact?
The results are strong estimates based on common psychrometric equations. Sensor quality, pressure, and extreme conditions can affect accuracy.