How do you calculate relative humidity from dry bulb temperature?
Use dry bulb temperature together with either wet bulb temperature or dew point to calculate relative humidity accurately. This calculator supports both methods and visualizes the result instantly.
Relative Humidity Calculator
Select a method, enter temperatures, and click calculate.
Used for the wet bulb method. Standard sea level pressure is about 1013.25 hPa.
Results
Enter your values and click calculate to see relative humidity, vapor pressure, saturation pressure, and psychrometric details.
How do you calculate relative humidity from dry bulb temperature?
When people ask, “how do you calculate relative humidity from dry bulb temperature,” the technically correct answer is that dry bulb temperature by itself is not enough. Relative humidity depends on both temperature and moisture content. Dry bulb temperature tells you how warm the air is, but it does not tell you how much water vapor the air contains. To calculate relative humidity, you need dry bulb temperature plus at least one additional moisture indicator, most commonly wet bulb temperature or dew point.
That distinction matters in weather observation, HVAC design, industrial drying, agriculture, clean-room operation, museums, data centers, and indoor air quality management. Two rooms can both be at 75°F, yet one can feel comfortable and the other muggy. The difference is moisture in the air, not temperature alone. Relative humidity expresses that moisture as a percentage of saturation at the measured temperature.
What dry bulb temperature means
Dry bulb temperature is the standard air temperature measured by a normal thermometer that is shielded from radiation and moisture. In meteorology and psychrometrics, it serves as the baseline temperature state of the air. The term “dry bulb” was introduced to distinguish it from the wet bulb thermometer used in sling psychrometers and aspirated psychrometers.
Dry bulb temperature is essential because the amount of water vapor air can hold increases rapidly as temperature rises. Warm air can hold much more moisture than cool air. That is why relative humidity can change significantly during the day even if the actual amount of vapor in the air remains nearly constant.
Why dry bulb temperature alone is not enough
Relative humidity is defined as:
Relative Humidity = (Actual Water Vapor Pressure ÷ Saturation Vapor Pressure at the Same Temperature) × 100
The denominator, saturation vapor pressure, comes from the dry bulb temperature. The numerator, actual vapor pressure, comes from moisture data such as dew point or wet bulb temperature. Without the numerator, relative humidity cannot be uniquely determined.
- If dry bulb temperature is 30°C and dew point is 10°C, relative humidity is low.
- If dry bulb temperature is 30°C and dew point is 24°C, relative humidity is high.
- The dry bulb temperature is identical in both cases, but the humidity is very different.
Method 1: Calculate relative humidity using dry bulb temperature and dew point
This is one of the cleanest and most reliable methods because dew point directly represents the actual moisture content of the air. The basic approach is:
- Measure dry bulb temperature.
- Measure or estimate dew point.
- Compute saturation vapor pressure at the dry bulb temperature.
- Compute saturation vapor pressure at the dew point. This equals the actual vapor pressure.
- Divide actual vapor pressure by saturation vapor pressure and multiply by 100.
A commonly used Magnus-type approximation for saturation vapor pressure in hPa is:
es(T) = 6.112 × exp((17.67 × T) ÷ (T + 243.5))
Where T is in Celsius. If Td is dew point, then:
RH = 100 × es(Td) ÷ es(T)
Example using the dew point method:
- Dry bulb temperature: 30°C
- Dew point: 20°C
- es(30) ≈ 42.43 hPa
- es(20) ≈ 23.37 hPa
- RH ≈ 100 × 23.37 ÷ 42.43 ≈ 55.1%
This shows how relative humidity is fundamentally a comparison between how much vapor is actually present and how much the air could hold at that temperature.
Method 2: Calculate relative humidity using dry bulb temperature and wet bulb temperature
The wet bulb method is traditional in field meteorology and HVAC work. Wet bulb temperature is measured with a thermometer whose bulb is wrapped in a moist wick and ventilated. Evaporation cools the bulb. The drier the air, the stronger the cooling and the lower the wet bulb temperature relative to the dry bulb temperature.
An approximate psychrometric equation for actual vapor pressure is:
e = es(Tw) – 0.00066 × (1 + 0.00115 × Tw) × P × (T – Tw)
Where:
- e = actual vapor pressure in hPa
- es(Tw) = saturation vapor pressure at wet bulb temperature
- T = dry bulb temperature in Celsius
- Tw = wet bulb temperature in Celsius
- P = station pressure in hPa
Then:
RH = 100 × e ÷ es(T)
Example using the wet bulb method:
- Dry bulb temperature: 30°C
- Wet bulb temperature: 24°C
- Pressure: 1013.25 hPa
- es(24) ≈ 29.83 hPa
- Psychrometric correction ≈ 4.08 hPa
- Actual vapor pressure ≈ 25.75 hPa
- es(30) ≈ 42.43 hPa
- RH ≈ 60.7%
The wet bulb method is very useful when a psychrometer is available and dew point is not directly measured.
Comparison table: saturation vapor pressure by dry bulb temperature
The table below shows how sharply saturation vapor pressure rises with temperature. This is why warm air can support much more moisture than cool air, and why relative humidity can fall quickly when air warms without adding moisture.
| Dry bulb temperature | Saturation vapor pressure | Approximate maximum moisture behavior | Practical implication |
|---|---|---|---|
| 0°C | 6.11 hPa | Low moisture-holding capacity | Cool air reaches saturation easily |
| 10°C | 12.27 hPa | About double the value at 0°C | A modest warming strongly lowers RH if moisture stays constant |
| 20°C | 23.37 hPa | Roughly 3.8 times the value at 0°C | Typical indoor conditions are sensitive to humidification control |
| 30°C | 42.43 hPa | Nearly 7 times the value at 0°C | Warm air can feel oppressive if dew point is high |
| 40°C | 73.95 hPa | Very high moisture-holding capacity | Industrial drying and heat stress calculations become critical |
Comparison table: same dry bulb temperature, different moisture values
The next table demonstrates why dry bulb temperature alone cannot tell you relative humidity. All examples use the same dry bulb temperature, but the dew point changes. The resulting relative humidity changes substantially.
| Dry bulb temperature | Dew point | Actual vapor pressure | Relative humidity |
|---|---|---|---|
| 30°C | 10°C | 12.27 hPa | 28.9% |
| 30°C | 15°C | 17.04 hPa | 40.2% |
| 30°C | 20°C | 23.37 hPa | 55.1% |
| 30°C | 24°C | 29.83 hPa | 70.3% |
| 30°C | 27°C | 35.65 hPa | 84.0% |
Step-by-step interpretation of the result
Once you calculate relative humidity, you should interpret it in context. Relative humidity is not just a number; it influences comfort, evaporation rate, mold risk, static electricity, and process stability.
- Below 30%: Air is dry. Static electricity and skin dryness become more common.
- 30% to 60%: Often considered a comfortable indoor operating range for many buildings.
- Above 60%: Condensation and mold risk can increase, especially near cool surfaces.
- Above 80%: Air is very humid and evaporation slows significantly.
It is also important to remember that relative humidity changes when air temperature changes, even if the actual amount of water vapor does not. For example, warming indoor winter air without humidification tends to lower RH dramatically.
Common mistakes when calculating relative humidity
- Using dry bulb temperature alone. This is the most common conceptual error.
- Mixing Celsius and Fahrenheit. Most psychrometric equations require Celsius.
- Ignoring pressure in wet bulb calculations. Pressure affects the psychrometric correction.
- Assuming weather-app humidity is enough for process control. Local microclimates can differ from station data.
- Confusing relative humidity with absolute humidity. RH is temperature dependent; absolute humidity is not expressed as a percentage of saturation.
Where these formulas are used in practice
These calculations are used in many professional settings. HVAC engineers use them to size cooling coils, humidifiers, and dehumidifiers. Meteorologists use them in weather analysis and forecasting. Agricultural specialists evaluate crop stress, greenhouse control, and storage conditions. Industrial operators monitor humidity in drying, coating, printing, food processing, semiconductor fabrication, and pharmaceutical manufacturing.
For museums and archives, humidity control protects paper, wood, canvas, and photographic materials. In hospitals and laboratories, humidity affects both human comfort and equipment performance. In data centers, humidity that is too low can increase electrostatic discharge risk, while humidity that is too high can increase condensation risk.
Authoritative resources and reference material
If you want to validate the science behind relative humidity and psychrometric calculations, these sources are excellent starting points:
- National Weather Service (.gov)
- National Oceanic and Atmospheric Administration, NOAA (.gov)
- Penn State Extension educational resources (.edu)
Final takeaway
If your question is literally “how do you calculate relative humidity from dry bulb temperature,” the best expert answer is this: you do not calculate it from dry bulb temperature alone. You calculate it from dry bulb temperature plus another moisture measurement. If you have dew point, use the vapor pressure ratio method. If you have wet bulb temperature and pressure, use a psychrometric formula. In both cases, the goal is the same: determine actual vapor pressure and compare it with saturation vapor pressure at the dry bulb temperature.
Use the calculator above whenever you have the necessary measurements. It will convert the temperatures as needed, calculate the humidity, and plot the result so you can see how actual vapor pressure compares with saturation conditions.