Relative humidity measures how much water vapour is currently in the air compared to the maximum amount the air can hold at the same temperature. The relative humidity formula is RH (%) = (actual vapour pressure ÷ saturation vapour pressure) × 100, where actual vapour pressure shows the moisture already present in the air and saturation vapour pressure shows the maximum moisture the air can hold before reaching saturation.
This formula helps explain why the same amount of moisture can feel very different at different temperatures. Since warm air can hold more water vapour than cool air, relative humidity changes whenever temperature changes, even if no moisture is added or removed.
Key Takeaways
- Relative humidity is calculated as actual vapour pressure divided by saturation vapour pressure, multiplied by 100.
- Saturation vapour pressure increases with temperature, which is why RH drops when you heat a space without adding moisture.
- RH can be calculated from temperature and dew point, or from dry bulb and wet bulb temperature readings.
- The Magnus formula is the standard approximation used in HVAC and meteorology for saturation vapour pressure.
- In operational settings, a calibrated hygrometer gives more reliable RH readings than manual calculation.
What Is Relative Humidity
Relative humidity, or RH, is the percentage that shows how close air is to being fully saturated with water vapour at a specific temperature. If the RH is 50%, the air contains half of the water vapour it could hold before reaching saturation at that temperature.
This matters because air temperature controls how much moisture the air can hold. Warmer air has a higher saturation point, while cooler air reaches saturation sooner. That is why RH can rise as air cools and drop as air warms, even when the actual amount of water vapour stays the same.
Relative humidity is different from absolute humidity. Absolute humidity measures the actual mass of water vapour in a volume of air, while relative humidity compares that moisture level against the air’s maximum capacity at the current temperature. This makes RH more useful for comfort, weather, HVAC, storage, and facility humidity control.
The Relative Humidity Formula
The relative humidity formula compares the moisture already present in the air with the maximum moisture the air can hold at the same temperature. It shows how close the air is to saturation, expressed as a percentage.
Formula:
RH (%) = (Actual Vapor Pressure ÷ Saturation Vapor Pressure) × 100
In this formula:
- RH (%) means relative humidity as a percentage.
- Actual vapor pressure means the current amount of water vapor pressure in the air.
- Saturation vapor pressure means the maximum water vapor pressure the air can hold at that temperature before condensation begins.
Breaking Down the Variables
Actual vapor pressure is the pressure created by water vapor already present in the air. It is usually expressed in pascals, hectopascals, or millibars, depending on the measurement system used.
Saturation vapor pressure changes with temperature. Warm air has a higher saturation vapor pressure because it can hold more water vapor before reaching saturation, while cooler air reaches saturation with less water vapor.
When actual vapor pressure equals saturation vapor pressure, relative humidity reaches 100%. At that point, the air is saturated, and any further cooling can cause water vapor to condense into liquid water on nearby surfaces.
The Magnus Formula: A Practical Approximation
The Magnus formula is commonly used to estimate saturation vapor pressure from temperature. It is useful because most real-world RH calculations start with temperature and dew point rather than directly measured vapor pressure.
Magnus approximation:
es(T) = 6.112 × exp((17.67 × T) ÷ (T + 243.5)) (August-Roche-Magnus approximation — coefficients per WMO Guide to Instruments and Methods of Observation, WMO-No. 8)
In this equation, es(T) is saturation vapor pressure in hPa, and T is temperature in degrees Celsius. When calculating RH from dew point, the dew point is used to estimate actual vapor pressure, while the air temperature is used to estimate saturation vapor pressure.
The simplified RH calculation becomes:
RH (%) = [es(dew point) ÷ es(air temperature)] × 100
This method gives a practical estimate for weather, HVAC, and facility humidity calculations without needing a full psychrometric chart. For broader technical context, the WMO Guide to Instruments and Methods of Observation provides a standard reference for meteorological humidity measurement.
How to Calculate Relative Humidity: Step-by-Step
Calculating relative humidity depends on the data available. The most common method uses air temperature and dew point, while field measurements often use dry bulb and wet bulb temperature. NOAA’s moisture calculator also uses temperature and dew point inputs to estimate humidity values in a practical calculation format.
Both methods compare actual moisture in the air against the saturation limit at the current temperature.
Worked Example 1: Using Temperature and Dew Point
Suppose the air temperature is 25°C and the dew point is 15°C. The dew point tells you the temperature at which the current water vapor in the air would reach saturation.
Here is the calculation process:
- Saturation vapor pressure at 15°C: about 17.04 hPa
- Saturation vapor pressure at 25°C: about 31.67 hPa
- RH (%) = (17.04 ÷ 31.67) × 100
- RH (%) = 53.8%
So, when the air temperature is 25°C and the dew point is 15°C, the relative humidity is about 54%.
Worked Example 2: Using Dry Bulb and Wet Bulb Temperature
Dry bulb temperature is the regular air temperature. Wet bulb temperature is measured with a wetted thermometer bulb, where evaporation cools the reading. The difference between the two values helps estimate how much moisture is in the air.
Suppose the dry bulb temperature is 25°C, the wet bulb temperature is 20°C, and standard atmospheric pressure is used. A psychrometric chart or standard wet bulb equation can estimate the actual vapor pressure from these readings.
Here is the simplified calculation process:
- Dry bulb temperature: 25°C
- Wet bulb temperature: 20°C
- Wet bulb depression: 5°C
- Estimated actual vapor pressure: about 19.95 hPa
- Saturation vapor pressure at 25°C: about 31.67 hPa
- RH (%) = (19.95 ÷ 31.67) × 100
- RH (%) = 63%
So, with a dry bulb temperature of 25°C and a wet bulb temperature of 20°C, the relative humidity is about 63%. This method is useful in HVAC, industrial monitoring, and field conditions where direct dew point data may not be available. The psychrometric relationship between wet bulb depression and vapor pressure is documented in the ASHRAE Handbook of Fundamentals, Chapter 1 (Psychrometrics).
Relative Humidity Calculator
A relative humidity calculator helps convert temperature and dew point into an RH percentage without doing each equation manually. It is useful when you need a quick estimate for comfort checks, HVAC review, storage conditions, or facility monitoring.
If an embedded calculator is not available, a static reference table can still help readers compare common temperature and dew point combinations. The values below show how RH changes when air temperature stays the same but dew point changes.
Here are example RH values using air temperature and dew point:
- 20°C air temperature with 10°C dew point: Approximate RH is 53%.
- 20°C air temperature with 15°C dew point: Approximate RH is 73%.
- 25°C air temperature with 10°C dew point: Approximate RH is 39%.
- 25°C air temperature with 15°C dew point: Approximate RH is 54%.
- 25°C air temperature with 20°C dew point: Approximate RH is 74%.
- 30°C air temperature with 20°C dew point: Approximate RH is 55%.
These examples show why temperature and dew point inputs must be considered together. A dew point of 15°C does not produce the same RH at 20°C as it does at 25°C because warmer air has a higher saturation vapor pressure.
What Affects Relative Humidity and Why It Changes
Relative humidity changes when temperature, moisture content, ventilation, or air movement changes. Since RH compares actual water vapor with the air’s maximum moisture capacity, the percentage can rise or fall even when no new moisture is added.
Why RH Drops When You Heat a Space
Heating a space lowers relative humidity because warm air has a greater capacity to hold water vapor. The actual amount of water vapor may stay the same, but the saturation vapor pressure increases, so the RH percentage drops.
This is why indoor air often feels dry in winter. Cold outdoor air enters the building, gets heated, and then shows a lower RH because the warmer air can hold more moisture than it currently contains. ASHRAE guidance on relative humidity in habitable spaces also reinforces why humidity control matters for indoor environments.
Here is the basic relationship:
- When temperature rises: Air moisture capacity increases, and relative humidity decreases if no moisture is added.
- When temperature falls: Air moisture capacity decreases, and relative humidity increases if moisture content stays the same.
- When temperature stays stable: RH mainly changes due to added or removed moisture, though ventilation, occupancy, equipment, and process conditions can still affect readings.
This concept matters in offices, warehouses, manufacturing areas, and other controlled spaces. Heating alone does not add moisture, so dry indoor air can create comfort issues, static concerns, and material-handling problems unless humidity is actively managed.
Relative Humidity Targets by Application
Different spaces need different RH targets because comfort, materials, equipment, and process quality all respond differently to moisture. A general home range is not enough for facilities that handle electronics, pharmaceuticals, printed materials, stored products, or controlled production environments.
Here are common RH targets by application:
- Residential living spaces: 30% to 50% RH for comfort, air quality, and mold prevention.
- Office environments: 30% to 60% RH for comfort, air quality, and static reduction.
- Data centers: 40% to 60% RH for equipment safety and static control.
- Pharmaceutical manufacturing: Product and process dependent for stability, handling, and GMP control.
- Cleanrooms: Process dependent for contamination control and material stability.
- Printing facilities: Around 45% to 55% RH for paper stability, static reduction, and print quality.
- Cold storage: Product dependent for moisture retention, condensation control, and storage life.
- Greenhouses: Crop and growth-stage dependent for transpiration, VPD, and disease control.
High relative humidity can increase condensation and mold risk in some environments, while low RH can contribute to static, material shrinkage, or comfort issues. The right target depends on the application, not just a universal “ideal” number.
When to Use a Hygrometer Instead of Calculating
The relative humidity formula is useful for learning how RH works, checking sample conditions, or validating a basic calculation. In real operating environments, however, a calibrated hygrometer or sensor is usually the better choice because humidity changes throughout the day.
A hygrometer gives real-time RH readings without requiring repeated manual calculations. In professional settings, teams may use capacitive humidity sensors, chilled mirror hygrometers, psychrometers, or sensor arrays depending on the accuracy and documentation required.
For facilities that need continuous monitoring and correction, humidity control systems help maintain target RH across larger spaces and changing operating conditions. This is especially important in storage rooms, laboratories, production areas, greenhouses, museums, and other spaces where small RH changes can affect materials, comfort, equipment, or process quality.
Final Thoughts
The relative humidity formula helps explain how temperature, dew point, and vapor pressure work together. Once you know that RH compares actual vapor pressure with saturation vapor pressure, it becomes easier to see why humidity rises when air cools and drops when air warms.
For facilities where maintaining precise RH is an operational requirement, Smart Fog’s non-wetting humidification systems are engineered to deliver stable, uniform humidity control with self-evaporating droplets.
FAQ
What is the relative humidity formula?
The relative humidity formula is RH (%) = (actual vapor pressure ÷ saturation vapor pressure) × 100. It compares the water vapor currently in the air with the maximum water vapor the air can hold at the same temperature.
How do you calculate relative humidity from temperature and dew point?
Calculate saturation vapor pressure at the dew point and at the air temperature. Then divide the dew point vapor pressure by the air temperature vapor pressure and multiply by 100 to get the RH percentage.
What does 100% relative humidity mean?
A relative humidity of 100% means the air is saturated at its current temperature. It cannot hold more water vapor in gaseous form, so further cooling may cause condensation on surfaces.
Why does relative humidity change with temperature?
Relative humidity changes because warm air can hold more water vapor than cool air. If moisture content stays the same, RH drops when temperature rises and increases when temperature falls.
What is the difference between relative humidity and absolute humidity?
Relative humidity compares current moisture to the air’s maximum moisture capacity at a given temperature. Absolute humidity measures the actual mass of water vapor in a specific volume of air.
Can you calculate relative humidity without a hygrometer?
Yes, relative humidity can be calculated using temperature and dew point or dry bulb and wet bulb temperature. However, a calibrated hygrometer is better for continuous, real-time humidity monitoring.
What is a good indoor relative humidity level?
A common indoor RH range is 30% to 50% for residential comfort and moisture control. Commercial and industrial spaces may need different targets based on equipment, materials, products, or process requirements.
Why does indoor air feel dry in winter?
Indoor air often feels dry in winter because cold outdoor air enters the building and is heated. Heating raises the air’s moisture capacity, which lowers relative humidity if no moisture is added.
