Calculate relative humidity (%), absolute humidity (g/m³), and actual vapor pressure from air temperature and dew point. Explore moisture levels in real time.
A real humidity calculation—relative %, absolute g/m³, and water vapor pressure—powered by temperature and dew point.
In the same unit as air temperature
This tool works from temperature + dew point. If you already have relative humidity instead, use our Dew Point Calculator to find dew point.
Right now, the air around you contains an invisible ingredient: water vapor. You cannot see it, but it influences how the air feels, when condensation forms, and how weather develops.
A number like 60% humidity sounds simple. But what does that percentage really represent? How many grams of water are floating in every cubic meter of air? And what exactly is vapor pressure?
The free UtilityGlen Humidity Calculator turns air temperature and dew point into three useful measurements: relative humidity, absolute humidity, and actual water-vapor pressure.
No complicated weather equations required. Enter two temperatures, explore the results, and discover the fascinating science of moisture in the air.
Rounded illustrative readings for 25°C air with a 16.7°C dew point.
The surprising truth
Relative humidity compares the air's actual water-vapor pressure with the saturation vapor pressure at the same temperature. It is not the percentage of air molecules made of water.
A humidity calculator estimates important properties of moist air using known atmospheric measurements.
UtilityGlen uses air temperature and dew point to estimate relative humidity, absolute humidity, and actual water-vapor pressure. These measurements are related, but each describes a different aspect of atmospheric moisture.
Understanding their differences is useful for interpreting weather conditions, studying condensation, comparing moisture levels, and learning atmospheric science.
Relative humidity describes how close the air is to saturation at its current temperature.
Best for: Comparing the degree of saturation, discussing indoor humidity, and interpreting weather reports.
Absolute humidity is the mass of water vapor contained in a unit volume of moist air.
Example water-vapor mass per cubic meter.
Best for: Understanding vapor mass per volume and comparing moisture concentration at a given temperature.
Actual vapor pressure is the partial pressure exerted by water vapor within the air mixture.
Roughly 1.9 kilopascals.
Best for: Atmospheric calculations, humidity equations, and understanding water-vapor behavior.
You only need two measurements: current air temperature and dew point. The calculator takes care of the humidity formulas.
Type the measured temperature or adjust the slider. Choose Celsius or Fahrenheit.
Supply the dew point in the same temperature unit as the air temperature.
Read relative humidity (%), absolute humidity (g/m³), vapor pressure (hPa), and the temperature-to-dew-point spread.
Try Dry Day, Comfortable, Humid Day or Saturated presets, then copy your results or reset the inputs.
Remember: This tool calculates relative humidity from temperature and dew point. It doesn't measure the humidity in your room automatically. You need readings from a weather report, thermometer/dew-point sensor, or other suitable source.
Relative humidity is expressed as a percentage. At 100% RH, the air is saturated with respect to liquid water at that temperature. Lower percentages indicate lower actual vapor pressure relative to saturation vapor pressure.
Air is relatively far from saturation. In indoor environments, very low relative humidity may contribute to a dry-air sensation.
Air has an intermediate degree of saturation. Perceived comfort depends on temperature, moisture level and other conditions.
The air is closer to saturation. Under suitable conditions, cooling can cause dew, fog or surface condensation.
Important: The scale is a visual explanation, not a universal comfort or health rating. A relative humidity reading of 70% can feel very different at 10°C than at 35°C.
Imagine a parcel of air at 30°C with a dew point of 15°C. Now cool it down to 20°C without adding or removing water vapor, and assume pressure stays approximately constant.
The actual vapor pressure stays approximately the same, but saturation vapor pressure decreases as temperature falls. That means the relative humidity increases.
The lesson: Relative humidity can rise even when no extra water vapor is added. Absolute humidity, however, may change slightly during cooling because air density changes, so it should not be treated as perfectly constant in every cooling process.
The UtilityGlen Humidity Calculator uses a Magnus-type saturation vapor-pressure relationship over liquid water. It estimates vapor pressure from dew point and compares it with the saturation vapor pressure at the current air temperature.
T is air temperature in °C. The result is in hectopascals (hPa). Apply the same relationship to dew point to estimate actual vapor pressure.
Td is dew point and T is air temperature, both in °C. The ratio compares actual vapor pressure with saturation vapor pressure.
AH is in g/m³, e(Td) is actual vapor pressure in hPa, and T is air temperature in °C.
The difference between air temperature and dew point indicates how far the air temperature is above saturation.
These equations are approximations for ordinary atmospheric conditions using saturation vapor pressure over liquid water. Results may differ from ice-based humidity definitions at subfreezing temperatures, and extreme conditions may require more specialized models. Accurate inputs are essential.
At a fixed air temperature of 25°C, changes in dew point correspond to different relative humidity readings. This table illustrates the relationship.
| Air temp | Dew point | Approx. RH | Spread |
|---|---|---|---|
| 25°C | 5°C | 28% | 20°C |
| 25°C | 10°C | 39% | 15°C |
| 25°C | 15°C | 54% | 10°C |
| 25°C | 16.7°C | 60% | 8.3°C |
| 25°C | 20°C | 74% | 5°C |
| 25°C | 25°C | 100% | 0°C |
Rounded reference values calculated using a Magnus-type vapor-pressure formula over liquid water. They are illustrative, not live atmospheric observations.
Atmospheric moisture affects weather, evaporation, condensation, and the conditions inside our homes. Understanding humidity helps explain many familiar experiences.
Humidity influences evaporation and how the air feels. On hot days, high moisture levels can make sweating less effective at cooling the body.
Indoor relative humidity can affect perceptions of dryness or dampness. Temperature, ventilation, insulation, and moisture sources also influence indoor conditions.
When the temperature of a surface falls below the dew point of nearby air, water vapor can condense into visible droplets.
Moisture influences plant transpiration and greenhouse conditions. Specialized horticultural planning may also use vapor-pressure deficit, which is different from actual vapor pressure.
Both describe atmospheric moisture, but they answer different questions.
“How close is this air to saturation right now?”
Its value depends on both the amount of water vapor and the current air temperature.
“At what temperature would this air reach saturation?”
Dew point offers a useful indication of atmospheric water-vapor content at comparable pressure.
Useful relationship: When air temperature and dew point are equal, relative humidity is approximately 100% under the assumptions of the calculation.
Clear answers to the most common questions about relative humidity, absolute humidity, vapor pressure, dew point, and moisture calculations.
Discover the hidden moisture around you with UtilityGlen's free Humidity Calculator. Enter air temperature and dew point, calculate relative and absolute humidity, explore vapor pressure, and understand the science behind your results.
UtilityGlen calculates humidity from temperature and dew point using a Magnus-type saturation vapor-pressure approximation over liquid water. Relative humidity is expressed as a percentage, absolute humidity in g/m³, and actual vapor pressure in hPa. Results are estimates based on the supplied conditions and are not a substitute for calibrated atmospheric measurements.
For more information about humidity, dew point and atmospheric science, visit:
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