Calculate relative humidity, dew point, or air temperature. Explore the relationship between moisture in the air and temperature.
Understanding relative humidity is essential for maintaining comfort, protecting your health, and preventing issues like mold growth or dry air. Our relative humidity calculator helps you instantly calculate relative humidity from temperature and dew point, or find the dew point when you know the temperature and humidity level. Whether you're adjusting your home HVAC system, planning outdoor activities, or studying atmospheric physics, this RH calculator gives you accurate results in seconds.
Instantly calculate relative humidity, dew point, or temperature. Understand the relationship between moisture in the air and temperature for comfort, health, and weather applications.
Calculate using Celsius, Fahrenheit, or Kelvin for temperature inputs
Find RH from temperature & dew point, or calculate any missing value
See instantly if humidity is too dry, comfortable, or too humid
Get detailed explanations of your results with our AI assistant
Whether you're adjusting your home HVAC, planning outdoor activities, or studying meteorology, our calculator provides accurate results based on the Magnus-Tetens formula used by professionals worldwide.
Relative humidity (RH) is a measure of how much moisture is present in the air compared to the maximum amount of water vapor the air can hold at a given temperature. It's expressed as a percentage from 0% to 100%. When you need to determine relative humidity, you're essentially finding out how "saturated" the air is with moisture.
For example, if the relative humidity is 50%, the air is holding half of the maximum moisture it could contain at that temperature. At 100% relative humidity, the air is fully saturated—any additional moisture will condense into dew, fog, or rain.
The key insight: warmer air can hold more water vapor than cooler air. This is why relative humidity changes throughout the day even when the actual moisture content stays the same.
When learning how to calculate humidity, it helps to understand these related concepts:
Dew point is often considered a better "feels like" humidity measure because it's an absolute value that doesn't change with temperature. A dew point of 65°F (18°C) or higher typically feels muggy regardless of the actual temperature.
Understanding how to work out relative humidity is important for many reasons:
| RH Level | Comfort | Effects & Risks |
|---|---|---|
| < 25% | Too Dry | Dry skin, respiratory irritation, static electricity, wood cracking |
| 25-30% | Dry | Minor discomfort, dry eyes, potential for slight irritation |
| 30-50% | Comfortable ✓ | Ideal range for health, comfort, and home protection |
| 50-60% | Slightly Humid | Acceptable, may feel warm. Monitor for mold in poorly ventilated areas |
| 60-70% | Humid | Mold risk increases, sweating less effective, may feel sticky |
| > 70% | Very Humid | High mold/mildew risk, significant discomfort, health concerns |
Our humidity calculator uses the widely-accepted Magnus-Tetens formula (with Alduchov-Eskridge coefficients) to calculate RH accurately. This is the same method used by meteorologists and engineers worldwide. Simply enter two of the three values (temperature, dew point, or relative humidity), and the calculator instantly computes the missing value.
When you need to know how to calculate relative humidity from temperature and dew point, the formula is:
This relative humidity formula is accurate for temperatures between -40°C and 50°C, covering virtually all conditions you'll encounter in daily life.
Select what you want to calculate: Relative Humidity, Dew Point, or Temperature
Enter the known values (temperature and/or dew point) in your preferred units
If calculating dew point or temperature, also enter the relative humidity percentage
Click Calculate to instantly get your result with a comfort level assessment
Use the AI explanation feature for detailed insights about your specific calculation
Let's work out the relative humidity when the air temperature is 95°F (35°C) and the dew point is 70°F (21.1°C).
Find the relative humidity for a warm summer day.
RH = 100 × exp[17.625 × Dp/(243.04 + Dp)] / exp[17.625 × T/(243.04 + T)]
= 100 × exp[17.625 × 21.1/(243.04 + 21.1)] / exp[17.625 × 35/(243.04 + 35)]
= 100 × exp[1.408] / exp[2.219]
≈ 44.48%
At 44.48% relative humidity, the air is moderately humid. This is within the comfortable range (30-50%) for indoor environments. However, combined with the high temperature of 95°F, it will still feel quite warm outdoors due to reduced sweat evaporation.
If you want to figure relative humidity without a calculator, follow these steps:
A psychrometer uses two thermometers—one with a wet wick (wet bulb) and one without (dry bulb). As water evaporates from the wet bulb, it cools down. The difference between readings helps determine relative humidity using psychrometric charts or tables.
When you need to find out relative humidity at home or work, several instruments can help:
For most homes and offices, maintaining 30-50% relative humidity provides the best balance of comfort and health. This range:
In winter, you may need to lower the target slightly (30-40%) to prevent condensation on cold windows. In summer, staying under 60% helps prevent that "sticky" feeling and mold growth.
Understanding what influences RH helps you better control your indoor environment:
Uses the industry-standard Magnus-Tetens equation with Alduchov-Eskridge coefficients for precise results
Calculate relative humidity, dew point, or temperature—whichever value you need
Support for Celsius, Fahrenheit, and Kelvin temperature units with automatic conversion
Instant feedback on whether humidity levels are comfortable, too dry, or too humid
Get detailed, personalized explanations of your calculations with our AI assistant
Q1. What is relative humidity and how is it calculated?
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Relative humidity (RH) is the ratio of the current amount of water vapor in the air to the maximum amount the air could hold at that temperature, expressed as a percentage. To calculate relative humidity, you need the air temperature and dew point, then apply the Magnus formula: RH = 100 × exp[17.625 × Dp/(243.04 + Dp)] / exp[17.625 × T/(243.04 + T)], where T is temperature and Dp is dew point in Celsius.
Q2. How do I calculate relative humidity with temperature and dew point?
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To calculate relative humidity from temperature and dew point: 1) Measure or obtain both values in °C. 2) Use our RH calculator by entering the temperature and dew point. 3) The calculator applies the Magnus-Tetens formula to give you the relative humidity percentage. For example, if temperature is 25°C and dew point is 15°C, the relative humidity is approximately 55%.
Q3. What does 100% relative humidity mean?
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When relative humidity reaches 100%, the air is completely saturated with water vapor—it's holding the maximum amount of moisture possible at that temperature. At this point, any cooling will cause condensation, forming dew, fog, or precipitation. This typically occurs when the air temperature equals the dew point temperature.
Q4. What happens to relative humidity when temperature increases?
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If the moisture content of the air stays constant while temperature increases, relative humidity decreases. This is because warmer air has a higher capacity to hold water vapor. For example, if you heat a room without adding moisture, the RH will drop, which is why indoor air often feels dry in winter when heating is on.
Q5. What is a good relative humidity for a house?
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The ideal indoor relative humidity for comfort and health is between 30-50%. This range minimizes dust mites and mold growth, reduces static electricity, keeps respiratory passages comfortable, and protects wooden furniture. In winter, aim for the lower end (30-40%) to prevent window condensation; in summer, stay under 60%.
Q6. How do you measure relative humidity at home?
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The easiest way to measure relative humidity at home is with a digital hygrometer, available at most hardware stores for under $20. Alternatively, you can use a psychrometer (wet and dry bulb thermometer) or measure temperature and dew point separately, then use an online calculator like ours to determine RH.
Q7. What is the difference between relative humidity and dew point?
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Relative humidity is a percentage showing how saturated the air is compared to its capacity at current temperature—it changes as temperature changes. Dew point is the temperature at which air would become saturated (100% RH)—it's an absolute measure that only changes when moisture content changes. Dew point is often considered a better indicator of how 'muggy' it feels.
Q8. Can relative humidity be over 100%?
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In rare cases called 'supersaturation,' relative humidity can briefly exceed 100%. This occurs in very clean air without condensation nuclei (tiny particles) or during rapid cooling. However, this is unstable and short-lived—the excess moisture will quickly condense. For practical purposes, 100% is the maximum.
Q9. Is 60% humidity too high indoors?
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60% relative humidity is on the high end for indoor comfort. While not immediately dangerous, sustained humidity above 60% can promote mold growth, dust mites, and make the air feel stuffy. Consider using a dehumidifier or improving ventilation if your indoor humidity regularly exceeds 55-60%.
Q10. How does relative humidity affect how hot it feels?
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High relative humidity makes hot temperatures feel even hotter because sweat evaporates more slowly from your skin, reducing your body's ability to cool itself. This is why the 'feels like' or 'heat index' temperature is often higher than the actual temperature on humid days. At 90°F with 70% RH, it can feel like 106°F.
Q11. What is the relative humidity formula?
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The most accurate formula for calculating relative humidity is the Magnus-Tetens equation: RH = 100 × exp[β × Dp/(λ + Dp)] / exp[β × T/(λ + T)], where T is air temperature (°C), Dp is dew point (°C), β = 17.625, and λ = 243.04°C. This formula is accurate for temperatures between -40°C and 50°C.
Q12. How do you find relative humidity without a hygrometer?
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Without a hygrometer, you can approximate relative humidity using the ice cube test (if a glass 'sweats' within 3 minutes in a room, humidity may be high), observe a psychrometer reading with two thermometers, or check local weather reports for outdoor conditions. For accurate indoor measurements, use our calculator with a thermometer and estimate dew point from weather data.
Q13. Why does my house feel humid even when AC is running?
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If your house feels humid with the AC on, the unit may be oversized (cooling too fast to properly dehumidify), the evaporator coil may be dirty, or there may be air leaks bringing in humid outdoor air. AC units remove moisture by condensing water vapor on cold coils—if the unit cycles too quickly, this process doesn't complete effectively.
Q14. At what humidity does mold grow?
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Mold typically begins to grow when relative humidity exceeds 60% and thrives above 70%. Mold spores are always present in the air, but they need moisture to grow. Keep indoor humidity below 50-55% and ensure good ventilation in bathrooms, kitchens, and basements to prevent mold growth.
Q15. How do I convert dew point to relative humidity?
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To convert dew point to relative humidity, you also need to know the current air temperature. Then use the Magnus formula or our calculator. Enter both the temperature and dew point, and the calculator will automatically compute the relative humidity percentage.