What is relative humidity and how does this calculator determine it?

Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount the air can hold at a given temperature, expressed as a percentage. This calculator uses standard psychrometric formulas, typically requiring inputs like air temperature and dew point (or wet-bulb temperature), to compute that percentage based on the saturation vapor pressure at your specified conditions.

What inputs do I need to provide for the Relative Humidity Calculator to work?

You will need to enter at least two key parameters, most commonly the air temperature (in Celsius or Fahrenheit) and the dew point temperature, though some versions can also use wet-bulb temperature and barometric pressure. If you only have dry-bulb and wet-bulb readings from a sling psychrometer, the calculator can derive relative humidity using the difference between those two values and the ambient pressure.

Can I use this calculator for both indoor and outdoor environmental assessments?

Yes, the Relative Humidity Calculator is versatile and works for any air sample, whether from a room, greenhouse, or outside, as long as the temperature and moisture inputs are measured at the same location and time. However, for outdoor use, it is important to account for local atmospheric pressure, which can vary with altitude, and the calculator allows you to adjust that for more accurate results in high-elevation areas.

How accurate is the Relative Humidity Calculator compared to a physical hygrometer?

When provided with precise inputs (e.g., calibrated thermometer and dew point data), the calculator is mathematically exact based on the Magnus formula or similar psychrometric approximations, typically within ±0.5% of professional hygrometer readings under standard conditions. However, real-world accuracy depends on the quality of your temperature and dew point measurements—if those have errors of even 0.5°C, the calculated relative humidity could deviate by several percentage points, so always use well-maintained sensors.

Relative Humidity Calculator

Last updated: 2026-09-09

Relative Humidity Calculator — Free online relative humidity calculator. Enter temperature and dew point to get instant results.

Inputs

Result

Enter values and press Calculate

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Common Examples — Click to Fill

	Temperature	Dew point
Light	10	5
Moderate	15	8
Strong	20	10
Severe	30	15
Extreme	40	20

TL;DR: To calculate relative humidity, divide the actual vapor pressure by the saturation vapor pressure at the current temperature and multiply by 100, but the simplest method is using the formula RH = 100 × (e_s(T_d) / e_s(T)), where T_d is the dew point and T is the air temperature — which is exactly what this calculator does instantly when you enter those two values.

What Is the Relative Humidity Calculator?

The Relative Humidity Calculator is a free online tool that computes the percentage of water vapor in the air relative to the maximum amount the air can hold at a given temperature. You input just two numbers — the current air temperature and the dew point temperature — and the calculator returns the relative humidity (RH) as a percentage. This is not an approximation; it uses the Magnus formula, a standard meteorological equation, to deliver accurate results for weather monitoring, HVAC system design, agricultural planning, and everyday comfort assessment.

Relative humidity is one of the most misunderstood weather metrics because it is temperature-dependent. Air at 25°C with a dew point of 15°C feels different from air at 35°C with the same dew point, even though the actual moisture content is identical. This calculator solves that confusion by giving you the exact percentage of saturation, which is what your body perceives as "mugginess" and what HVAC engineers use to prevent mold growth or static electricity buildup. Anyone from a homeowner checking a hygrometer to a greenhouse manager optimizing plant transpiration will find this tool essential.

The critical insight is that you do not need a psychrometer or wet-bulb thermometer to find relative humidity — the dew point alone, combined with air temperature, mathematically determines it. Because dew point is an absolute measure of moisture (it does not change with temperature), this calculator eliminates the guesswork and provides a value that is scientifically valid across all atmospheric conditions.

How to Use the Calculator

Using the Relative Humidity Calculator takes less than ten seconds. Follow this numbered sequence exactly as the input fields appear:

Enter Air Temperature: Type the current air temperature in the first input field. You can enter any positive or negative value. The calculator accepts decimals for precision (e.g., 23.5).
Enter Dew Point: Type the dew point temperature in the second input field. This is the temperature at which the air would become fully saturated. It must be lower than or equal to the air temperature — if the dew point exceeds the air temperature, condensation would already have occurred and the calculation becomes physically invalid.
Click "Calculate": Press the Calculate button. The tool processes both values immediately using the Magnus formula.
Read the Result: The output displays the relative humidity as a percentage (e.g., 58.3%). A reading of 100% means the air is fully saturated (fog or rain), while 0% would mean perfectly dry air (virtually impossible on Earth).
Reset if Needed: If you want to try different values, use the Reset button to clear both fields and start over.

Formula and Calculation Method

The calculator uses the standard meteorological approximation known as the Magnus formula, which relates saturation vapor pressure to temperature. The relative humidity is the ratio of the actual vapor pressure (determined by the dew point) to the saturation vapor pressure (determined by the air temperature), multiplied by 100.

The formula is expressed as:

RH = 100 × [ exp((17.625 × T_d) / (243.04 + T_d)) / exp((17.625 × T) / (243.04 + T)) ]

Where:

T = air temperature in degrees Celsius
T_d = dew point temperature in degrees Celsius
exp = the exponential function (e raised to the power of the argument)
17.625 and 243.04 are empirical constants optimized for atmospheric conditions

In plain language, the formula does this: it first calculates how much water vapor the air currently holds (based on the dew point) and then calculates the maximum water vapor the air could hold (based on the air temperature). The ratio of those two values, expressed as a percentage, is the relative humidity.

Worked Example: Suppose the air temperature is 30°C and the dew point is 20°C. Plug the numbers into the formula:

For the dew point (numerator): exp((17.625 × 20) / (243.04 + 20)) = exp(352.5 / 263.04) = exp(1.340) = 3.820
For the air temperature (denominator): exp((17.625 × 30) / (243.04 + 30)) = exp(528.75 / 273.04) = exp(1.937) = 6.938
Divide: 3.820 / 6.938 = 0.5506
Multiply by 100: RH = 55.1%

This means the air at 30°C with a 20°C dew point is 55.1% saturated. The air feels humid but not oppressive — water will evaporate readily, but you will notice perspiration not drying quickly.

Practical Examples

Here are three realistic scenarios demonstrating how the calculator behaves across different conditions. The table below summarizes the inputs and outputs.

Scenario	Air Temp (°C)	Dew Point (°C)	Relative Humidity	What It Means
Dry Desert Afternoon	40	10	16.1%	Extremely dry. Skin dries fast, static electricity common, wildfire risk heightened.
Comfortable Spring Day	20	12	60.3%	Comfortable. Humans feel fine, but mold can start growing in poorly ventilated spaces.
Tropical Rainforest	28	26	89.6%	Very muggy. Clothes feel damp, perspiration barely evaporates, heat stress is likely.

Notice the pattern: as the difference between air temperature and dew point shrinks, the relative humidity rises. When the two numbers are equal, RH = 100% and fog or precipitation occurs. If your dew point is within 3°C of the air temperature, the air feels oppressive regardless of the actual temperature.

Another useful example involves temperature changes. If the dew point stays fixed at 15°C but the air temperature drops from 25°C to 20°C, the relative humidity increases from 54% to 74% — even though no moisture was added or removed. This is why condensation forms on cold windows at night: the RH near the glass reaches 100% as the temperature drops to the dew point.

Tips for Accurate Results

To get the most reliable relative humidity reading from this calculator, follow these specific guidelines aligned with the input fields and the formula's assumptions:

Verify units are Celsius: The Magnus formula constants (243.04 and 17.625) are calibrated for degrees Celsius. If your thermometer reports Fahrenheit, convert to Celsius first using C = (F − 32) × 5/9. Entering Fahrenheit values will produce wildly incorrect results.
Ensure dew point ≤ air temperature: The dew point can never exceed the air temperature in stable atmospheric conditions. If you see this, your dew point reading is wrong or you swapped the two fields. The calculator will still process it, but the result will be above 100%, which is unphysical without supersaturation (which only occurs in clouds).
Do not round intermediate steps: The exponential function is sensitive to input rounding. If you are doing the calculation by hand alongside the calculator, carry at least four decimal places through each step before rounding the final percentage. Rounding the dew point from 15.4°C to 15°C changes the result by roughly 1.5 percentage points.
Check the validity range: The Magnus constants are accurate between −40°C and +50°C for both temperature and dew point. Outside this range, the formula deviates from true values by more than 1%. For polar or industrial extremes, consult a psychrometric chart instead.
Use a properly calibrated dew point sensor: The calculator is only as good as your inputs. Electronic hygrometers require periodic calibration with a salt solution kit. A 1°C error in dew point translates to approximately 3–5% error in relative humidity at typical room temperatures.
Take both readings at the same location and time: Temperature and dew point can vary by several degrees within a room. Measure both within a few minutes of each other, away from heat sources, air conditioning vents, or humidifiers.

Frequently Asked Questions

1. Why does relative humidity change with temperature if the moisture in the air is the same?

Relative humidity is a ratio, not an absolute measure. Warm air has a higher capacity for water vapor than cold air. At 30°C, air can hold up to about 30 grams of water vapor per cubic meter, but at 10°C, that capacity drops to about 9 grams. If you have 9 grams of water vapor in both cases, the 30°C air is at 30% RH while the 10°C air is at 100% RH. This is why the same dew point produces different relative humidities at different temperatures. The actual amount of moisture (absolute humidity) does not change — only the saturation point does. This is why meteorologists prefer dew point for communicating "how much water is in the air" and relative humidity for communicating "how close to saturation we are."

2. Can I use this calculator for weather forecasting?

Yes, but with context. This calculator tells you the instantaneous relative humidity, which is useful for predicting fog formation (RH near 100%), dew on grass (RH near 100% at ground level), and human comfort (RH between 40% and 60% is ideal). However, it does not predict future humidity — that requires knowing how the temperature will change. If you know the forecasted high temperature and you measure the current dew point, you can use this calculator to estimate the minimum relative humidity that will occur that day (since RH is lowest at maximum temperature). For precipitation forecasting, you need additional data like cloud cover and frontal boundaries, but the RH value is a strong indicator of convective thunderstorm potential when it exceeds 70% in the lower atmosphere.

3. What is the difference between relative humidity, absolute humidity, and dew point?

Absolute humidity is the mass of water vapor per unit volume of air (grams per cubic meter). It does not change with temperature unless moisture is added or removed. Dew point is the temperature to which air must be cooled to become saturated (reach 100% RH) at constant pressure. It is also an absolute measure — a dew point of 15°C always represents the same moisture content. Relative humidity is the percentage of the air's moisture content relative to its maximum capacity at the current temperature. The key relationship: if you know any two of these three values (temperature, dew point, and RH), you can calculate the third. This calculator specifically converts temperature and dew point into relative humidity. For example, a dew point of 15°C always represents about 10.5 grams of water per cubic meter, regardless of whether the air temperature is 20°C or 35°C — but the relative humidity will be very different (74% vs. 32%).