Is the Dew Point Calculator free to use?

Yes, our Dew Point Calculator is completely free to use with no registration required.

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Dew Point Calculator

Calculate dew point temperature from air temperature and relative humidity using the Magnus formula. Understand humidity levels, comfort ratings, and absolute humidity with our free online dew point calculator.

Air Temperature (°C)

Relative Humidity (%)

Quick Calculate

Dew Point

17°C

Temperature at which condensation begins

Relative Humidity

60%

Absolute Humidity

0.1 g/m³

Comfort Level

Slightly Humid

Air feels slightly humid. Some people may start to notice moisture in the air.

Dew Point Formula (Magnus Formula)

T_d = (b × α(T, RH)) / (a - α(T, RH))

Where:

α(T, RH) = (a × T) / (b + T) + ln(RH/100)
a = 17.27
b = 237.7°C
T = air temperature (°C)
RH = relative humidity (%)

The Magnus formula is an approximation that works well for temperatures between -40°C and 50°C. It is widely used in meteorology and HVAC applications.

Understanding Humidity

Relative Humidity

Compares current moisture to the maximum the air can hold at that temperature. Higher percentages mean more moisture relative to capacity.

Absolute Humidity

The actual mass of water vapor in a given volume of air, measured in grams per cubic meter (g/m³). Unlike relative humidity, it does not change with temperature at fixed volume.

Dew Point

The temperature at which air becomes saturated and water vapor begins to condense. It is a direct measure of moisture content and is more reliable than relative humidity for assessing comfort.

Why Dew Point Matters

Dew point indicates how muggy the air feels. A dew point below 10°C (50°F) is comfortable, while above 20°C (68°F) feels oppressive. Unlike relative humidity, dew point does not fluctuate much throughout the day.

Dew Point Chart (°C)

Temp (°C)	20%	30%	40%	50%	60%	70%	80%	90%
0°C	-20	-15	-12	-9	-7	-5	-3	-1
5°C	-16	-11	-7	-5	-2	0	2	3
10°C	-12	-7	-3	0	3	5	7	8
15°C	-8	-2	2	5	7	10	12	13
20°C	-4	2	6	9	12	14	16	18
25°C	0	6	10	14	17	19	21	23
30°C	5	11	15	18	21	24	26	28
35°C	9	15	19	23	26	29	31	33

Color coding: blue = very dry, green = comfortable, yellow = slightly humid, red = humid/muggy. Dew point values shown in °C.

How to Use

1
Select your unit system — Imperial (°F) or Metric (°C)
2
Choose your input mode: enter either relative humidity (%) or dew point temperature
3
Enter the current air temperature — the calculator supports a wide range from -40°C to 50°C
4
If using relative humidity mode, enter the humidity percentage (0-100%) — this is the most common way to calculate dew point from weather data
5
If using dew point mode, enter the dew point temperature directly — useful when weather reports show dew point instead of relative humidity
6
View your dew point temperature, relative humidity, and absolute humidity in g/m³
7
Check the comfort level indicator to understand how the air feels — from Very Dry to Very Humid
8
Use the quick calculate buttons for common indoor and outdoor humidity scenarios to quickly compare conditions
9
Review the dew point chart below to see how dew point changes across different temperature and humidity combinations for planning outdoor activities or HVAC settings

Examples

Good Examples

Comfortable indoor conditions

22°C air temperature with 50% relative humidity → dew point of 11°C — Comfortable: ideal for most indoor activities and sleeping

Muggy summer day

32°C air temperature with 85% relative humidity → dew point of 29°C — Very Humid: oppressive conditions, limit outdoor exertion

Dry winter air

18°C indoor temperature with 25% relative humidity → dew point of -2°C — Very Dry: consider using a humidifier to prevent skin irritation and respiratory discomfort

Weather report interpretation

Air temperature 28°C, dew point 20°C → relative humidity ≈ 62%, absolute humidity ≈ 17.3 g/m³ — Slightly Humid: noticeable but tolerable for outdoor exercise

Comparing indoor vs outdoor comfort

Indoor: 23°C, 45% RH → dew point 10°C (Comfortable). Outdoor: 30°C, 75% RH → dew point 25°C (Humid). The difference explains why stepping outside feels oppressive

HVAC system optimization

Target dew point of 10-15°C for ideal indoor comfort. If current reading is 22°C dew point, the air conditioning should reduce humidity to bring it within the comfort zone

Bad Examples

Confusing dew point with relative humidity

A dew point of 20°C always means the same moisture content regardless of air temperature, but relative humidity at 20°C dew point varies from 47% at 30°C to 100% at 20°C

Ignoring dew point for outdoor exercise

A dew point above 20°C significantly impairs the body's cooling ability — exercise performance drops and heat illness risk increases even if the air temperature seems moderate

Assuming low relative humidity means comfortable conditions

At 35°C with 30% RH, the relative humidity seems low but the dew point is 15°C — the heat itself is the main concern, not moisture

Common Mistakes

Confusing dew point with relative humidity — dew point is an absolute measure of moisture that does not change with temperature, while relative humidity varies throughout the day as temperature changes
Assuming a low relative humidity means the air is dry — in hot weather, even 40% RH can have a high dew point indicating substantial moisture in the air
Not realizing that dew point is the best indicator of comfort — meteorologists prefer dew point over relative humidity because it directly measures how muggy the air feels
Forgetting that dew point can never exceed the air temperature — when they are equal, relative humidity is 100% and condensation or fog forms
Confusing absolute humidity with relative humidity — absolute humidity measures the actual mass of water vapor per unit volume (g/m³), while relative humidity is a percentage of saturation
Not accounting for indoor humidity in winter — cold outdoor air holds less moisture, so when heated indoors without a humidifier, the relative humidity drops and dew point falls, causing dry skin and respiratory issues
Assuming dew point calculations are the same across all formulas — the Magnus formula used here is accurate for -40°C to 50°C, but different formulas may give slightly different results at extreme temperatures
Using dew point and wet bulb temperature interchangeably — they measure different things; dew point indicates moisture content, while wet bulb temperature accounts for evaporative cooling potential

Frequently Asked Questions

What is the dew point formula?

The Magnus formula for dew point is: $T_d = \frac{b \cdot \alpha(T, RH)}{a - \alpha(T, RH)}$ , where $\alpha(T, RH) = \frac{a \cdot T}{b + T} + \ln\left(\frac{RH}{100}\right)$ , with constants $a = 17.27$ and $b = 237.7\text{°C}$ . This formula is accurate for temperatures between -40°C and 50°C and is widely used in meteorology and HVAC applications.

What is a comfortable dew point for indoor air?

A dew point between 10°C and 15°C (50°F to 59°F) is considered ideal for indoor comfort. Below 5°C (41°F), the air feels very dry and can cause skin irritation and respiratory discomfort. Above 20°C (68°F), the air feels muggy and uncomfortable. The US Occupational Safety and Health Administration recommends maintaining indoor relative humidity between 20-60%, which typically corresponds to dew points in the 5-16°C range.

What is the difference between dew point and relative humidity?

Dew point is the temperature at which air becomes saturated with water vapor — it directly measures the amount of moisture in the air. Relative humidity is the percentage of moisture in the air compared to the maximum it can hold at the current temperature. Unlike relative humidity, dew point does not change throughout the day as temperature changes, making it a more reliable indicator of how muggy the air feels.

At what dew point does it feel muggy?

Most people start to notice humidity when the dew point reaches 16°C (60°F). At 20°C (68°F), it feels oppressive and uncomfortable. Above 24°C (75°F), the air feels tropical and outdoor activities become difficult. For reference, a dew point below 10°C (50°F) feels comfortable and refreshing, while below 0°C (32°F) feels very dry.

How to calculate dew point from temperature and humidity step by step?

Step 1: Measure the air temperature in °C. Step 2: Measure the relative humidity as a percentage. Step 3: Calculate α = (17.27 × T) / (237.7 + T) + ln(RH/100). Step 4: Calculate dew point = (237.7 × α) / (17.27 - α). For example, at 25°C and 60% RH: α = (17.27 × 25)/(237.7 + 25) + ln(0.6) = 1.644 + (-0.511) = 1.133. Dew point = (237.7 × 1.133)/(17.27 - 1.133) = 269.3/16.137 = 16.7°C.

Dew point vs humidity — which is better for measuring comfort?

Dew point is generally a better measure of comfort than relative humidity. This is because dew point directly measures moisture content and stays relatively constant throughout the day, while relative humidity fluctuates with temperature. A dew point of 20°C feels equally muggy whether the air temperature is 25°C or 35°C, but the relative humidity would be very different in those two scenarios.

What is absolute humidity and how does it relate to dew point?

Absolute humidity measures the actual mass of water vapor in a given volume of air, expressed in grams per cubic meter (g/m³). It is calculated using the formula: $\text{AH} = \frac{6.112 \times e^{\frac{17.67T}{T+243.5}} \times RH \times 2.1674}{273.15 + T}$ . Unlike relative humidity, absolute humidity does not change with temperature at a fixed volume. It is directly related to dew point — a higher dew point means higher absolute humidity.

How does dew point affect weather and precipitation?

When the air temperature drops to the dew point, the air becomes saturated and water vapor begins to condense, forming dew, fog, or clouds. If the dew point is below 0°C (32°F), frost forms instead of dew. High dew points indicate moist air that can produce more precipitation. Weather forecasters use dew point to predict fog, dew, frost, and the likelihood of afternoon thunderstorms.