Dew Point Calculator

Formula

γ(T, RH) = ln(RH / 100) + (b × T) / (c + T) Td = (c × γ) / (b − γ)

Td represents the calculated dew point temperature in degrees Celsius (°C). T is the dry-bulb ambient air temperature in degrees Celsius (°C). RH is the relative humidity expressed as a percentage between 1 and 100. ln is the natural logarithm function. The constants b and c are empirical Magnus parameters optimized for water vapor: b = 17.62 and c = 243.12 °C.

Worked example

Air temperature

25

Relative humidity

60

• Dew point: 16.7 °C
• In Fahrenheit: 62 °F
• Comfort: Humid

What the Dew Point Calculator Measures

The dew point calculator computes the temperature at which ambient air reaches complete water saturation (100% relative humidity), causing water vapor to condense into dew, fog, or condensation. Unlike relative humidity, which fluctuates inversely with temperature throughout the day, the dew point provides an absolute indicator of the actual moisture mass contained within the air parcel.

This tool is designed for homeowners monitoring indoor comfort, HVAC technicians balancing air handling equipment, painters determining safe drying conditions, and runners assessing heat stress risks. It translates ambient temperature and relative humidity into both Celsius and Fahrenheit dew points alongside an intuitive comfort index.

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Step-by-Step Worked Example

Consider a typical warm indoor or outdoor environment with the following conditions:

• Air Temperature ($T$): 25 °C
• Relative Humidity ($RH$): 60%

1. Calculate the Intermediate Factor ($\gamma$)

Using the Magnus formula constants $b = 17.62$ and $c = 243.12$ °C:

$$\gamma = \ln\left(\frac{RH}{100}\right) + \frac{b \cdot T}{c + T}$$

Substitute the known values:

$$\gamma = \ln(0.60) + \frac{17.62 \cdot 25}{243.12 + 25}$$ $$\gamma = -0.510826 + \frac{440.5}{268.12}$$ $$\gamma = -0.510826 + 1.642921 = 1.132095$$

2. Calculate the Dew Point ($T_d$)

Apply the intermediate factor $\gamma$ to the inverted Magnus relation:

$$T_d = \frac{c \cdot \gamma}{b - \gamma}$$ $$T_d = \frac{243.12 \cdot 1.132095}{17.62 - 1.132095}$$ $$T_d = \frac{275.2349}{16.4879} \approx 16.693 \text{ °C}$$

Rounded to one decimal place, the dew point is 16.7 °C.

3. Convert to Fahrenheit and Assess Comfort

Convert the Celsius result to Fahrenheit:

$$T_{d(^{\circ}\text{F})} = (16.7 \cdot 1.8) + 32 = 62.06 \text{ °F} \approx 62 \text{ °F}$$

Evaluating this value against standard human comfort thresholds places 16.7 °C in the Humid category.

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Comparison of Varying Temperatures at 60% Relative Humidity

To see how changes in ambient temperature affect absolute moisture content when relative humidity stays constant:

• Lower Input (20 °C, 60% RH): Dew point is 12.0 °C (53.6 °F) — Comfortable
• Base Example (25 °C, 60% RH): Dew point is 16.7 °C (62.0 °F) — Humid
• Higher Input (30 °C, 60% RH): Dew point is 21.4 °C (70.5 °F) — Very humid / oppressive

Even though relative humidity is fixed at 60% across all three scenarios, the air at 30 °C holds substantially more water vapor, shifting conditions from mild comfort to oppressive stickiness.

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How to Interpret Dew Point Ranges

Meteorological and HVAC professionals rely on standardized dew point tiers to gauge perceived outdoor comfort and indoor mold risk:

| Dew Point Range (°C) | Dew Point Range (°F) | Human Sensation | Practical Implication | | :--- | :--- | :--- | :--- | | Below 10 °C | Below 50 °F | Dry / Crisp | Dry skin; low risk of mold or condensation indoors. | | 10 °C to 15.9 °C | 50 °F to 60 °F | Comfortable | Ideal conditions for physical work and residential spaces. | | 16 °C to 20.9 °C | 61 °F to 69 °F | Humid / Muggy | Sweat evaporation slows; air feels noticeably sticky. | | 21 °C and above | 70 °F and higher | Oppressive | High heat-exhaustion risk during prolonged exertion. |

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Practical Applications

• Condensation Control: Industrial coatings, painting projects, and epoxy applications typically specify that surface temperatures must remain at least 3 °C (5 °F) above the ambient dew point to guarantee proper adhesion.
• HVAC Operations: Dehumidifiers remove moisture by cooling indoor air below its dew point across a cold coil, condensing water vapor out of the stream before reheating the dried air.
• Agricultural Frost Forecasting: If night skies are clear and wind is calm, the overnight minimum air temperature frequently drops near the evening dew point. If the dew point is below 0 °C (32 °F), frost forms rather than dew.

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When Not to Rely on the Magnus Approximation

The Magnus formula provides an empirical curve fit calibrated for normal barometric pressures near sea level (roughly 950 to 1,050 hPa) and ambient temperatures between -40 °C and 50 °C. Do not rely exclusively on basic Magnus estimations under high-altitude conditions, inside pressurized chambers, or during hyperbaric testing, where variations in total atmospheric pressure alter water vapor saturation pressure.

Comparison

Scenario	Input	Result
Lower input	Air temperature: 20	Dew point: 12 °C
Base example	Air temperature: 25	Dew point: 16.7 °C
Higher input	Air temperature: 30	Dew point: 21.4 °C

Common mistakes

• Inputting temperature values in Fahrenheit directly into the Magnus formula, which produces erroneous values because the constants b (17.62) and c (243.12) require Celsius.
• Assuming relative humidity alone indicates how sticky the air feels without factoring in absolute moisture through dew point.
• Confusing wet-bulb temperature with dew point temperature; wet-bulb reflects evaporative cooling, whereas dew point reflects condensation onset.
• Believing dew point can exceed the dry-bulb air temperature under normal atmospheric conditions, which is thermodynamically impossible.
• Overlooking barometric pressure extremes, as standard Magnus approximation parameters assume normal sea-level atmospheric pressures.

Checklist

• Verify the input temperature is measured in degrees Celsius before running the calculation.
• Ensure relative humidity is an integer or decimal between 1% and 100%.
• Use shielded, calibrated sensors to prevent direct solar radiation from skewing ambient temperature.
• Check surface temperatures nearby if calculating condensation risk on cold pipes or window panes.
• Convert Fahrenheit ambient readings to Celsius prior to using the standard Magnus constants.
• Recalculate whenever relative humidity or air temperature changes throughout the daily weather cycle.

Calculators Percentage Editorial Team

Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.

Last reviewed: November 12, 2026

This calculator uses the empirical Magnus formula approximation for standard atmospheric pressure. Environmental measurements are intended for general HVAC and meteorological guidance, not safety-critical engineering.