Dew Point Calculator

Dew point is the temperature at which air would become saturated if cooled at constant pressure without changing its water-vapor content. It is a temperature, whereas relative humidity is a percentage. A room at 25°C and 60% relative humidity has an estimated dew point of 16.70°C under this calculator’s model.

That estimate gives a useful reference for a cool window, pipe or wall: a surface near or below the dew point can favor condensation. Dew point alone does not tell you whether rain will fall, or diagnose a building’s moisture problem.

1. Select Dew point, then choose Celsius or Fahrenheit.
2. Enter the air temperature and relative humidity from your measurements. Results update immediately.
3. Read the dew point in both units and the temperature difference called dew point depression.
4. Open Condensation Check to compare a measured surface temperature, if needed.

To solve a different problem, choose Relative humidity or Air temperature. Each mode leaves only the two known variables editable. Switching units converts the temperatures, and switching modes carries forward a valid calculated result. Reset restores the labeled example; Copy, Print and CSV include the model assumptions.

Start with temperature in Celsius and humidity as a percentage. For 25°C and 60%, the Magnus equation gives γ ≈ 1.13305. Substituting into the dew point equation gives 16.69766°C, shown as approximately 16.70°C (62.06°F).

These four examples use the same coefficients as the interactive calculator:

This tool uses a = 17.625 and b = 243.04°C in a Magnus approximation over liquid water. T is air temperature, Td is dew point, ln is the natural logarithm, and RH is a percentage:

The same coefficients appear in the US EPA humidity algorithm. This calculator limits both air temperature and dew point to −40°C through 50°C (−40°F through 122°F). It refuses results outside that range rather than extrapolating. This range is a model boundary, not an instrument accuracy claim.

At 0% RH, ln(0) is undefined, so the calculator cannot report a finite dew point. Very low positive humidity may also put the dew point below the supported range. Full numerical precision is kept internally; the displayed answer is rounded.

Relative humidity compares actual water-vapor pressure with saturation vapor pressure at the current temperature. If an air parcel warms without gaining or losing moisture at the same pressure, its relative humidity falls while its dew point stays approximately the same. Cooling does the reverse until saturation is reached.

This is why “60% humidity” needs a temperature to be meaningful. Sixty percent at 25°C does not represent the same moisture content as 60% at 5°C. The National Weather Service explanation of dew point and humidity discusses this distinction in weather observations.

Select Relative humidity and enter air temperature and dew point in the same unit. The tool converts them to Celsius before evaluating:

RH = 100 × exp[aTd / (b + Td) − aT / (b + T)]

Air at 25°C with a rounded dew point of 16.7°C gives approximately 60.01% RH. Using the unrounded forward result returns 60% within numerical precision. In this ordinary unsaturated-air model, dew point cannot exceed air temperature; that would imply RH above 100%.

Finding air temperature instead

The Air temperature mode uses g = aTd / (b + Td) − ln(RH / 100), then T = bg / (a − g). For a dew point of 16.69766°C and RH of 60%, it returns approximately 25°C. The same temperature limits apply to the inverse result.

Convert a temperature using °F = °C × 9/5 + 32, or °C = (°F − 32) × 5/9. Thus, 25°C is 77°F and 16.70°C is approximately 62.06°F. The Temperature unit control converts every temperature input, including an optional surface temperature, while retaining its unrounded physical value.

Negative temperatures are accepted inside the documented range. Below freezing, the formula still refers to saturation over liquid water. Do not interpret a negative dew point as a calculated frost point.

Dew point depression is the air temperature minus the dew point. For 25°C air and a 16.70°C dew point, the difference is about 8.30°C. A smaller difference means the air is closer to saturation at its current moisture content.

A temperature difference uses a different conversion from a temperature reading: an 8.30°C difference is about 14.94°F. Multiply by 1.8, without adding 32. At 100% relative humidity, the model gives a zero difference and dew point equals air temperature.

For the 25°C, 60% example, a surface measured at 14°C is cooler than the estimated 16.70°C dew point. The Condensation Check therefore reports “Condensation may occur.” A surface above the estimated dew point does not meet that simple temperature condition for the entered air.

The comparison describes a possible local condition. Air movement, temperature gradients, moisture supply, coatings and sensor errors affect what actually happens. Below freezing, ice saturation needs separate consideration. This educational surface condensation estimate is not a certified HVAC engineering assessment, a mold prediction or a complete building-moisture analysis.

Higher dew points often feel more humid because they indicate more water vapor at a given pressure. Comfort still depends on air temperature, airflow, activity, clothing and individual preference. Two people in the same room may describe its conditions differently.

This calculator shows the measured quantities without assigning a universal “safe” or “comfortable” threshold. A weather comfort chart is a descriptive guide, not a personal health assessment or an HVAC design target.

Dew point describes saturation relative to liquid water. Frost point describes saturation relative to ice. Their saturation vapor pressures differ below freezing, so the two temperatures cannot be used interchangeably. Supercooled liquid droplets and deposited ice are also different physical outcomes.

This calculator deliberately uses the liquid-water relation throughout its stated range. For frost-sensitive work, use an ice-based model and appropriate measurements. The National Physical Laboratory humidity reference distinguishes water and ice vapor-pressure formulations. Browse the Science & Engineering category for published CalcVibes tools.