Relative Humidity Calculator

How to use this relative humidity calculator

What this calculator does. This tool connects three atmospheric quantities: Air temperature, Dew point, and Relative humidity. Give it any two valid quantities and it solves the third using a Magnus approximation. That makes it useful for checking a weather observation, translating a dew-point reading into relative humidity, or estimating the dew point implied by a temperature and RH reading. It is a compact thermodynamic approximation, not a forecast, a substitute for a calibrated hygrometer, or a complete indoor-air-quality assessment. NOAA explains that relative humidity compares the water vapor present with the maximum possible at the current temperature, while dew point describes the temperature at which saturation occurs; see NOAA's humidity and dew point overview.

When to use it. Use it when you have a temperature and dew-point observation but need RH; when a sensor gives temperature and RH but you need dew point for condensation awareness; when a specification gives dew point and RH but you want the corresponding air temperature; or when you want to sanity-check how a change in temperature alters RH while the moisture state represented by dew point stays fixed.

How to calculate. The calculator opens with a complete demonstration: 95 °F air temperature and 70 °F dew point, producing 44.48% relative humidity, with a valid Excel workbook ready immediately. To use your own values:

1. Choose Temperature unit. Fahrenheit is shown first; switching to Celsius converts both temperature fields rather than merely changing their labels.
2. Edit any two of the three quantities. The two most recently edited fields are treated as your known inputs, and the remaining field receives the green Calculated badge.
3. Read the large primary result and the three summary values. They update from one canonical calculation, so the displayed temperature, dew point, and RH remain internally consistent.
4. Select Download Excel to export the current validated state as a real .xlsx workbook. Select Reset to clear the demonstration and all calculated output. After Reset, Excel export stays disabled until you enter a complete valid pair again.

Input guide. Temperature unit is a required display/control choice and accepts Fahrenheit (°F) or Celsius (°C); changing it converts both current temperature values. Air temperature is a numeric temperature, required whenever it is one of the two drivers. Enter plain decimal notation such as 95 or 35.0; the supported approximation range is – 40 to 122 °F ( – 40 to 50 °C). A common mistake is entering a unit symbol inside the field or using a decimal comma such as 1,5; this calculator uses a decimal point and rejects ambiguous formatting. Dew point follows the same numeric and unit rules. In ordinary unsaturated air the dew point is at or below air temperature; NOAA's National Weather Service gives the underlying dew point definition. Relative humidity is a percentage above 0 and up to 100. Enter 44.48, not 0.4448. Values above 100 are outside this calculator's supported unsaturated/saturated-air domain. Higher RH at a fixed temperature raises the calculated dew point; at a fixed dew point, higher RH corresponds to a lower calculated air temperature.

Output guide. Primary result is whichever quantity is currently being solved: it can be a temperature in the selected unit or a percentage. Air temperature, Dew point, and Relative humidity repeat the complete solved state so you can read all three together. A 100% RH result represents saturation in this model; a dew point equal to air temperature is the same boundary expressed as temperature. Lower RH means a larger separation between air temperature and dew point. These are calculated identities within the approximation, not recommendations about comfort, weather safety, or building operation.

Worked example. The startup values are 95 °F air temperature and 70 °F dew point. Converted to Celsius, those are 35 °C and about 21.11 °C. Substituting them into RH = 100 × exp[a·Td/(b+Td) – a·T/(b+T)], with a = 17.625 and b = 243.04 °C, gives 44.48% after display rounding. This matches the first-open result and the corresponding workbook cells.