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Relative Humidity Calculator

Relative humidity equals actual vapor pressure divided by saturated vapor pressure times 100

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Relative Humidity (Vapor Pressure)

Relative humidity is the ratio of actual water vapor pressure to the saturated vapor pressure at the same temperature, expressed as a percentage. This approach uses direct pressure measurements.

RH = (e / eₛ) × 100

Dewpoint Equation

The Magnus-formula dewpoint method relates temperature, dewpoint temperature, and relative humidity. It is widely used in meteorology when a thermometer and hygrometer provide temperature and dewpoint readings.

RH = 100 × exp(17.271·Tᵈ/(237.7+Tᵈ)) / exp(17.271·T/(237.7+T))

How It Works

Relative humidity (RH) is the ratio of actual water vapor in the air to the maximum it could hold at that temperature, expressed as a percentage. This calculator supports two approaches: the vapor-pressure ratio (RH = AVP/SVP × 100) and the Magnus-formula dewpoint method, which relates temperature, dewpoint, and RH.

Example Problem

The actual vapor pressure is 12 mbar and the saturated vapor pressure is 20 mbar. What is the RH?

  1. RH = (12 / 20) × 100 = 60%

At 60% RH, the air holds 60% of the maximum moisture it could carry at that temperature.

When to Use Each Variable

  • Solve for RH (Vapor Pressure) — when you have actual and saturated vapor pressure readings, e.g., using data from a weather station psychrometer.
  • Solve for Actual Vapor Pressure — when you know RH and saturated vapor pressure, e.g., calculating moisture content for HVAC load analysis.
  • Solve for Saturated Vapor Pressure — when you know RH and actual vapor pressure, e.g., verifying a sensor calibration against known conditions.
  • Solve for RH from Dewpoint — when you have temperature and dewpoint readings, e.g., converting weather report dewpoint data to relative humidity.
  • Solve for Dewpoint Temperature — when you know temperature and RH and need the dewpoint, e.g., predicting condensation on cold surfaces.
  • Solve for Temperature — when you know dewpoint and RH and need the air temperature, e.g., back-calculating conditions from logged humidity data.

Key Concepts

Relative humidity expresses how close the air is to saturation as a percentage. At 100% RH, the air cannot hold more moisture and condensation begins. The saturation vapor pressure increases exponentially with temperature, which is why warm air can hold much more moisture than cold air. The Magnus formula provides an accurate approximation for the temperature-vapor pressure relationship.

Applications

  • HVAC engineering: controlling indoor humidity for occupant comfort and preventing condensation on cold surfaces
  • Meteorology: forecasting fog, dew, and precipitation based on temperature and dewpoint measurements
  • Agriculture: monitoring greenhouse humidity to prevent plant diseases like mildew and botrytis
  • Museum conservation: maintaining stable humidity to protect artwork, documents, and artifacts from moisture damage
  • Electronics manufacturing: controlling humidity in clean rooms to prevent static discharge and corrosion

Common Mistakes

  • Comparing RH values at different temperatures — 50% RH at 30°C contains far more moisture than 50% RH at 10°C because warm air holds more water vapor
  • Confusing relative humidity with absolute humidity — RH is a percentage of capacity, not a measure of actual moisture content; use dewpoint or mixing ratio for absolute comparisons
  • Assuming constant RH means constant comfort — a room at 50% RH and 20°C feels comfortable, but 50% RH at 35°C feels oppressively muggy because the absolute moisture content is much higher

Frequently Asked Questions

What is a comfortable indoor humidity level?

Most people find 30–50% RH comfortable. Below 30% can dry out skin and mucous membranes; above 60% encourages mold growth and dust mites.

What is the difference between dewpoint and relative humidity?

Dewpoint is the temperature at which the air becomes fully saturated and condensation begins. RH expresses how close the air currently is to that saturation point as a percentage. A dewpoint of 20°C with an air temperature of 25°C gives about 73% RH.

Why does relative humidity change throughout the day?

RH rises as temperature drops (the air’s capacity decreases) and falls as temperature climbs. That is why mornings often feel damp and afternoons feel dry even though the actual moisture content may not change much.

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