Relative Humidity Calculator

Units
Given

How to use it
  1. Select US or Metric units.
  2. Under Given, choose dry bulb and wet bulb (a sling psychrometer) or dry bulb and dew point.
  3. Enter the two temperatures, and the altitude if it is not near sea level.
  4. The relative humidity updates as you type, with the dew point or wet bulb and the humidity ratio.
  5. Or click or tap a point on the chart in the results to enter that dry bulb and humidity. Moving the pointer over the chart reads out the RH, wet bulb, dew point and humidity ratio at that point.

Results

Enter the dry bulb.

Click or tap the chart to enter that dry bulb and humidity.

  • Relative humidity, %
  • Wet bulb, °F
  • Enthalpy, Btu/lb
  • Specific volume, ft³/lb
Drawn for sea level. About this chart The curved lines are relative humidity; the saturation curve is 100 %. The dew point is the dry bulb where a level line from the state meets the saturation curve.

Uses the ASHRAE Fundamentals ch. 1 equations, the same as the psychrometric chart calculator, which also gives enthalpy and specific volume.

Relative humidity table

Relative humidity at sea level, %, from a sling psychrometer reading: find the dry bulb, then the wet-bulb depression (dry bulb minus wet bulb). Worked out with the same equations as the calculator. For other altitudes, or a reading between the columns, use the calculator above.

Relative humidity, %, at sea level, by dry bulb and wet-bulb depression
Dry bulb, °F2 °F4 °F6 °F8 °F10 °F12 °F15 °F20 °F25 °F
40846853382815–––
5087746250392812––
60897868584940276–
659080716253443214–
7090817364564837204
75918274665952412511
80928376686154453016
85928477706357483421
90928578716559503725
95938679736761534028
100938680746862554231
105938781756964564534
110948781767065584736

A dash means the depression is too large for that dry bulb: the air would be below about 1 % RH.

How relative humidity is calculated

Relative humidity is the water vapor pressure in the air as a percentage of the saturation vapor pressure at the same temperature:

RH = 100 × pw / pws(tdb)

  • pw = partial pressure of the water vapor in the air
  • pws(tdb) = saturation vapor pressure at the dry bulb temperature

From dry bulb and dew point. The dew point is the temperature at which the air's vapor would saturate, so pw = pws(tdp) and RH = 100 × pws(tdp) / pws(tdb). Altitude does not enter.

From dry bulb and wet bulb. The humidity ratio W comes from the wet bulb by the psychrometer equation (ASHRAE Handbook—Fundamentals ch. 1, eq. 35, or eq. 37 with ice on the wick below 32 °F), which depends on the air pressure and so on altitude. The vapor pressure is then pw = p × W / (0.621945 + W).

Saturation pressures use the Hyland–Wexler equations (Fundamentals ch. 1, eqs. 5 and 6), over ice below 32 °F and over water above. The air pressure at altitude is the standard atmosphere (eq. 3).

Example. At 75 °F dry bulb and 62.5 °F wet bulb at sea level, RH is 49.8 % and the dew point 55.0 °F. At 75 °F dry bulb and a 55 °F dew point, RH is 49.8 %.

Relative humidity questions

How do I calculate relative humidity from dry bulb and wet bulb?

Find the humidity ratio from the wet bulb with the psychrometer equation, turn it into a vapor pressure, and divide by the saturation pressure at the dry bulb. The calculator above does this; the table gives sea-level values for a range of common readings.

What is the difference between relative humidity and dew point?

Dew point measures how much water is in the air; it does not change as the air warms or cools until it reaches the dew point. Relative humidity measures how close the air is to saturation at its present temperature, so the same air has a lower RH when it is warmer.

Why does altitude change the result?

With a wet bulb reading, the wet bulb depends on the air pressure as well as the moisture, so the same dry and wet bulb readings mean slightly wetter air at altitude: 75 °F and 62.5 °F give about 50 % RH at sea level and about 53 % at 5,280 ft. With a dew point, altitude makes no difference to RH.

What else can I calculate from these readings?

Enthalpy, specific volume and the other properties, with a chart, are in the psychrometric chart calculator. For cooling loads and condensate between two conditions, use the psychrometric 2-condition calculator.

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