Relative Humidity Calculator: RH from Vapour Pressure, Dew Point, Absolute Humidity and Humidity Ratio

Find the relative humidity and dew point from the water vapour pressure and the temperature.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the air temperature and either the vapour pressure or the relative humidity.

The calculator finds the saturation vapour pressure at that temperature and the relative humidity RH = pv over psat times 100, or the vapour pressure if you gave the RH.

It also gives the dew point, the absolute humidity in grams per cubic metre and the humidity ratio in grams per kilogram of dry air.

Formulas used

  • RH = \frac{p_v}{p_{sat}}\times100
  • p_{sat} = 0.61094\,e^{17.625T/(T + 243.04)}
  • T_d = \frac{243.04\ln(p_v/0.61094)}{17.625 - \ln(p_v/0.61094)}
  • W = 0.622\frac{p_v}{P - p_v}

Worked example

With the default numbers in the calculator above:

  1. Saturation vapour pressure at this temperature (Magnus formula): 3.16 kPa
  2. Relative humidity: 63.26 %
  3. Dew point: the temperature at which this air becomes saturated and starts to condense: 17.53 C
  4. Absolute humidity: the mass of water vapour in each cubic metre of air: 14.53 g/m3
  5. Humidity ratio: the mass of water for each kilogram of dry air: 12.52 g per kg of dry air

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the example in the poster?

With a vapour pressure of 2.0 kPa and a saturation pressure of 3.17 kPa (air at about 25 C), the relative humidity is 63.1 percent.

What is the dew point?

The temperature to which the air must be cooled for its water vapour to start to condense.

Why does RH change with temperature?

The saturation pressure rises fast with temperature, so the same water gives a lower RH in warmer air and a higher RH in cooler air.

Where is the wet bulb temperature?

In the psychrometrics calculator, which gives the full set of air properties.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Alduchov and Eskridge (1996), Improved Magnus form approximation of saturation vapor pressure; ASHRAE Handbook: Fundamentals (psychrometrics).

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