Dew Point Calculator

What a dew point calculator tells a grower

The dew point is the temperature at which the water already in the air starts to turn into liquid. Any leaf, wall, window or duct at or below it gets wet, and wet surfaces are where mould and leaf diseases start. Botrytis, the grey mould behind bud rot, thrives at 85% humidity and above.

The calculator compares the dew point with your coldest surface. Within 2 °C (3.6 °F) of the dew point counts as high risk, and within 4 °C (7.2 °F) as moderate. No published margin exists for this, so these are our rules of thumb, with room for sensor error and for surfaces colder than you measured. It also rates mould risk from humidity alone: moderate from 70%, high from 85%. Alongside the dew point you get absolute humidity in g/m³ and vapour pressure in kPa, the actual amount of water in the air.

Dew point formula, with real numbers

The calculator uses the Magnus form with the same constants as the vapour pressure formula on the rest of the site. Take a room at 24 °C (75.2 °F) and 65% RH:

• γ = ln(0.65) + 17.27 × 24 ÷ (24 + 237.3) = 1.1554
• Dew point = 237.3 × 1.1554 ÷ (17.27 − 1.1554) = 17.0 °C (62.6 °F)

If the coldest surface is a wall at 18 °C (64.4 °F), it sits only 1.0 °C (1.8 °F) above the dew point: high risk. The result shows that the wall starts to condense once humidity passes about 69%, and the advice is to bring humidity below about 57% to keep a 3 °C (5 °F) margin.

It also works the other way round. As a humidity dew point calculator, choose Find: Humidity, enter 25 °C (77 °F) and a 15 °C (59 °F) dew point, and it returns about 54% RH. Choose Temperature to find the air temperature instead.

Lights-off humidity: where condensation starts

Most dew point tools stop at one reading. In a grow room the risky moment is lights-off. The water in the air stays the same, so the dew point does not move, but the room cools towards it and humidity climbs.

Enter the lights-off temperature under Lights-off. In the room above, cooling from 24 °C to 19 °C (66.2 °F) lifts humidity to about 88%, high for mould, and leaves at 19 °C sit less than 2 °C above the dew point. Start the same room at 55% RH instead and the night reading is about 75%, with a margin of 4.6 °C (8.3 °F).

• Lower humidity before the lights go off, not after.
• Keep the night temperature closer to the day temperature.
• Keep air moving over the canopy and cold walls.
• Run the dehumidifier or exhaust fan into the dark period. The dehumidifier size calculator and the exhaust fan calculator help you size equipment that can keep up.

Common dew point mistakes

• Only measuring the air. Condensation forms on the coldest surface, not in the middle of the room. Measure an outside wall, a window or the top leaves at lights-off.
• Watching humidity instead of the dew point. Humidity changes every time the room warms or cools. The dew point only changes when water is added or removed.
• A hygrometer next to the light or the humidifier. It reads that spot, not the room.
• Converting wet and dry bulb readings by hand. The dry bulb and wet bulb calculator turns them into humidity and dew point.

Want every value at once, to pin on the wall? The printable dew point chart lists the dew point for each temperature and humidity.