Dew Point Calculator

The temperature at which your air, cooled, would condense.

Compute dew point

Temperature unit

°F °C

Air temperature

Relative humidity (%)

Dew point —°F Surfaces below this will condense

Spread (T − Tdew) —°F Higher = drier; lower = wetter feel

Why dew point matters

Dew point is the temperature your air would need to be cooled to in order to reach 100% saturation. At any temperature below the dew point, water condenses out of the air — onto leaves, walls, sensors, and fixtures.

For growers this matters because:

• Night setpoint — if you drop room temperature below the dew point of your daytime air, condensation forms on leaves, inviting botrytis and powdery mildew. Night temp should stay safely above dew point.
• Cold spots — areas of the room colder than dew point (poorly insulated walls, near uninsulated ducts, stratified ceiling) become wet surfaces. Mold and pathogen amplification follows.
• Dehumidifier sizing — capacity is best understood as "how many degrees of dew-point reduction per hour" rather than "% RH" — RH percentage is temperature-dependent and misleading.
• HVAC design — air conditioner cooling coils must be below dew point to remove moisture. Sensible-only cooling (no dehumidification) just lowers temp without dropping moisture, raising RH and dew-point margin together.
• Greenhouse glazing — when greenhouse skin temperature drops below dew point, condensation fogs the glass and drops onto canopy. Anti-drip films and proper venting manage this.

Dew point spread

The "spread" (air temperature minus dew point) is a useful single-number indicator:

• Spread > 15°F (8°C): dry conditions; minimal condensation risk; high VPD
• Spread 10–15°F (5–8°C): moderate; typical indoor cultivation
• Spread 5–10°F (3–5°C): humid; condensation risk on cold surfaces; watch for botrytis
• Spread < 5°F (3°C): near-saturated; high condensation risk; significant pathogen pressure

The math

The calculator uses the Magnus-Tetens approximation:

α = (17.27 × T) / (237.7 + T) + ln(RH / 100)
T_dew = (237.7 × α) / (17.27 − α)

where T is in °C, RH is in percent, and T_dew comes back in °C.

Accurate to within 0.4°C for temperatures from 0°C to 60°C and RH 1% to 100%. Simpler approximations (subtract twice the spread coefficient, etc.) are less accurate; this is the standard form used in meteorology.

Free under CC BY 4.0. Cite as "OAT Dew Point Calculator (openagriculturetechnology.com)".