Dew Point Calculator.
Set the temperature and relative humidity to get the dew point instantly, plus the spread to condensation and your grow-room mold risk. The dew point is the temperature where air can hold no more water and starts wetting every cool surface.
How to use the dew point calculator
Move the sliders and the dew point updates live. The readout turns from green to amber to red as the air nears saturation, because the closer the dew point creeps to your surface temperature, the sooner condensation and mold set in. Growers: pair this with the VPD calculator to manage the same moisture from the plant's side.
How to calculate the dew point
The dew point is the temperature the air must cool to before its water vapor condenses into liquid. It comes from just two readings, temperature and relative humidity, through the Magnus equation, the same saturation-vapor-pressure math behind vapor pressure deficit.
Step by step
- 1Convert your air temperature to Celsius: T = (°F − 32) × 5 ÷ 9.
- 2Find the gamma term: γ = ln(RH ÷ 100) + (17.27 × T) ÷ (237.3 + T).
- 3Solve for the dew point: Td = (237.3 × γ) ÷ (17.27 − γ), in °C.
- 4Convert back if you want °F: °F = °C × 9 ÷ 5 + 32.
Dew point vs relative humidity
Relative humidity is a ratio tied to the current temperature, so 60% at 90°F holds far more water than 60% at 60°F. The dew point states the actual moisture directly and does not move when you only change the temperature. That is why it is the more reliable number for condensation, comfort, and mold. In psychrometrics, the science of moist air, the dew point sits alongside the wet-bulb temperature as one of the two absolute moisture readings; below freezing it becomes the frost point, where moisture deposits as frost instead of dew.
Dew point key facts
- Definition: the dew point is the temperature air must cool to, at constant pressure, for its water vapor to condense into liquid.
- Formula (Magnus): Td = (237.3 × γ) ÷ (17.27 − γ), where γ = ln(RH/100) + (17.27 × T)/(237.3 + T), T in °C.
- Mold rule: keep the grow-room dew point below about 60°F and every surface a few degrees above it.
- Spread rule: a gap under about 4°F between surface temperature and dew point means condensation is likely.
- Comfort: below 55°F dry and pleasant, 60–65°F sticky, above 70°F oppressive.
- Limit: the dew point can never exceed the air temperature; they meet at 100% RH.
- Related: same temperature-plus-humidity inputs as vapor pressure deficit (VPD); below 32°F it is called the frost point.
Dew point and mold risk in a grow room
Mold does not care about your air temperature, it cares about wet surfaces. Wherever a leaf, wall, or duct cools to the dew point, invisible condensation forms and powdery mildew follows. In a tent the canopy and walls run coolest at lights-off, exactly when humidity climbs, so the dew point is the number that predicts a mold outbreak before you can see it.
Two rules keep a room safe: keep the dew point below about 60°F, and keep every surface at least a few degrees above it. This calculator flags the risk as the spread narrows. Manage the drying power on the plant's side with the VPD calculator, scan every temperature and humidity combination on the dew point chart, and if the dew point sits high you likely need more dehumidification or air exchange.
Dew point comfort chart
Human comfort tracks the dew point far better than relative humidity. Here is how each range feels, and what it means for a grow space.
| Dew point | How it feels | Grow-room note |
|---|---|---|
| Below 50°F | Dry, crisp, comfortable | Very mold-safe; may be too dry for clones |
| 50–55°F | Pleasant | Comfortable working range |
| 55–60°F | Slightly humid | Watch cool surfaces at lights-off |
| 60–65°F | Sticky, noticeable | Elevated mold risk; add dehumidification |
| 65–70°F | Uncomfortable, muggy | High mold risk; condensation likely |
| Above 70°F | Oppressive | Severe; expect condensation and mildew |
Dew point questions
How do you calculate the dew point?
With the Magnus equation. Using temperature T in Celsius and relative humidity RH, compute γ = ln(RH/100) + (17.27×T)/(237.3+T), then the dew point equals (237.3×γ)/(17.27−γ) in °C. The calculator above does it live as you move the sliders.
What does a 70 dew point mean?
A 70°F dew point means the air holds a lot of moisture and feels oppressive outdoors. Indoors it is a warning sign: any surface at or below 70°F will grow condensation, so mold and powdery mildew risk is high.
What is an uncomfortable dew point?
Comfort tracks the dew point, not relative humidity. Below 55°F feels dry and pleasant, 55 to 60 is comfortable, 60 to 65 turns sticky, 65 to 70 is muggy and uncomfortable, and above 70 is oppressive.
What is worse, humidity or dew point?
Dew point is the more honest gauge. Relative humidity is a percentage relative to the current temperature, so 60% at 90°F holds far more water than 60% at 60°F. The dew point states the actual moisture directly, which is why meteorologists and growers rely on it.
What dew point causes mold in a grow room?
Mold forms wherever a surface cools to the dew point. Leaves and walls run coolest at lights-off, so keep the dew point below about 60°F and every surface a few degrees above it. A dew point above 60°F, or a spread under about 4°F, means condensation and powdery mildew are likely.
What is the difference between dew point and relative humidity?
Dew point is absolute: the temperature at which air saturates and starts to condense. Relative humidity is a ratio, the percentage of moisture held versus the most possible at the current temperature. Warm the air and RH drops while the dew point stays put.
How is dew point related to VPD?
Both come from temperature and relative humidity. VPD measures the drying power the air exerts on the plant; the dew point marks where that same air condenses on a surface. Use VPD to dial in transpiration and the dew point to avoid condensation and mold.
Can the dew point be higher than the air temperature?
No. The dew point can at most equal the air temperature, which happens at 100% relative humidity, when fog, dew, or condensation is already forming. The gap between them, the spread, is what this calculator reports: the smaller the spread, the closer the air is to saturation.
What is the frost point?
When the dew point falls below freezing, 32°F or 0°C, moisture skips the liquid phase and deposits as frost instead of dew. That temperature is called the frost point, and it is why cold greenhouse glazing frosts over on winter nights.