The temperature where air gives up its water.
A dew point calculator that finds the exact temperature where air saturates and vapour turns to dew, fog or frost — with frost point, comfort level, and altitude-adjusted figures for where you actually are.
Td = (243.04·γ) / (17.625 − γ)
Why dew point tells the truth when humidity lies
Relative humidity sounds precise, but it shifts the moment the temperature does. Dew point holds still. It is the one number that tells you how much water is actually in the air — and what that water is about to do.
What the number means
The dew point is the temperature you would have to cool the air to before it became completely saturated — 100% full of water vapour. Cool it one degree further and the air can no longer hold all that moisture, so the excess condenses out as dew on the grass, fog in the valley, or frost on the windscreen.
That is why two cities at "70% humidity" can feel nothing alike. A cold morning at 70% holds very little actual water; a tropical afternoon at 70% is drenched in it. The dew point cuts through that confusion because it is tied to the water itself, not to a ratio that drifts with the thermometer.
The formula doing the work
Behind the readout is the Magnus–Tetens approximation, using the refined constants from Alduchov and Eskridge (1996). It is the standard relationship meteorologists use to connect temperature, humidity, and saturation, and it stays accurate across the everyday range from −40°C to +50°C.
- Pick your inputs. Temperature with humidity for an ordinary reading, wet-bulb if you have a psychrometer, or reverse to find the humidity that hits a target dew point.
- Choose units. Metric or imperial — everything converts cleanly.
- Read the gap. The saturation track shows how far the air is from condensing. A narrow gap means dew, fog, or window-sweat is close.
Where it actually matters
Dew point is not trivia. It decides whether a vineyard loses its buds to frost overnight, whether a server room's cooling pipes drip onto the racks, and whether a house grows mould behind the drywall. The same threshold governs all three.
Dew point comfort scale, in full
The same dew point feels the same everywhere on earth, regardless of the air temperature around it. That is what makes this scale so useful — it maps directly onto how the air actually feels and behaves. Values are rounded; the band edges are approximate.
| Dew point | How it feels | What happens around you |
|---|---|---|
| Below 10°C / 50°F | Dry and crisp | Sweat evaporates the instant it forms; lips and skin can feel a little dry |
| 10–15°C / 50–59°F | Comfortable | The pleasant zone — cool surfaces stay dry, no muggy feeling |
| 16–18°C / 60–64°F | Becoming sticky | Air starts to feel heavy; you notice it on a still day |
| 18–21°C / 65–69°F | Oppressive | Sweat lingers, sleep gets uncomfortable, windows may fog |
| Above 21°C / 70°F | Dangerous | Cooling by sweat largely fails — real heat-stress territory |
Putting dew point to work
Once you can read the dew point, three everyday problems become much easier to head off before they start.
Stopping damp and mould
Mould needs moisture on a surface. When a wall or window is colder than the indoor dew point, moisture settles there and mould follows. Keeping that figure below the temperature of your coldest surface is the whole game.
Protecting plants from frost
When the dew point drops below 0°C it becomes the frost point. If the air then cools to meet it, ice forms directly on leaves and buds — even if the official temperature reads a degree or two above freezing.
Setting up air conditioning
An air conditioner cools by pulling moisture out of the air. Tracking the dew point — not just the temperature — tells you whether it's actually drying the room or just chilling damp air that still feels clammy.
The terms, plainly
A quick reference for the words that come up around dew point and humidity.
- Dew point
- The temperature the air must cool to before it becomes fully saturated and water starts to condense out. A direct measure of how much moisture the air actually holds.
- Relative humidity
- How full the air is compared with the most it could hold at that temperature. Because it moves with temperature, the same air can read 50% in the afternoon and 90% by night.
- Wet-bulb temperature
- The lowest temperature you can reach by evaporating water into the air. Measured with a wet cloth over a thermometer; it sits between the dew point and the air temperature.
- Frost point
- The dew point when it falls below freezing. Moisture skips the liquid stage and deposits straight as frost crystals.
- Absolute humidity
- The actual mass of water vapour in a given volume of air, usually grams per cubic metre — a raw count rather than a ratio.
- Vapour pressure
- The share of total air pressure that comes from water vapour alone. It rises as the air holds more moisture, up to a saturation ceiling set by temperature.
Why altitude changes the picture
The dew point temperature itself depends on temperature and humidity, not on how high you are. But two things that matter in practice — the mixing ratio and the cloud base height — do shift with elevation, because the air pressure around you drops as you climb.
At 1,500 m the pressure is roughly 845 hPa instead of the 1,013 hPa you get at sea level. That changes how much water vapour a kilogram of air actually carries, which is why this calculator asks for your elevation. Enter it once and the pressure-dependent figures adjust to where you really are, rather than assuming everyone stands at sea level.
If you leave elevation at zero, you get standard sea-level values — exactly what a basic calculator would give you.
Who checks the dew point, and why
The same number means very different things depending on what you're doing. A few of the people who rely on it:
🎨Spray painters & finishers
Paint and clear-coat blush or trap moisture if applied when the surface is near that point. A safe rule is to keep the surface at least 3°C above it before spraying.
Watch the gap, not the humidity🌿Greenhouse growers
When leaf temperature drops to the dew point, condensation settles on plants and invites mildew and botrytis. Growers heat or vent to hold the air a few degrees clear of it overnight.
Dew point under leaf temp🪵Woodworkers
Wood swells and glue-ups fail in damp air. A workshop dew point below roughly 12°C keeps timber stable and finishes curing cleanly, especially before applying water-based coats.
Stable timber zone✈️Pilots
The spread between temperature and dew point estimates cloud base — roughly 125 m of height for every 1°C of spread. A narrowing spread is an early warning of fog or a lowering ceiling.
Spread → cloud base🍺Brewers & fermenters
Cold tanks and cellar walls sweat when humidity climbs above their surface temperature, dripping onto equipment. Keeping that level low protects both hygiene and labels.
Cellar surfaces stay dry🏢HVAC technicians
Comfort and condensation both come down to this number, not thermostat setting. Cooling a room without dropping that just makes clammy air colder — the mixing ratio shows what's really being removed.
Dry the air, not just chill itCommon questions
What's a comfortable indoor dew point?
Between about 10°C and 15°C (50–60°F). In that band sweat evaporates freely and cold surfaces stay dry, so there's no condensation feeding mould.
Why do my windows sweat in winter?
The glass has dropped below your indoor dew point. Warm, moist room air touches the cold pane, can't hold its water there, and leaves it behind as condensation. Lowering indoor humidity raises the margin and stops it.
Can the dew point be higher than the air temperature?
No. At most they're equal — that's 100% humidity and fully saturated air. Add any more moisture and it immediately condenses out, so the dew point can never overtake the temperature.
How is dew point different from wet-bulb?
Dew point is where water condenses out of the air. Wet-bulb is the lowest temperature you can reach by evaporating water into the air. In dry air the wet-bulb sits between the two; as humidity climbs they converge.
When does dew become frost?
When the dew point falls below 0°C it's called the frost point. The moisture skips the liquid stage and deposits straight as ice crystals — which is why a still, clear night can leave frost even when the air above is a touch warmer.
Does altitude change the dew point?
The dew point temperature itself doesn't change with height — it's set by temperature and humidity. But the air pressure does drop as you climb, which shifts the mixing ratio and the cloud base estimate. That's why this calculator lets you enter your elevation, so those pressure-dependent figures match where you actually are.
What dew point is safe for painting?
Keep the surface at least 3°C (about 5°F) above that point while painting and until the coat sets. If the surface cools that far, moisture condenses under or into the finish and causes blushing or poor adhesion — check the gap figure, not just the humidity reading.
What is the dew point spread?
The spread, or dew-point depression, is simply the air temperature minus that value. A wide spread means dry air with plenty of cooling room before condensation; a narrow spread means fog, dew or saturation is close. Pilots multiply it by roughly 125 m per degree to estimate cloud base.
Sources
- US National Weather Service (NOAA) — why dew point beats relative humidity for comfort.
- ASHRAE — psychrometrics and indoor condensation standards.
This tool gives theoretical psychrometric values for education and general guidance. Real condensation can vary with local pressure and altitude. Verify against NWS or ASHRAE data for safety-critical work.