Understanding your result
The headline is the dew point, with the comfort band and the condensation verdict for the surface in the sub-line. The stats give the comfort band, the moisture content in grains per pound and the relative humidity right at the surface. The table adds the condensation answer with the margin in degrees, and the room humidity at which that surface would start to sweat.
The relative humidity at the surface is the figure for mould: even when a surface stays above the dew point, the air touching it can be above 80% relative humidity, the level at which mould grows over time. The room humidity that would condense is the practical target: keep the room below it and the surface stays dry.
For the opposite problem, dry winter air, the humidifier size calculator sizes the fix without pushing the dew point above the window temperature; for damp basements, the dehumidifier size calculator. Hot, humid summer air is rated by the heat index calculator, and a house that stays clammy with the air conditioning on may have an oversized unit, which the size check tests.
How we calculate this
The Magnus approximation with the Alduchov and Eskridge coefficients (17.625 and 243.04 °C) is the standard meteorological formula and is accurate to a few tenths of a degree from well below freezing to the hottest summer air. Moisture content uses the psychrometric definition at sea-level pressure. The comfort bands are the common National Weather Service description of dew points. Grains per pound and the other units are on the unit conversions page, and the humidity targets on the ventilation and exhaust minimums page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Saturation vapour pressure | Magnus, 6.1094 exp(17.625 T ÷ (243.04 + T)) hPa | Alduchov and Eskridge (1996) |
| Pressure | 1,013.25 hPa (sea level) | Standard atmosphere |
| Comfort bands | < 40 dry, < 55 comfortable, < 60 humid, < 65 sticky, < 70 uncomfortable, ≥ 70 oppressive (°F) | Common NWS description |
| Indoor RH guidance | 30 to 50% | ENERGY STAR and EPA indoor air guidance |
| Mould threshold | About 80% RH at the surface | Building science guidance (sustained surface RH) |
Assumptions last reviewed October 8, 2026.
The calculator does not account for altitude, which changes the moisture content slightly, or for the time it takes surfaces to wet and dry; brief condensation that dries quickly does little harm, while weeks of high surface humidity do. The guide to humidity, dew point and comfort explains the targets and how air conditioning, dehumidifiers and ventilation move them, and the guide to dehumidifier sizing covers damp spaces.
Two worked examples
A summer room and a cold surface
72 °F at 50% relative humidity, checking a 45 °F surface (a cold-water pipe or a single-pane window in a cold snap).
- Dew point: 52.3 °F, comfortable
- Moisture content: 58.2 grains per lb
- The 45 °F surface is below the dew point: it condenses
- That surface stays dry only if the room is below about 38% relative humidity
A cold-water pipe at 45 °F in a 50% RH basement will drip; pipe insulation solves it more cheaply than drying the air to 38%.
A winter room and a window
70 °F at 40% relative humidity, checking the inside of a window at 35 °F on a very cold night.
- Dew point: 44.6 °F, comfortable
- Moisture content: 43.4 grains per lb
- The 35 °F glass is well below the dew point: it condenses, and may frost
- The glass stays dry only below about 28% relative humidity
This is why winter humidity guidance falls as the weather gets colder: 40% is comfortable but too high for this window on this night. The humidifier should be turned down to 30% or lower during a cold snap, or the window upgraded.
Where to find your inputs
Temperature and humidity. A hygrometer, many thermostats, or a weather station; cheap hygrometers can be off by 5% RH, so check one against another.
Surface temperature. An infrared thermometer pointed at the glass, pipe, duct or wall; on a window, measure the bottom edge of the glass, which is coldest.
Common mistakes
- Using relative humidity to judge comfort. 50% at 85 °F is far muggier than 50% at 70 °F; dew point tells you.
- Running a humidifier at 45% in a cold snap. The windows will fog and frost.
- Measuring the middle of the window. The bottom edge is colder and condenses first.
- Dehumidifying the whole house to stop one pipe sweating. Insulate the pipe.
- Ignoring cold corners. Mould grows where surface humidity stays high, not where the room’s average is.
- Trusting one hygrometer. Calibrate or compare; small errors matter near the dew point.
Questions people ask
- What is the dew point?
- The temperature to which air must be cooled, at constant moisture content, for its water vapour to start condensing. At 72 °F and 50% relative humidity the dew point is about 52 °F, so any surface colder than that, such as a cold-water pipe or an air conditioning duct, will sweat. Unlike relative humidity, the dew point stays the same when the air warms or cools without gaining or losing moisture.
- What dew point is comfortable?
- Most people find a dew point below about 55 °F comfortable, 55 to 60 °F noticeably humid, 60 to 65 °F sticky, 65 to 70 °F uncomfortable and above 70 °F oppressive. Below about 40 °F the air feels dry. At a 72 °F indoor temperature, the comfortable range of 30 to 50% relative humidity corresponds to dew points of about 39 to 52 °F.
- Why do my windows fog in winter?
- The inside surface of the glass is colder than the room's dew point. On a cold night the inside of a double-pane window may be 40 to 45 °F; at 70 °F and 40% relative humidity the dew point is about 45 °F, so the glass sweats. Lower the indoor humidity, improve air movement across the glass, or upgrade to low-e or triple glazing, which keeps the inside surface warmer.
- Why does my basement wall or cold-water pipe sweat in summer?
- Because it is colder than the dew point of the humid summer air. A basement wall at 60 °F sweats whenever the basement air's dew point is above 60 °F, which in humid climates is most of the summer. Dehumidifying the basement to a dew point below the wall temperature, usually 50% relative humidity at 65 to 70 °F, stops it; insulating pipes stops them sweating.
- What humidity causes mould?
- Mould generally needs relative humidity of about 80% or more at the surface for days at a time, which is why cold corners, closets on outside walls and the backs of furniture against cold walls grow mould even when the room's humidity is moderate. The calculator shows the relative humidity right at the surface you enter, which is the number that matters for mould.
- How is dew point calculated?
- With the Magnus approximation, which gives the saturation vapour pressure of water at a temperature. The calculator converts to Celsius, computes γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T), and then the dew point as 243.04 γ ÷ (17.625 − γ), using the coefficients of Alduchov and Eskridge. It is accurate to a few tenths of a degree over the range people live in.