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Dew point calculator

The dew point is the single most useful number in a damp house: any surface colder than it is actively collecting water. Room humidity tells you how the air feels; the dew point tells you what the walls are doing.

Checked against EPA, ASHRAE and DOE guidance Updated 3 sources How we check this

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Units:

Normal margin

Only genuinely cold surfaces will reach the dew point.

Dew point
47.5 °F
Margin to dew point
22.5 °F
Moisture in the air
48 gr/lb

What the dew point tells you

Air holds water as vapor, and how much it can hold depends almost entirely on its temperature. Cool that air and its capacity falls; cool it far enough and it can no longer hold what it has. The temperature where that happens is the dew point, and any water above that limit leaves the air as liquid — on your windows, on the cold corner behind the dresser, inside the wall if it gets that far.

This is why condensation is not really a humidity problem. It is a temperature problem that humidity sets the threshold for. A room at 70 °F and 45% humidity has a dew point around 47 °F, so nothing in a normally heated house will condense. Push that same room to 65% humidity and the dew point climbs past 57 °F — now the window glass on a cold night is wet every morning.

The margin is what matters

Compare the dew point to the coldest surface in the room, not to the room temperature. An uninsulated exterior wall typically runs 3–7 °F below room air; a window reveal or a single-pane pane on a freezing night runs much colder than that.

A margin above 10 °F means you are comfortably clear. Under 5 °F means cold surfaces are already condensing. Under 2 °F means the room itself is close to saturation and everything cool is wet.

Using it to find the real problem

An inexpensive infrared thermometer turns this from theory into a diagnosis. Read the dew point here, then point the thermometer at the corner that keeps growing mold, at the window reveal, at the wall behind the sofa. Any reading below the dew point figure is a surface that is actively collecting water.

That single comparison tells you which fix you need: if surfaces are cold, you need insulation and heat; if the dew point itself is high, you need ventilation or dehumidification.

cold warm → how much water the air CAN hold water actually in your air — this does not change dew point capacity meets content → condensation plenty of spare capacity here
Cooling air does not take water out of it — it takes away the room the water had to sit in. The dew point is where it runs out.

A worked example

A worked example, using a reading most people would call unremarkable: a bedroom at 68 °F and 55% relative humidity on a January morning.

  1. Start with the room reading

    68 °F and 55% RH. Nothing about that looks alarming, and a hygrometer in the middle of the room would not prompt anybody to act.

  2. The dew point comes out at 51 °F

    That is the temperature at which this air has to give the water back. Every surface in the room colder than 51 °F is condensing at that moment, whether or not you can see it.

  3. Now measure the window

    Single-pane glass on a 30 °F night typically sits somewhere near 43 °F — eight degrees below the dew point, so the pane runs with water. A modern double-pane unit might read 58 °F and stay dry.

  4. And the coldest wall corner

    An uninsulated exterior corner at 60 °F is above the dew point, so it does not condense — but the air touching it reads about 73% relative humidity, which is close enough to the 80% mold threshold to be worth watching through a cold spell.

How to read the result

The dew point is useful precisely because it does not move when you change the temperature. Turn the heating up and the relative humidity falls, which looks like an improvement on the hygrometer while exactly the same amount of water remains in the air. The dew point is unchanged, and so is the list of surfaces that are wet.

That makes it the right number for two decisions. The first is which surfaces to worry about: measure them with an infrared thermometer and compare. The second is whether outdoor air will dry your house — compare the outdoor dew point with your indoor one, and ignore both percentages, because they are describing different air at different temperatures.

The calculation uses the Magnus–Tetens approximation for saturation vapor pressure, which is accurate to roughly 0.7 °F across the range a house ever sees. The limiting factor on the answer is your hygrometer, not the arithmetic.

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Common questions

What is a good dew point for indoors?

For a comfortable home in the 30–50% humidity band at normal room temperature, the dew point lands roughly between 40 °F and 52 °F. What matters more than the absolute figure is that it stays several degrees below your coldest interior surface.

Is dew point the same as humidity?

No. Relative humidity is a percentage of what the air could hold at its current temperature, so it changes as the air warms or cools even though the water content has not. Dew point is an absolute measure — it only changes when moisture is actually added or removed.

Why do my windows fog up in the morning?

Overnight the glass cools below the dew point of your indoor air, so water leaves the air onto the coldest surface in the room. Either lower the humidity or raise the glass temperature — better glazing, or moving heat toward the window.

Can the dew point be higher than the air temperature?

No. At most it equals it, which is the definition of 100% relative humidity — fog, or a surface running with water. If a calculator or a sensor ever reports a dew point above the air temperature, the humidity reading is wrong, usually because the sensor is saturated or has just been moved from a much wetter place and has not settled.

What dew point feels comfortable indoors?

Most people find anything under about 55 °F comfortable, 60 to 65 °F noticeably humid, and above 70 °F oppressive. That scale is why Gulf Coast summers feel the way they do: outdoor dew points sit in the low seventies for months. Indoors, holding a dew point around 50 °F keeps a house comfortable and keeps most surfaces safely above condensing.

Sources

  1. US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
  2. ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
  3. US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions