Dew Point Calculator

Dew point is the temperature at which air becomes saturated, so that any further cooling forces water vapor to condense into liquid. It is a temperature, but it is really a measure of how much water the air is carrying: the more vapor, the higher the temperature at which condensation starts.

Because it is tied to the amount of vapor rather than to the air's capacity, dew point does not move when you heat or cool a space. Warm a room by twenty degrees and the dew point is unchanged. Add moisture — a wet carpet evaporating, a kettle, a leaking crawlspace — and the dew point rises immediately.

That stability is what makes dew point one of the few readings you can compare across rooms, across days and across temperatures without converting anything. If today's dew point is lower than yesterday's, water genuinely left the building.

Read down to your air temperature and across to your relative humidity, and the cell is the dew point in °F. Any surface in the room colder than that figure will collect condensation, which is the whole reason to carry the number in your head.

Two things fall straight out of the table. First, at 50% relative humidity the dew point sits roughly thirty degrees below the air temperature at every row, which is a usable field shortcut when you only need to know whether a slab is at risk. Second, the spacing tightens as humidity climbs: from 80% upward the dew point is within a few degrees of the air temperature, so almost anything cooler than the room starts sweating.

For a value between the rows, use the calculator above rather than interpolating — the relationship is a curve, not a straight line. To see the same numbers plotted against wet-bulb and grains, use the psychrometric chart.

The honest calculation has three steps: saturation vapor pressure at the air temperature, actual vapor pressure from relative humidity, then the temperature at which that actual pressure would itself be saturation. Only the third step is awkward, because the saturation equation cannot be rearranged to give temperature directly.

Most calculators solve that with the Magnus approximation — a curve fit that produces a dew point directly from temperature and relative humidity. It is fast, it is good to a few tenths of a degree in ordinary indoor conditions, and it drifts further from the truth at very high, very low or very dry conditions.

This calculator inverts the ASHRAE Hyland-Wexler saturation equation numerically instead: it searches for the temperature whose saturation vapor pressure matches the measured vapor pressure, converging to well below the resolution of any field meter. The result can differ from a Magnus-based calculator by a few tenths of a degree. Neither difference matters more than the calibration of the instrument you took the reading with, but the exact method is the one shown in the formula panel above.

Below 32°F the calculation switches to the saturation equation over ice, which is the physically correct curve when condensation would form as frost.

Relative humidity tells you how full the air is; dew point tells you how much water is in it. Two rooms at 50% relative humidity can hold wildly different amounts of moisture if their temperatures differ, and only the dew point makes that visible.

Air at 90°F and 50% relative humidity has a dew point around 69°F. Air at 60°F and 50% relative humidity has a dew point around 41°F. Same percentage, very different air. Cool the first room to 70°F without removing any water and its relative humidity is nearly 100% — the number changed dramatically, the water content did not.

For restoration this is the same argument as grains per pound, expressed as a temperature instead of a mass. Dew point and grains rise and fall together; use whichever is quicker to read on your meter and stay consistent about it. The GPP calculator shows both from the same inputs.

The rule that matters on a jobsite is simple: any surface colder than the air's dew point will collect water, and no amount of airflow will dry it while that stays true. Enter a surface temperature in the calculator above and it tells you how far above or below the dew point that surface sits.

Concrete slabs are the usual culprit. A slab sits near ground temperature all year, so in warm humid weather it can be fifteen degrees below the dew point of the air above it. Point air movers at it and you are pushing humid air onto a cold plate; condensation forms as fast as you evaporate. The fix is to lower the grains in the air first, or to warm the slab, or both.

Windows and exterior walls do the same thing in cold weather, from the other direction: warm humid indoor air meets a cold assembly and sweats on the inside face. Ductwork and cold water lines running through a drying chamber will drip onto ceilings that were never part of the original loss. Every one of those is secondary damage, and every one is predictable from a surface temperature and a dew point.

This is also why heating a wet building without dehumidification backfires. Heat drives evaporation, evaporation raises the dew point, and the raised dew point finds the coldest assembly in the structure. Heat and vapor removal have to move together. See structural drying for how that balance is managed across a job, and Categories and Classes for how the type of loss changes the plan.

Mold does not need standing water; it needs a surface that stays damp, and a surface held below the dew point stays damp indefinitely. That is the connection between a psychrometric reading and a microbial problem.

The practical consequence is that chronically cold surfaces in humid buildings — the inside face of an uninsulated exterior wall, the back of furniture pushed against it, a closet on a north wall, a cold basement floor — grow mold without any leak at all. Nothing dripped. The air simply kept condensing on them.

On a water loss, the same mechanism is what turns a slow response into a bigger job. Keeping surface temperatures above the dew point and getting grains down early is a mold-prevention measure as much as a drying measure.

Once growth is established, the work moves from drying into remediation, which is a different standard and a different certification. Our sister site covers it: the IICRC S520 mold remediation standard and AMRT certification.