Dew Point Calculator

Air temperature:

Use the temperature from the same room, weather station, duct, or enclosure as the relative humidity reading.

Relative humidity:

Enter 1-100%; the default 60% gives an immediate indoor moisture example.

Surface temperature to compare:

Set the surface temperature you care about; condensation can begin when it is at or below the dew point.

Check inputs

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Station pressure:

Leave 1013.25 hPa for sea-level planning, or enter local station pressure for moisture-mass estimates.

hPa

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Cold surfaces can collect moisture even when the surrounding air does not feel especially damp. A window pane, duct, pipe, product package, camera lens, coil, or enclosure wall may be several degrees cooler than the room air. Dew point explains why that difference matters: it is the temperature at which the water vapor already in the air reaches saturation.

Relative humidity is a percentage, not a fixed moisture amount. It compares current water vapor with the maximum water vapor the air could support at the current temperature. Warmer air has a higher saturation limit, so the same relative humidity can mean very different moisture loads on a warm afternoon and a cool morning. Dew point is often easier to compare because it moves with the actual vapor pressure rather than only with the percent reading.

When air cools to its dew point, relative humidity reaches 100%. More cooling usually pushes water out of the air as dew, fog, frost, or liquid on a nearby surface. The practical question is therefore not only how humid the room or weather station reads, but how close the coldest important surface is to the saturation temperature.

Dew point situations and common mistakes
Situation	Why dew point helps	Common mistake
Weather comfort	A higher dew point means more moisture is available to slow sweat evaporation.	Treating a low relative humidity reading in hot air as automatically dry-feeling.
Windows, pipes, and ducts	The cold surface, not the average room air, decides where condensation starts.	Comparing dew point only with the thermostat temperature.
Storage and handling	Cold goods moved into humid air can develop moisture before the whole room changes.	Trusting a sensor far from the product or enclosure wall.
HVAC and drying	Moisture removal depends on cooling air below dew point or lowering vapor pressure.	Assuming cooler supply air always means a lower moisture load.

Temperature scale comparing cold surface, dew point, safe surface, and air temperature margin.

A few terms keep the result from being misread. Dry-bulb temperature is the ordinary air temperature measured away from direct wetting, sun, or radiant heat. Dew point depression is the gap between air temperature and dew point. Surface margin is the gap between a surface and the dew point. A small positive margin can still be risky when sensors are uncalibrated, airflow is uneven, or the surface cools overnight.

Dew point is a screening value, not a full moisture investigation. Air pressure, material temperature, sensor placement, local cold spots, and ice-versus-water behavior can all matter in demanding work. The estimate is most useful when it prompts the right follow-up: compare the coldest relevant surface, confirm the readings are from the same air mass, and decide whether dehumidification, insulation, warming, or better measurement is needed.

How to Use This Tool:

Use measurements from the same room, duct, enclosure, weather station, or product area. Dew point comparisons become unreliable when the air reading and surface reading come from different airflow or height zones.

Enter Air temperature and choose C or F. Switching units converts both temperature fields so the dew point and surface comparison stay in the same scale.
Enter Relative humidity from 1% through 100%. If the summary shows Check inputs, fix any out-of-range humidity or nonnumeric temperature before reading the tables.
Enter Surface temperature to compare for the cold surface that matters, such as a window, coil, pipe, stored item, lens, or enclosure wall.
Open Advanced only when Station pressure should influence the mass-based moisture rows. Dew point, surface margin, and the curve come from air temperature and relative humidity.
Read the summary first. The dew point is the primary value, while the air-to-saturation spread and surface margin show how much cooling room remains.
Use Moisture Metrics for vapor pressure, absolute humidity, mixing ratio, and specific humidity. Use Condensation Brief for the surface, air saturation, comfort, and formula-scope labels.
Open Dew Point Curve when you want a sensitivity check from 5% to 100% relative humidity at the same air temperature. The result is ready when the summary no longer shows Check inputs and the active tab fills with rows or a curve.

Interpreting Results:

Dew point is the temperature where the current moisture content would saturate the air. Dew point depression is air temperature minus dew point. Larger depression means more cooling is needed before saturation; a very small value means fog, dew, or surface wetting can appear after a small temperature drop.

Surface margin is the most important condensation check. Positive values mean the compared surface is warmer than the dew point. A value of 0 C or less means the surface is at or below the modeled condensation point. Do not treat a positive margin as proof of safety when the margin is only a degree or two and the probe placement is uncertain.

Dew point result bands and actions
Result cue	Boundary	How to act on it
`Condensing now`	`Surface margin <= 0 C`	Warm the surface, dry the air, isolate the cold surface, or recheck with a calibrated probe.
`Very tight margin`	`0 C < margin < 2 C`	Treat the result as fragile because small sensor error or nighttime cooling can remove the margin.
`Narrow margin`	`2 C <= margin < 5 C`	Monitor drift and compare the coldest surface, not just a convenient surface.
`Near saturation`	`Dew point depression < 1 C`	Expect fog, dew, or wetting after only slight cooling.
`Wide spread`	`Dew point depression >= 8 C`	Air is farther from saturation at the measured moisture level.

Surface RH estimate shows the modeled relative humidity right next to the compared surface if moisture content stays unchanged. A value near or above 100% supports the same warning as a zero or negative surface margin. Comfort labels use dew point in Fahrenheit, so they help with weather feel, but condensation decisions still depend on the surface margin.

Technical Details:

Water-vapor pressure controls the dew point. Saturation vapor pressure rises rapidly as temperature rises, so warm air can hold much more water vapor before reaching saturation. Relative humidity supplies the fraction of that saturation pressure that is actually present.

The calculation uses the Magnus vapor-pressure approximation over water. Temperatures are converted to Celsius for the equation, then converted back to the selected display unit. Station pressure is used for mixing ratio and specific humidity because those rows compare water vapor with air mass; it does not change the dew point from temperature and relative humidity.

Formula Core:

The core equation estimates saturation vapor pressure, scales it by relative humidity, then solves for the saturation temperature of that same vapor pressure.

\begin{array}{lll} e_{s} (T) & = & 6.112 \exp (\frac{17.625 T}{243.04 + T}) \\ e & = & e_{s} (T) \frac{RH}{100} \\ γ & = & \ln (\frac{RH}{100}) + \frac{17.625 T}{243.04 + T} \\ T_{d} & = & \frac{243.04 γ}{17.625 - γ} \end{array}

Here T is air temperature in Celsius, RH is relative humidity percent, e_s(T) is saturation vapor pressure in hPa, e is actual vapor pressure, and Td is dew point in Celsius. At 25 C and 60% relative humidity, saturation vapor pressure is about 31.63 hPa, actual vapor pressure is about 18.98 hPa, and dew point is about 16.7 C.

Derived Moisture Fields:

Derived moisture calculations used with dew point
Field	Rule	Use
`Dew point depression`	`Air temperature - dew point`	Shows the cooling gap before the air reaches saturation.
`Surface margin`	`Surface temperature - dew point`	Compares a surface directly with the condensation point.
`Surface RH estimate`	`Actual vapor pressure / saturation pressure at the surface`	Approximates saturation risk in the boundary air near that surface.
`Condensation RH threshold`	`Surface saturation pressure / air saturation pressure`	Estimates the room relative humidity where that surface reaches dew point.
`Absolute humidity`	`216.7 * actual vapor pressure / (air temperature C + 273.15)`	Reports approximate water-vapor mass per cubic meter.
`Mixing ratio`	`621.945 * vapor pressure / (station pressure - vapor pressure)`	Compares water vapor with dry-air mass in `g/kg`.
`Specific humidity`	`622 * vapor pressure / (station pressure - 0.378 * vapor pressure)`	Compares water vapor with total moist-air mass in `g/kg`.

The dew point curve repeats the same equation at 5% relative humidity increments from 5% through 100% while holding air temperature constant. It is a sensitivity view, not a forecast, and it should be compared with the current relative humidity and the surface temperature being checked.

Limitations, Privacy, and Accuracy Notes:

The numeric calculation runs in the browser after the page loads. Air temperature, relative humidity, surface temperature, and station pressure do not need a server lookup for dew point, surface margin, chart rows, tables, or JSON output.

Air temperature must stay from -80 C to 80 C, relative humidity from 1% to 100%, surface temperature from -100 C to 120 C, and station pressure from 300 hPa to 1100 hPa.
The Magnus approximation is practical for common weather, building, storage, and equipment checks, but calibrated psychrometric instruments may be needed for compliance or safety decisions.
Ice surfaces, very cold laboratory ranges, pressurized vessels, and poorly placed sensors can produce real conditions that differ from the modeled result.

Worked Examples:

These cases use the same formulas as the live result and show how surface margin changes the practical decision.

Indoor wall check

With 25 C air and 60% relative humidity, dew point is about 16.7 C. A 20 C surface has a margin near +3.3 C, so it is above dew point but still close enough to watch if the wall cools overnight. The condensation threshold for that surface is about 73.8% room relative humidity.

Warm room, cool window

At 77 F and 75% relative humidity, dew point is about 68.5 F. A window at 68 F is slightly below the dew point, so the surface margin is negative and the Condensing now cue is a reasonable first warning.

Validation cleanup

If relative humidity is entered as 105% or station pressure as 1200 hPa, the summary remains at Check inputs. Correct the value into the accepted range before using the curve, tables, or copied output.

FAQ:

Can dew point be higher than air temperature?

Not with relative humidity from 1% through 100%. At 100% relative humidity, dew point and air temperature match.

Why can condensation appear below 100% room humidity?

Relative humidity is measured at the air temperature. A colder surface has a lower saturation limit, so air near that surface can reach saturation while the room average remains below 100%.

Why does station pressure not change the dew point?

Dew point is calculated from air temperature and relative humidity. Station pressure affects Mixing ratio and Specific humidity, which compare water vapor with air mass.

What should I do with a negative surface margin?

Treat the surface as at condensation risk. Warm the surface, reduce relative humidity, add insulation, improve airflow, or allow cold items to warm before exposing them to humid air.

Why does the curve change only with relative humidity?

The curve holds the current air temperature constant and recalculates dew point at each relative humidity step. Change the air temperature input to produce a different curve.

Glossary:

Dew point: The temperature where the current water vapor would saturate the air if cooled without changing moisture content.
Relative humidity: Current vapor pressure compared with saturation vapor pressure at the same air temperature.
Saturation vapor pressure: The water-vapor pressure at which air reaches saturation for a given temperature.
Dew point depression: The temperature gap between air temperature and dew point.
Surface margin: The compared surface temperature minus the dew point.
Mixing ratio: Water vapor mass compared with dry-air mass, reported here in grams per kilogram.
Specific humidity: Water vapor mass compared with total moist-air mass, reported here in grams per kilogram.

References:

Dew Point vs. Humidity, NOAA National Weather Service.
Dew and Dew Point Glossary Entries, NOAA National Weather Service.
Improved Magnus Form Approximation of Saturation Vapor Pressure, O. A. Alduchov and R. E. Eskridge, U.S. Department of Energy OSTI record, 1997.