💧 Dew Point, Wet Bulb & Psychrometrics Calculator
Get dew point, wet bulb, frost point and absolute humidity, the pressure dew point in ppm for compressed air, and the surface temperature mold starts at.
Noticeably humid
Getting sticky, but tolerable for most.
Surfaces in this air
A surface has to sit 4.7°C below this air temperature to start growing mold and 8.2°C below it to start running with water. That gap is why a wall can be moldy and completely dry to the touch — 80% surface humidity is the limit BS EN ISO 13788 uses, and mold gets there first.
How much water is actually in it
Dew point measures how much moisture is actually in the air; relative humidity only says how close the air is to saturation at its current temperature. That is why 70% humidity means nothing on its own — at 5°C it is dry air, at 30°C it is oppressive — while a dew point of 21°C is muggy anywhere on earth. Saturation vapor pressure comes from the Magnus form with the Alduchov–Eskridge coefficients, which tracks Sonntag (1990) to within 0.34% from −40°C to 50°C. The wet bulb is solved from the psychrometric equation rather than an empirical fit; the comfort labels are a forecasting convention, not a measurement.
What Dew Point, Wet Bulb & Psychrometrics Calculator Does
The dew point is the temperature to which air must cool before it becomes saturated and water starts condensing out. It is a direct measure of how much moisture the air actually holds, which is why forecasters watch it and why it predicts discomfort far better than relative humidity does.
Relative humidity is a ratio, not a quantity. It tells you how close the air is to saturation at its current temperature, so it changes through the day even when nothing about the moisture changes — the same air can read 90% at dawn and 45% at noon. That is why "70% humidity" means nothing on its own: at 5°C it is dry air, at 30°C it is oppressive. A dew point of 21°C is muggy anywhere on earth.
This calculator also answers the three questions the dew point is usually a step towards, and which the calculators ranking for it leave out. The wet-bulb temperature, which is what a thermometer wrapped in a wet wick settles at and the number that actually governs heat stress and evaporative cooling. The moisture content in ppm and the pressure dew point, which is the same physics applied to compressed air and dry gas. And the surface temperature at which a wall starts growing mold, which is several degrees warmer than the temperature at which it starts running with water.
Everything runs in your browser. The saturation-vapor-pressure curve is the Magnus form with the Alduchov–Eskridge (1996) coefficients; measured against Sonntag (1990) it stays within 0.34% from −40°C to 50°C over water and 0.11% from −60°C to 0°C over ice. The wet bulb is solved from the psychrometric equation as the National Weather Service publishes it, rather than from an empirical shortcut — the difference is quantified below.
How to Use Dew Point, Wet Bulb & Psychrometrics Calculator
- Pick a mode: humidity to dew point, dew point to humidity, or compressed air and gas
- Enter the air temperature and either the relative humidity or the dew point, in °C or °F
- Read the dew point, wet bulb and frost point, with absolute humidity, mixing ratio and ppm
- Check the surface panel for the temperature mold risk starts at and the temperature condensation starts at
- For compressed air, enter the pressure dew point and the line pressure to convert it to any other pressure and to ppm
Formula Used by Dew Point, Wet Bulb & Psychrometrics Calculator
The Magnus formula, and its inverse
es(T) = 6.1094 · e^(17.625·T / (243.04 + T)); Td = (b·γ)/(a − γ) where γ = (a·T)/(b + T) + ln(RH/100)
- es(T)
- saturation vapor pressure in hPa — the most the air could hold at T
- T
- air temperature in °C
- RH
- relative humidity in percent
- Td
- dew point in °C — always at or below T
- a, b
- 17.625 and 243.04 over water; 22.587 and 273.86 over ice, which gives the frost point
Worked example
24°C air at 60% relative humidity.
- γ = (17.625 × 24)/(243.04 + 24) + ln(0.60) = 1.583962 − 0.510826 = 1.073136
- Td = (243.04 × 1.073136)/(17.625 − 1.073136) = 260.813/16.551864
Result: 15.8°C. Any surface below that will condense water — and running it backwards returns exactly 60%, which is the check that the two directions agree.
Wet-bulb temperature, solved not approximated
e = es(Tw) − P × 0.00066 × (T − Tw) × (1 + 0.00115 × Tw)
- Tw
- wet-bulb temperature in °C — what a wetted, ventilated thermometer reads
- e
- actual vapor pressure of the air, hPa
- P
- station pressure in hPa (1013.25 at sea level)
- the 0.00115 term
- the psychrometer correction — it is why this models a real instrument rather than an idealized one
Worked example
30°C air at 50% relative humidity, sea level.
- e = 0.50 × es(30) = 0.50 × 42.367 = 21.183 hPa
- Tw cannot be isolated — it appears inside the exponential of es(Tw) — so it is found by bisection
- Checked by running the NWS closed-form inversion backwards from Tw
Result: 22.1°C, against a dew point of 18.4°C and a dry bulb of 30°C. The inversion returns the input humidity to within 0.02 percentage points across −5…50°C and 10–100% RH.
Pressure dew point — why compressed air is a different number
e₂ = e₁ × P₂ / P₁, then invert the Magnus form on e₂
- P₁, P₂
- total pressures in bar ABSOLUTE (gauge + 1.01325)
- e₁, e₂
- water vapor partial pressure before and after
- what is conserved
- the vapor mole fraction — the ppmv figure does not change, only the partial pressure
Worked example
A dryer rated to a −40°C pressure dew point, measured in a 7 barg line.
- Over ice, es(−40°C) = 0.128340 hPa, and the line is at 8.01325 bar absolute
- Expanded to 1.01325 bar: e = 0.128340 × 1.01325/8.01325 = 0.016229 hPa
- Inverting the over-ice Magnus form on that pressure
Result: −57.0°C at atmospheric pressure, and 16.0 ppmv either way. This is why a dryer is always specified at line pressure: quoting the same air as "−57°C" would be true and would mislead everyone.
How dew point feels
A common forecasting convention. It is subjective and strongly affected by acclimatization — the same dew point reads differently in Florida and in Maine.
| Dew point | Description |
|---|---|
| Below 2°C (35°F) | Very dry — static, chapped lips, dry sinuses |
| 2–10°C (35–50°F) | Dry and crisp |
| 10–16°C (50–60°F) | Comfortable — the range most people prefer |
| 16–18°C (60–65°F) | Noticeably humid |
| 18–21°C (65–70°F) | Humid and sticky |
| 21–24°C (70–75°F) | Very humid, uncomfortable |
| Above 24°C (75°F) | Oppressive — sweat barely evaporates |
The same relative humidity, different air
Why relative humidity alone tells you nothing about how humid it feels. Computed at 70% RH throughout.
| Air temp | RH | Dew point | Feels |
|---|---|---|---|
| 5°C | 70% | 0.0°C | Dry |
| 15°C | 70% | 9.6°C | Comfortable |
| 25°C | 70% | 19.1°C | Sticky |
| 35°C | 70% | 28.7°C | Oppressive |
Surfaces in a 20°C room — mold comes before condensation
Mold risk starts where the surface humidity reaches 80%, the threshold BS EN ISO 13788 applies; liquid water appears only at the dew point. Both columns are surface temperatures, and the gap between them is why a moldy wall can be dry to the touch. At 80% room humidity the two meet at room temperature and no surface is safe.
| Room RH | Mold risk below | Condensation below | Cold spot needed |
|---|---|---|---|
| 30% | 5.1°C | 1.9°C | 14.9°C below the room |
| 40% | 9.3°C | 6.0°C | 10.7°C below the room |
| 50% | 12.6°C | 9.3°C | 7.4°C below the room |
| 60% | 15.4°C | 12.0°C | 4.6°C below the room |
| 70% | 17.9°C | 14.4°C | 2.1°C below the room |
| 80% | 20.0°C | 16.4°C | none — every surface qualifies |
Dew point as moisture content, at 1 atmosphere
The conversion people ask for and no consumer calculator performs. Below 0°C these are frost points over ice, which is the convention the instrument industry uses. ppmw is by weight in air (molar mass 28.9635 g/mol against water at 18.01528).
| Dew / frost point | Vapor pressure | ppm by volume | ppm by weight |
|---|---|---|---|
| −70°C | 0.00262 hPa | 2.58 | 1.61 |
| −60°C | 0.01082 hPa | 10.7 | 6.64 |
| −40°C | 0.12834 hPa | 127 | 78.8 |
| −20°C | 1.0313 hPa | 1,018 | 633 |
| 0°C | 6.1094 hPa | 6,030 | 3,750 |
| 10°C | 12.260 hPa | 12,100 | 7,530 |
| 20°C | 23.334 hPa | 23,030 | 14,320 |
One dryer, five line pressures
A −40°C pressure dew point means a different amount of water at every pressure, which is why the pressure must be quoted with the number. Atmospheric equivalent is the same gas after expansion.
| Line pressure | Bar absolute | ppmv in the line | Atmospheric equivalent |
|---|---|---|---|
| 0 barg | 1.013 | 127 | −40.0°C |
| 4 barg | 5.013 | 25.6 | −53.3°C |
| 7 barg | 8.013 | 16.0 | −57.0°C |
| 10 barg | 11.013 | 11.7 | −59.3°C |
| 13 barg | 14.013 | 9.2 | −61.1°C |
Wet bulb, dew point and dry bulb at 35°C
All three describe the same air. The wet bulb is the one that matters for heat stress, because it is the coolest a sweating body or an evaporative cooler can reach.
| Relative humidity | Dew point | Wet bulb | Evaporative cooling available |
|---|---|---|---|
| 20% | 8.7°C | 19.1°C | 15.9°C |
| 40% | 19.4°C | 24.1°C | 10.9°C |
| 60% | 26.1°C | 28.3°C | 6.7°C |
| 80% | 31.0°C | 31.9°C | 3.1°C |
| 100% | 35.0°C | 35.0°C | none |
How to Read Your Result
Use it to stop condensation, not just to complain about the weather
Condensation on windows, walls or pipes means those surfaces are below the indoor dew point. The two fixes are lowering the dew point — ventilation, extraction over showers and cooking, a dehumidifier — or raising the surface temperature with insulation. Wiping treats the symptom.
Mold does not wait for water
Surface relative humidity, not liquid water, is what mold needs, and the working threshold is around 80%. In a 20°C room at 60% humidity, a surface has to reach 12.0°C before it will stream — but it only has to reach 15.4°C to sit at 80% surface humidity. That is a 3.4°C band in which a wall is growing mold and is dry to the touch, and it is where cold corners behind furniture and uninsulated window reveals live. The EPA makes the same point from the other direction: keep indoor humidity below 60% and the cold spots stop mattering.
The wet bulb is the number that decides whether heat is survivable
Sweat cools you by evaporating, and evaporation stops when the air is saturated — so the coolest a sweating body can get is the wet-bulb temperature, not the dew point and not the dry bulb. At 35°C the difference is stark: at 20% humidity the wet bulb is 19.1°C and you have 16°C of cooling available, while at 80% humidity it is 31.9°C and you have 3°C. Evaporative ("swamp") coolers work on exactly the same limit, which is why they are useless in humid climates.
Below freezing it is a frost point, and that is a different number
Saturation over ice is lower than over supercooled water, so the two curves separate below 0°C. Air at −10°C and 60% humidity has a dew point of −16.3°C over water and a frost point of −14.6°C over ice — 1.8°C apart on the same air. Most calculators report one figure and do not say which curve produced it. This one reports both and labels them.
Compressing air makes water, and the arithmetic says how much
Squeezing air multiplies the vapor partial pressure without changing the temperature, so the dew point rises. Ordinary 20°C air at 60% humidity has a dew point of 12.0°C; take it to 7 barg and that dew point becomes 47.8°C — far above the air's own temperature. The difference has nowhere to go but the receiver, which is why every compressor has a drain and why a dryer is a separate machine.
Dew point can never exceed air temperature
If it did, the air would be holding more water than it can, which is what 100% humidity means. A dew point equal to the air temperature is fog, cloud or dew forming, and it is also the point where dew point, wet bulb and dry bulb all converge — the calculator rejects anything higher because it is not a physical state.
Limitations & Accuracy Notes
- The Magnus form is an empirical fit. Measured here against Sonntag (1990) it holds to 0.34% over water from −40°C to 50°C and 0.11% over ice from −60°C to 0°C; outside those ranges it has not been checked and should not be trusted.
- The wet bulb uses the NWS psychrometric equation, which models a properly ventilated wet thermometer. The thermodynamic (adiabatic-saturation) wet bulb is a slightly different quantity — the two agree to within 0.31°C across 0–50°C and 20–99% humidity, and neither is the "wrong" one.
- Many calculators use Stull's 2011 empirical fit for wet bulb instead. Measured against the psychrometric equation it is out by up to 0.86°C inside its own stated range of −20…50°C and 5–99% RH, and by nearly 4°C in the cold, dry corner Stull's own abstract excludes. Treat a wet bulb from an unnamed formula as ±1°C.
- Atmospheric figures assume 1013.25 hPa. Altitude changes the mixing ratio, the ppmv figure and the wet bulb; the compressed-air mode takes pressure as an input, the weather modes do not.
- Absolute humidity treats water vapor as an ideal gas. The error is well under a tenth of a percent at surface conditions and grows near saturation at high temperature.
- The 80% surface-humidity mold threshold is a design criterion for building fabric, not a biological guarantee. Real mold growth also depends on the material, how long the condition lasts, and what is on the surface — a monthly mean matters more than an instant.
- The comfort labels are a forecasting convention, not a measurement, and depend heavily on acclimatization.
- Accuracy is limited by your inputs. Cheap hygrometers are commonly several percent out, which moves the dew point by a degree or more — and at low humidity a 2% humidity error moves the frost point by several degrees.
- Not a substitute for a calibrated hygrometer where the number carries consequences: instrument air to a specification, a compressed-gas acceptance test, or a damp survey on a building someone is buying.
Frequently Asked Questions
What is dew point?
How do you calculate the dew point?
What does a dew point of 70°F mean?
What is a normal dew point?
How do I convert dew point to ppm?
Is the wet bulb the same as the dew point?
What dew point causes mold on walls?
What is the dew point?
Why is dew point better than relative humidity?
What dew point feels comfortable?
Can the dew point be higher than the air temperature?
What does dew point tell me about fog?
Is my data stored?
References & Further Reading
- NOAA National Weather Service glossary — dew point — The formal definition the calculation implements
- NWS — relative humidity and dewpoint from wet bulb (PDF) — The psychrometric equation used here, including the 0.00066 and 0.00115 coefficients, and the closed-form inversion used to check it
- NOAA Weather Prediction Center — dew point and relative humidity — NOAA's own calculator for the same conversion, used as a cross-check
- Alduchov & Eskridge (1996) — Improved Magnus form approximation of saturation vapor pressure — Journal of Applied Meteorology 35(4) 601–609. Source of the 17.625/243.04 water and 22.587/273.86 ice coefficients
- Stull (2011) — Wet-bulb temperature from relative humidity and air temperature — J. Applied Meteorology and Climatology 50(11) 2267–2269. The empirical fit compared against above; the abstract states its −1.0 to +0.65°C error range and its exclusion of cold, dry conditions
- US EPA — Mold Course, Chapter 2: why and where mold grows — Names 60% indoor relative humidity as a moisture problem and cold corners behind furniture as where condensation and mold appear