Dew Point Calculator
Get the dew point, frost point, wet bulb, absolute humidity and condensation risk the moment you type. Solve in any direction, pick the formula, and see how humid the air really feels.
One scale for every field, so your inputs can never be mismatched.
Enter the air temperature and relative humidity.
Or tap a quick start chip to see the calculator in action.
Pinned readings will appear here once you have a result.
Created by James Caldwell
Last updated: October 3, 2026
What Is the Dew Point?
The dew point is the temperature air has to be cooled to, at constant pressure, before it reaches 100% relative humidity and water starts to condense out of it. The US National Weather Service defines it exactly that way. Cool a surface below that temperature and moisture appears on it, which is why a cold drink beads up on a summer afternoon and a window fogs on a winter morning.
What makes the dew point useful is that it is a direct measure of how much water vapour the air is actually carrying. Relative humidity only tells you how close the air is to its own limit, and that limit moves as the temperature changes. The dew point does not move when you heat or cool the air. Warm a room from 10 °C to 25 °C without adding or removing moisture and the relative humidity collapses, but the dew point stays exactly where it was.
That is why a 21 °C room at 45% humidity and a 32 °C afternoon at 65% humidity feel nothing alike. The first has a dew point near 8.6 °C, which is comfortable. The second sits near 24.6 °C, which is oppressive. Same idea, very different air.
Dew Point vs Relative Humidity: Which One Tells You How It Feels
The dew point does, and it is not a close contest. The National Weather Service puts it plainly: if you want a real judge of how dry or humid it will feel outside, look at the dew point instead of the relative humidity, because the higher the dew point, the muggier it feels.
The reason is physiological. You cool yourself by evaporating sweat, and evaporation depends on how much more vapour the surrounding air can absorb. That capacity is set by the absolute amount of moisture already in the air, which is what the dew point measures. A high dew point means sweat evaporates slowly and you stay hot. Relative humidity cannot tell you that on its own, because 90% humidity on a freezing morning is pleasant air and 90% humidity at 27 °C is not.
| Air temp | Relative humidity | Dew point | How it feels |
|---|---|---|---|
| 0 °C | 90% | -1.4 °C | Very dry air, despite the high humidity reading |
| 21 °C | 45% | 8.6 °C | Comfortable |
| 30 °C | 40% | 14.9 °C | Hot but dry, sweat still works |
| 30 °C | 70% | 23.9 °C | Oppressive, the same heat now feels far worse |
Notice the last two rows. The temperature is identical and only the moisture changes, yet one is tolerable and one is dangerous for anyone working outside.
How to Calculate Dew Point (Formula)
Dew point is found in two steps: work out how much vapour pressure the air currently holds, then ask what temperature would make that pressure a saturated one.
Step 1: saturation vapour pressure
The Magnus formula gives the saturation vapour pressure in hectopascals for an air temperature in Celsius:
Those coefficients come from Alduchov and Eskridge (1996), who refitted the classic Magnus equation by least squares. Lawrence (2005) showed that this pair, 17.625 and 243.04 °C, gives dew points accurate to about 0.35 °C from -40 °C to 50 °C, which is why it is the default in the calculator above.
Step 2: actual vapour pressure
Relative humidity is simply the ratio of actual to saturation vapour pressure, so multiplying gives the pressure the vapour really exerts:
Step 3: invert the curve
The dew point is the temperature at which would be the saturation value. Because the Magnus form inverts algebraically, no iteration is needed. Define an intermediate term and solve:
A worked example at 20 °C and 50% humidity
- , so
So 20 °C air at 50% relative humidity has a dew point of 9.26 °C, or 48.7 °F. For reference, the saturation pressure at 20 °C is 23.33 hPa and the actual vapour pressure is 11.67 hPa, exactly half of it, as 50% humidity requires.
Going the other way: humidity from dew point
If you measured the dew point instead and want the relative humidity, take the ratio of the two saturation pressures:
The calculator above runs this in all three directions, so you can also recover the air temperature from a known dew point and humidity.
Dew Point Comfort Chart
These are the bands the calculator uses. They follow the thresholds widely used in US forecasting and weather writing, and the gauge in the results panel plots your reading against them.
| Dew point °C | Dew point °F | Band | What you notice |
|---|---|---|---|
| Below 0 | Below 32 | Very dry | Static shocks, dry skin and airways, wood and paper shrink |
| 0 to 10 | 32 to 50 | Dry | A bit dry for some, easy on buildings and stored materials |
| 10 to 16 | 50 to 60 | Comfortable | The target range, sweat evaporates freely |
| 16 to 18 | 60 to 64 | Getting sticky | Muggy evenings, clammy during activity |
| 18 to 21 | 64 to 70 | Humid and unpleasant | Sleep and hard exercise both get harder |
| 21 to 24 | 70 to 75 | Oppressive | Sweat barely evaporates, limit exertion |
| Above 24 | Above 75 | Severe | Heat illness risk rises sharply, treat outdoor work as hazardous |
The National Weather Service uses a simpler three-way split for summer: 55 °F and below is dry and comfortable, 55 °F to 65 °F is becoming sticky with muggy evenings, and 65 °F and above means lots of moisture in the air, becoming oppressive. The finer bands above sit inside those same boundaries.
Perception is personal, and people acclimatise. Someone used to a humid summer will shrug at a dew point that a desert resident finds unbearable.
What Is a Good Dew Point?
For general comfort, a dew point between 10 °C and 16 °C (50 °F to 60 °F) is the range most people describe as pleasant. Below about 0 °C the air is dry enough to crack skin and shrink timber, and above about 18 °C it starts to feel muggy.
Indoors, the usual control target is relative humidity rather than dew point. OSHA recommends, as guidance rather than an enforceable rule, keeping office temperature between 68 °F and 76 °F with humidity between 20% and 60%. At the warm end of that band, 60% humidity at 24 °C puts the dew point near 15.7 °C, still inside the comfortable range.
For stored collections the target is tighter. Conservation guidance commonly specifies 45% to 55% relative humidity for mixed museum and archive collections, with limits on how fast it is allowed to swing, although individual institutions set their own bands. The calculator checks your inputs against both of these ranges.
Dew Point and Condensation: Why Windows and Pipes Sweat
This is the most practical use of a dew point calculation. Any surface colder than the dew point of the air touching it will collect water. That single rule explains fogged windows, dripping cold-water pipes, damp patches in the corners of exterior walls, and condensation inside ductwork.
It also tells you there are two ways to stop it: warm the surface, or dry the air so the dew point drops below the surface temperature. Running a dehumidifier and adding insulation are the same fix approached from opposite ends.
Mould does not need visible condensation. The relative humidity in the thin layer of air against a cold surface is higher than the humidity in the middle of the room, because that air is colder while carrying the same amount of vapour. ISO 13788 treats a monthly mean surface relative humidity above 80% as the point where mould growth becomes a risk, and that can happen while the surface is still several degrees above the dew point.
Enter a surface temperature under Advanced options and the calculator reports the humidity against that surface, whether it is condensing, and how much margin is left before the 80% threshold.
Dew Point vs Frost Point
Below freezing, water vapour can saturate with respect to liquid water or with respect to ice, and the two give different answers. The frost point is always the higher of the two, because the stronger bonding between water molecules at an ice surface means less vapour pressure is needed to reach saturation.
In air at -10 °C and 70% relative humidity, the dew point over supercooled water is -14.4 °C while the frost point over ice is -12.9 °C, a gap of about 1.5 °C. Weather reporting conventionally quotes the dew point over water even in freezing conditions, which is why the calculator defaults to that and shows the frost point alongside it. Refrigeration and compressed-gas drying usually want the ice figure instead, which is what the over-ice switch gives you.
Dew Point vs Wet Bulb Temperature
Both are temperatures below the air temperature, and they are easy to confuse. The dew point is how far you would have to cool the air to saturate it. The wet bulb is how cool a wet surface actually gets when evaporation is carrying heat away from it, so it always sits between the dew point and the air temperature.
At 20 °C and 50% humidity, the dew point is 9.26 °C and the wet bulb is about 13.7 °C. The wet bulb is the number that matters for evaporative coolers, cooling towers and heat stress limits, because it is the floor that evaporation can reach. The dew point is the number that matters for condensation and for how humid the air feels.
The calculator uses the Stull (2011) empirical fit for wet bulb. It is published as valid near standard sea-level pressure for air between -20 °C and 50 °C at 5% to 99% relative humidity, with a mean absolute error under 0.3 °C and worst-case errors from -1 °C to +0.65 °C. Outside that range the calculator marks the wet bulb as out of range rather than showing a number that cannot be trusted.
What the Dew Point Spread Tells You About Fog and Cloud Base
The spread, sometimes called the dew point depression, is just the air temperature minus the dew point. It is a direct measure of how close the air is to saturating.
A small spread means fog and mist are near. As the spread closes toward zero, relative humidity approaches 100% and visible moisture forms. Pilots watch an evening spread of a couple of degrees as a warning that overnight radiation cooling will produce fog by dawn.
The spread also estimates the base of fair-weather cumulus. Rising air cools at roughly 3 °C per 1,000 ft while its dew point falls at only about 0.5 °C per 1,000 ft, so the gap closes with height. The resulting FAA rule of thumb is about 400 ft of cloud base for every 1 °C of surface spread:
A 20 °C afternoon with a 9.26 °C dew point has a 10.7 °C spread, putting cumulus bases near 4,300 ft. Treat this as a field estimate only. It applies to cumuliform cloud formed by surface heating in temperate conditions, and it does not hold in the tropics, in very cold air, or for layered cloud arriving from elsewhere.
Other Measures of Moisture in the Results
Dew point answers how the air feels and when it will condense. These other figures answer how much water is actually present, which is what you need for sizing equipment.
Absolute humidity is the mass of vapour in a cubic metre of air. It follows from the ideal gas law applied to the vapour alone, using the specific gas constant for water vapour of 461.5 J/(kg·K):
At 20 °C and 50% humidity that works out to 8.62 g/m³. It is the number to use when you want to know how many litres a dehumidifier could pull from a room.
Mixing ratio is the mass of vapour per kilogram of dry air, and unlike absolute humidity it does not change when the air expands or is heated, which makes it the preferred measure in HVAC work:
Vapour pressure deficit is saturation pressure minus actual pressure. Growers use it because it drives how fast plants transpire, and it is a better guide than humidity alone. Enthalpy is the total heat content of the air per kilogram of dry air, which is what air-conditioning load calculations are built on. Equilibrium moisture content is the moisture level that wood, paper and other hygroscopic materials settle at in given conditions, calculated here with the Hailwood-Horrobin fit from the USDA Wood Handbook. At 68 °F and 50% humidity it is about 9.3%.
How to Use This Dew Point Calculator
- Choose what to calculate. The default solves for dew point from temperature and humidity. You can also solve for relative humidity from a measured dew point, or for the air temperature that a known dew point and humidity imply.
- Pick a temperature scale. Celsius, Fahrenheit or Kelvin applies to every temperature field at once, and switching converts the values you have already entered, so the inputs cannot end up mismatched.
- Enter your two known values. Results appear as you type. The quick start chips load realistic conditions if you just want to see how the numbers behave.
- Open Advanced options if you need them. Switch the saturation vapour pressure formula, report over ice below freezing, set the barometric pressure for your altitude, or add a surface temperature to check condensation and mould risk.
- Pin readings to compare. The record and compare table holds up to eight results side by side, which is useful for checking one room against another or a setpoint against the conditions it produces.
Reset Calculator clears every input, the pinned readings and the results in one click.
Frequently Asked Questions
What is the dew point in simple terms?
It is the temperature at which air becomes full of water vapour and starts giving it back as liquid. Cool anything below the dew point and it gets wet.
How do you calculate dew point from temperature and humidity?
Compute the saturation vapour pressure at the air temperature with the Magnus formula, multiply it by the relative humidity as a fraction to get the actual vapour pressure, then invert the formula to find the temperature at which that pressure would be saturated. The worked example above does this for 20 °C and 50% humidity, giving 9.26 °C.
Can the dew point be higher than the air temperature?
No. At 100% relative humidity the dew point equals the air temperature, and that is the ceiling. A reported dew point above the temperature means one of the two readings is wrong, usually an uncalibrated or slow-responding humidity sensor.
What dew point is uncomfortable?
Most people start to notice stickiness around 16 °C (60 °F), find 18 °C to 21 °C (64 °F to 70 °F) genuinely unpleasant, and describe anything above 21 °C (70 °F) as oppressive. Above 24 °C (75 °F) the heat illness risk during exertion rises sharply.
Does the dew point change when the air temperature changes?
Not on its own. The dew point only moves when moisture is added or removed. Heating a sealed room drops the relative humidity but leaves the dew point untouched, which is exactly why the dew point is the more stable measure of how much water the air holds.
What is a dangerous dew point?
Dew points above about 24 °C (75 °F) are the ones to treat as hazardous, because sweat evaporates so slowly that the body struggles to shed heat. The calculator also shows the US National Weather Service heat index whenever the air is 80 °F or warmer, which combines temperature and humidity into an apparent temperature.
Why does my window fog up on the inside?
Because the glass is colder than the dew point of the indoor air. Enter your room temperature and humidity, then put the glass temperature into the surface temperature field, and the calculator will tell you how far below the dew point the glass sits. Either warm the glass, with better glazing, or lower the indoor dew point by ventilating or dehumidifying.
Is dew point the same as humidity?
No. Dew point is a temperature and measures the absolute amount of vapour present. Relative humidity is a percentage and measures how close the air is to its own saturation limit at its current temperature. Two rooms can share a relative humidity reading and hold very different amounts of water.
What is the difference between dew point and frost point?
They are the same idea measured against different surfaces: dew point against liquid water, frost point against ice. Below freezing the frost point is the higher number, by roughly 1.5 °C at -10 °C and 70% humidity. Weather reports quote the dew point over water by convention.
How accurate is this dew point calculator?
The default Magnus coefficients are accurate to about 0.35 °C for air between -40 °C and 50 °C. For work outside that range, or where vapour pressure accuracy matters more, switch to the Arden Buck formula under Advanced options. In practice the accuracy of your temperature and humidity sensors will matter more than the choice of formula. A 2 percentage point error in relative humidity moves the dew point by roughly 0.55 °C in typical room conditions, which is larger than the difference between any two of the formulas offered here.