Dew Point Calculator
Dew point calculator — enter the air temperature and relative humidity to get the dew point and a plain-English comfort rating, using the Magnus formula. Dew point is the truest measure of how humid and sticky the air actually feels. Runs entirely in your browser.
Dew Point Calculator
How to Use the Dew Point Calculator
Enter the temperature
Type the air temperature and pick °C or °F.
Enter the humidity
Add the relative humidity as a percentage.
Read the dew point
See the dew point in both units instantly.
Check the comfort band
See whether the air feels dry, sticky or oppressive.
The Number That Really Tells You How Muggy It Is
Ask most people how humid it is and they will quote the relative humidity, but that number is quietly misleading. Relative humidity is a percentage measured against the current temperature, so the same 70% can mean crisp and pleasant on a cool morning or thick and suffocating on a hot afternoon. The dew point cuts through that confusion. It is the temperature to which the air would have to cool for its water vapour to saturate and start condensing, and because it is an absolute measure of how much moisture the air actually holds, it lines up almost perfectly with how sticky the day feels. This calculator turns a temperature and a relative humidity into that far more useful figure using the Magnus formula, the standard approximation used in meteorology and building engineering.
The comfort bands the tool applies follow the US National Weather Service's dew-point guide — up to about 13°C (55°F) comfortable, 13–18°C (55–65°F) increasingly sticky, above 18°C (65°F) oppressive — subdivided a little more finely here: below roughly 13°C dew point the air feels dry and refreshing; into the mid-teens it becomes noticeably sticky; from about 18°C it is muggy; and beyond 21°C it turns oppressive, with tropical nights of 24°C and above feeling genuinely brutal. The physics behind that discomfort is straightforward. Your body sheds heat mainly by evaporating sweat, and evaporation slows dramatically when the surrounding air is already saturated. A high dew point therefore disables your primary cooling system, which is why a humid heatwave is so much more dangerous than a dry one at the same temperature — and why athletes and coaches track dew point closely.
Dew point matters beyond personal comfort, too. Pilots watch the gap between temperature and dew point — the “spread” — because when it narrows, fog and low cloud can form rapidly and cut visibility. Building engineers use it to keep surfaces above the dew point and prevent the hidden condensation that breeds mould and rots structures. One reassuring rule keeps the whole picture honest: the dew point can never rise above the air temperature. When the two meet, the air is fully saturated, humidity is 100%, and dew, fog or rain appears. The Magnus formula used here, with the coefficients Alduchov and Eskridge fitted in 1996, reproduces saturation vapour pressure over water to within about 0.4% between −40 and +50°C — a few tenths of a degree in dew point, which is ample for planning a run, a hike or an event. And as with every RECATOOLS calculator, the maths runs entirely in your browser, so your inputs never leave your device.
Relative humidity tells you how full the air is for its temperature; dew point tells you how much water is actually there — and that is what you feel.
Dew point can never exceed the air temperature
Dew point is the temperature at which air becomes saturated.
It tracks how the air feels better than humidity alone.
Below ~13°C dew point feels comfortable; above ~21°C, oppressive.
Dew point can never exceed the air temperature.
When they’re equal, humidity is 100% — fog or dew forms.
High dew points make sweat evaporate slowly, so you overheat.
Pilots use dew point to anticipate fog and clouds.
The Magnus formula links temperature, humidity and dew point.
Tropical nights with a 24°C+ dew point feel brutal.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- The dew point is the temperature to which air would have to be cooled, at constant pressure, for it to become fully saturated with water vapour. At that temperature the vapour pressure equals the saturation vapour pressure for water, so condensation and evaporation balance and dew, fog or condensation begins to form. Air does not "hold" water — saturation depends on temperature alone. It is a direct measure of how much water is actually in the air.
- Relative humidity is a percentage relative to the current temperature, so 70% humidity feels very different on a cool morning than on a hot afternoon. Dew point is an absolute measure of moisture, so it maps cleanly onto how sticky the air feels regardless of temperature. That is why meteorologists and athletes watch dew point rather than humidity.
- This tool uses the Magnus formula, which relates temperature and relative humidity to dew point through an exponential approximation of how saturation vapour pressure changes with temperature: γ = ln(RH/100) + b·T/(c + T), then Td = c·γ/(b − γ), with b = 17.625 and c = 243.04 °C — the coefficients Alduchov and Eskridge fitted in the Journal of Applied Meteorology in 1996. It is accurate across the everyday range of weather conditions and is a standard method in meteorology and HVAC engineering.
- The bands follow the US National Weather Service’s guide (about 55°F / 13°C and 65°F / 18°C as the two main thresholds), split a little more finely. As a rough guide: below about 13°C the air feels dry and pleasant; 13–16°C is comfortable to slightly sticky; 16–18°C starts to feel humid; 18–21°C is uncomfortably muggy; and above 21°C is oppressive, with 24°C and higher feeling genuinely brutal. The calculator labels your result with one of these bands.
- Your body cools itself by evaporating sweat. When the dew point is high, the air is already laden with moisture, so sweat evaporates slowly and your cooling system struggles. That is why a hot day with a high dew point feels far more punishing — and is more dangerous — than an equally hot but drier day.
- No. The dew point can equal the air temperature, which happens at 100% relative humidity, but it can never exceed it. When the two are equal the air is saturated, which is when dew, fog or cloud can form. It does not mean rain: rain needs lifting and droplet growth aloft, and it often falls through air well below saturation at ground level. If a calculation ever suggested a dew point above the temperature, the humidity input would be impossible.
- Pilots use the temperature–dew point spread to anticipate fog and cloud formation: when the spread is small, visibility can drop quickly. Building engineers use dew point to prevent condensation on cold surfaces and inside walls, which causes mould and damage. In both cases the absolute moisture content matters more than relative humidity.
- Yes. Switch the unit selector to Fahrenheit and the calculator converts internally, showing the dew point in Fahrenheit with the Celsius equivalent alongside. Toggling the unit also converts the temperature you have already entered so the underlying conditions stay the same.
- The Magnus formula is an approximation, but a very good one: Alduchov and Eskridge (1996) report their over-water coefficients reproduce saturation vapour pressure to within about 0.4% between −40 and +50°C, which is a few tenths of a degree in dew point. At temperature extremes the error grows slightly, but for planning a run, a hike, an event or assessing comfort it is more than precise enough. The result is rounded to 0.1°.
- Those are the band edges the tool applies: under 10°C dry, 10–13°C comfortable, 13–16°C becoming sticky, 16–18°C humid, 18–21°C very humid and oppressive, 21–24°C oppressive and miserable, 24°C and above severe. The two main breaks — about 13°C (55°F) and 18°C (65°F) — are the US National Weather Service’s; the finer steps between them are this tool’s own, so treat the labels as a guide to how the air will feel rather than a health threshold.
Method & sources
How it computes
Dew point from air temperature T (°C) and relative humidity RH (%) by the Magnus formula with the Alduchov–Eskridge (1996) coefficients: γ = ln(RH/100) + b·T/(c + T), Td = c·γ/(b − γ), b = 17.625, c = 243.04 °C; Fahrenheit input is converted first and the result is shown in both units. The result is labelled with a comfort band adapted from the US National Weather Service dew-point guide.
What this tool implements
- Magnus-form saturation vapour pressure over water with Alduchov & Eskridge coefficients b = 17.625, c = 243.04 °C (relative error in saturation vapour pressure ≤ 0.4% for −40 °C to +50 °C).
- Comfort bands adapted from the NWS guide (≤ 55 °F / 13 °C comfortable, 55–65 °F / 13–18 °C becoming sticky, ≥ 65 °F / 18 °C oppressive), subdivided at 10, 13, 16, 18, 21 and 24 °C; the page says the finer steps are the tool's own.
- Dew point can never exceed air temperature; RH must be 1–100%. Result rounded to 0.1 °.
Sources
- Alduchov OA, Eskridge RE. Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology 1996;35(4):601-609. doi:10.1175/1520-0450(1996)035<0601:IMFAOS>2.0.CO;2
- US National Weather Service, La Crosse WI. Dew Point vs Humidity — dew-point comfort levels: https://www.weather.gov/arx/why_dewpoint_vs_humidity
What can make this go out of date
- none at runtime
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