Free dew point calculator
Enter the air temperature and relative humidity to get the dew point — the temperature at which moisture condenses — in °C and °F, with a dry-to-oppressive comfort read, using the Magnus formula, updated live, as you type.
On this page13 sections
Magnus formula (Sonntag 1990 coefficients). Valid for T ∈ [−40, +50]°C, RH ∈ (0, 100].
Results are estimates. Consult a professional.
What is the dew point?
The dew point is the temperature to which air must cool, at constant pressure, before the water vapor in it starts to condense into liquid — dew on the grass, fog over a field, or droplets on a cold glass. This dew point calculator takes the two readings every weather app gives you, air temperature and relative humidity, and returns that condensation temperature in both Celsius and Fahrenheit.
Unlike relative humidity, the dew point is an absolute measure of how much moisture the air is actually holding. A higher dew point means more water vapor in the air, full stop — it does not shift around as the temperature rises and falls through the day. That is why meteorologists reach for it when they want to describe how muggy the air will truly feel.
Why dew point beats relative humidity for comfort
Relative humidity is a percentage of how full the air is relative to the most it could hold at that exact temperature — and that ceiling changes with temperature. As the air warms through the morning, its capacity grows, so the same amount of moisture reads as a lower relative humidity even though nothing about the actual water content changed. The number drops while the air feels no drier.
Because the dew point fixes the moisture itself, it tracks how sticky the air feels far more closely than relative humidity does. The U.S. National Weather Service puts it plainly: to judge how "dry" or "humid" it will feel outside, look at the dew point rather than the relative humidity.
National Weather Service (La Crosse, WI) — guidance that dew point, not relative humidity, is the better indicator of how humid the air feels, with the comfort thresholds used below.How the dew point calculator works
The calculator uses the Magnus formula, the standard closed-form way to convert air temperature and relative humidity into a dew point. It works in two steps: first it builds an intermediate term, γ, from the temperature and humidity, then it solves that term for the dew point temperature.
The inputs explained
Only two numbers go in, and both come straight off a weather app or a home hygrometer. Understanding what each one does to the result makes the dew point easy to read at a glance.
Air temperature
The ordinary, dry-bulb temperature a weather report gives. It sets the ceiling: the dew point can rise toward this value but never above it. Enter the current temperature in °C.
Relative humidity
The percentage of moisture the air is holding relative to its maximum at that temperature, from just above 0% up to 100%. The higher the humidity, the closer the dew point sits to the air temperature. At 100% the two are equal — the air is saturated and condensation is already forming.
A worked example using the dew point calculator
Sam checks the weather before a run: the air temperature is 30 °C (86 °F) and relative humidity is 60%. The humidity percentage alone does not say how oppressive it will feel, so Sam wants the dew point.
Step 1 — Enter the two readings
Air temperature 30 °C, relative humidity 60%. Both sit well inside the formula's valid range, so the estimate is reliable.
Step 2 — Build the γ term
Plugging the numbers into the first equation gives γ = ln(60/100) + (17.625 × 30) / (243.04 + 30) = 1.426.
Step 3 — Solve for the dew point
The second equation returns Td = (243.04 × 1.426) / (17.625 − 1.426) = 21.4 °C, which is 70.5 °F.
Dew point comfort levels: dry, comfortable, muggy, oppressive
Once you have the dew point, this scale turns it into how the air will feel. The bands are a general guide for most people — those acclimated to humid climates tolerate higher dew points, and direct sun or exertion make any band feel worse.
| Dew point | How it feels | What to expect |
|---|---|---|
| Below 13 °C (55 °F) | Dry | Crisp, pleasant air; sweat evaporates freely and the air feels light. |
| 13–16 °C (55–61 °F) | Comfortable | Still pleasant for most people; humidity is barely noticeable. |
| 16–18 °C (61–64 °F) | Slightly humid | Air begins to feel sticky, especially in the sun or during exercise. |
| 18–21 °C (64–70 °F) | Muggy | Noticeably humid and uncomfortable; evenings stay sticky. |
| Above 21 °C (70 °F) | Oppressive | Heavy, tropical air; sweat barely evaporates and heat stress builds. |
The progression follows National Weather Service guidance — as the dew point rises, the air moves from dry and comfortable, through sticky and muggy, to oppressive. The five bands here are the common meteorologist comfort scale; the °C values are the thresholds used by this calculator's reference scale, with Fahrenheit equivalents alongside.
Why the dew point can never exceed the air temperature
The dew point can climb toward the air temperature but cannot pass it. The reason is physical, not a quirk of the formula: cooling air to its dew point is what produces 100% relative humidity. If the dew point were higher than the current air temperature, the air would already be holding more water than it can carry — supersaturation, which is unstable and resolves almost instantly into fog, cloud, or dew.
This is also a quick sanity check on any dew point you read elsewhere: a reported dew point above the current air temperature is a data error, not a real condition.
Common dew point mistakes to avoid
- Confusing dew point with relative humidity. A 90% relative humidity at 10 °C is a chilly, low-dew-point day; a 50% humidity at 32 °C can feel far muggier. Read the dew point, not the percentage, to judge comfort.
- Reading the daily-high humidity instead of the current value. Relative humidity swings through the day as temperature changes. Enter the current reading, not the morning peak, or the dew point will be off.
- Assuming a high dew point needs a high temperature. Moisture, not heat, sets the dew point. A mild 24 °C morning at high humidity can carry a dew point in the oppressive range.
- Treating the bands as exact lines. Comfort is a gradient. A dew point of 21 °C versus 22 °C is a small step, not a switch from fine to unbearable — use the bands as a guide, not a verdict.
For related views of the same moist air, see the heat index calculator for how hot it feels and the wet-bulb temperature calculator for the limit of evaporative cooling.
How accurate is this dew point calculator?
The calculator uses the Magnus formula with the recommended coefficients (a = 17.625, b = 243.04 °C), the standard method for converting temperature and humidity to dew point. Across the everyday range of −40 to +50 °C it is accurate to within about ±0.4 °C — well inside the precision any weather decision needs. Outside that range, in extreme cold or heat, the error grows, but those conditions are far from normal comfort questions.
Two practical notes. First, the result is only as good as your inputs: a humidity reading from a cheap sensor or a value taken hours earlier will shift the dew point accordingly. Second, the comfort bands are general guidance — acclimation, sun, wind, and activity all change how a given dew point feels. Treat the dew point as a solid number and the comfort label as a rule of thumb.
Frequently asked questions about the free dew point calculator
About this dew point calculator
This dew point calculator runs entirely in your browser. Nothing you enter is sent anywhere — the dew point is computed locally from the Magnus formula as you adjust the air temperature and relative humidity, and the result updates instantly.
It is one of our weather calculators. Browse the full set of free calculators for more humidity, heat, and climate tools.