Dew Point Calculator
Enter any two of air temperature, relative humidity, and dew point to solve the third. The result is scored on the classic mugginess scale — dew point is the number meteorologists use for comfort, because unlike relative humidity it doesn’t swing with the afternoon temperature.
All temperatures use the unit selected above. Dew point can never exceed the air temperature (100% humidity means they are equal).
What the dew point tells you
The dew point is the temperature the air would have to cool to before its water vapor starts condensing — onto grass at dawn, a cold drink, or your bathroom mirror. It measures the absolute amount of moisture: a 70°F dew point is muggy whether the afternoon hits 80°F or 95°F, which is why forecasters quote it for comfort. Relative humidity, by contrast, compares moisture to the maximum the current temperature can hold, so it looks artificially low on hot afternoons and hits 100% on cool clear nights.
When the dew point is at or below freezing, vapor deposits as frost instead of dew — and pilots and growers watch that “frost point” closely.
How it’s calculated
Magnus formula with the Alduchov–Eskridge coefficients (b = 17.625, c = 243.04°C): γ = ln(RH/100) + bT/(c+T); dew point = cγ/(b−γ). Inverted forms solve RH = 100·e^(bT_d/(c+T_d) − bT/(c+T)) and air temperature. Accurate to about ±0.35°C for −45°C to +60°C. Moisture content (absolute humidity) uses the saturation vapor pressure at T: AH = 216.7 × e/(273.15+T) g/m³.
Formulas assume standard sea-level pressure; readings inside steam rooms, pressurized cabins, or at high altitude will drift slightly.
Dew point comfort scale
| Dew point | How it feels |
|---|---|
| Below 50°F (10°C) | Dry to comfortable |
| 50–60°F (10–16°C) | Comfortable — ideal for most people |
| 60–65°F (16–18°C) | Noticeably humid, sticky for some |
| 65–70°F (18–21°C) | Muggy — uncomfortable for most |
| 70–75°F (21–24°C) | Oppressive; strenuous activity gets risky |
| 75°F+ (24°C+) | Miserable — tropical, dangerous with heat |
General guide widely used by NWS forecast offices; sensitivity varies by person and acclimatization.
Worked example
An 86°F day at 60% relative humidity has a dew point of 70.5°F (21.4°C) — the bottom of the “oppressive” band, with about 18 g of water vapor per m³ of air. Flip the mode: 86°F with a 70°F dew point returns 59% RH, and RH 60% with a 70°F dew point implies an air temperature of about 85.5°F. Same triangle, any missing corner.
Common mistakes
- Judging summer comfort by relative humidity alone — 40% RH at 100°F is a sweltering 71°F dew point.
- Entering a dew point higher than the air temperature (impossible; the tool flags it).
- Mixing units — the °F/°C selector applies to both temperature fields.
- Using outdoor dew point for indoor decisions; HVAC and dehumidifiers change indoor moisture quickly.
Where it is used
- Judging how muggy a run, ride, or match will feel before heading out.
- Sizing dehumidifiers and diagnosing condensation or mold-prone rooms.
- Aviation preflight checks — small temperature/dew-point spreads warn of fog and carb icing.
- Growers watching frost point on clear, calm nights.
Frequently asked questions
What is a comfortable dew point?
Most people feel great below 60°F, notice stickiness in the low 60s, call 65–70°F muggy, and find 70°F+ oppressive. Air-conditioned buildings typically hold dew points near 50–55°F.
Why use dew point instead of relative humidity?
RH changes all day as temperature changes even when moisture doesn’t; dew point tracks the moisture itself. A 65% RH reading could be crisp (at 60°F) or brutal (at 95°F) — the dew points, 48°F vs 81°F, tell the real story.
Can the dew point exceed the air temperature?
No. At 100% relative humidity they are equal and condensation (dew, fog) begins. If you enter a higher dew point, the calculator caps RH at 100% and flags the input.
What formula is used and how accurate is it?
The Magnus approximation with Alduchov–Eskridge constants (17.625, 243.04°C), accurate to roughly ±0.35°C across −45°C to +60°C — tighter than any consumer humidity sensor, which is typically ±2–3% RH.
What indoor humidity should I aim for?
A common indoor guideline (used by OSHA’s technical manual and most HVAC pros) is 68–76°F with 20–60% RH; many mold-prevention guides suggest keeping RH under about 50% in summer. That corresponds to indoor dew points around 45–55°F.