What dew point actually tells you
Dew point is the temperature air would need to cool to before it becomes fully saturated with water vapor and starts condensing into dew. The higher the dew point, the more actual moisture is sitting in the air right now — which is why it's a far better "how humid does it feel" number than relative humidity.
Relative humidity is deceptive because it's relative to temperature: 90% humidity at a chilly 5°C carries very little water compared to 90% humidity at a sweltering 30°C, even though both read the same percentage. Dew point sidesteps that problem entirely — it's an absolute measure of moisture that doesn't shift just because the temperature does.
The Magnus formula, worked through
This calculator uses the Magnus approximation, a well-tested formula meteorologists rely on for everyday forecasting because it stays accurate to within about 0.4°C across the temperature and humidity ranges people actually experience.
α = (17.27 × T) ÷ (237.7 + T) + ln(RH ÷ 100)
Td = (237.7 × α) ÷ (17.27 − α)
Worked example: at 25°C and 60% relative humidity, α works out to about 1.6, and the dew point comes out around 16.7°C — squarely in the "comfortable" range on the scale below.
The same two constants — 17.27 and 237.7°C — rearrange to solve for relative humidity if you know temperature and dew point, or for temperature if you know dew point and humidity, which is exactly what the other two modes on this calculator do.
The dew point comfort scale
Meteorologists and HVAC professionals use roughly the same bands to describe how muggy the air will feel, regardless of the actual air temperature:
| Dew point | Feel |
|---|---|
| Below 10°C (50°F) | Dry |
| 10–16°C (50–61°F) | Comfortable |
| 16–18°C (61–64°F) | Slightly humid |
| 18–21°C (64–70°F) | Humid |
| 21–24°C (70–75°F) | Very humid |
| Above 24°C (75°F) | Extremely humid / oppressive |
This calculator's results panel colour-codes your result against this exact scale, so you don't need to look the bands up separately.
Where dew point actually gets used
- Weather forecasting: a high dew point is what makes a 30°C day feel oppressive versus pleasant, which is why forecasts increasingly report it alongside temperature.
- Condensation and mould risk: any surface that cools to or below the dew point — a cold window, an AC duct, a basement wall — will collect moisture, which over time invites mould growth.
- Frost forecasting: when dew point is expected to drop to or below 0°C overnight, frost on car windshields and outdoor plants becomes likely.
- HVAC and dehumidifier sizing: cooling systems need to pull air below its dew point to actually remove moisture, not just lower the temperature — a distinction that affects how a system is sized and run.
Dew point is one piece of how weather actually feels on your skin. For the other two common "feels like" numbers, see the Heat Index Calculator for hot, humid days, and the Wind Chill Calculator for cold, windy ones.