The dew point is about 44.6 °F, so the glass is only 0.4 °F above it: on the edge of fogging and far too close for painting.
How the dew point calculator works
DewPointCalc uses the Magnus formula with the Alduchov and Eskridge constants a = 17.625 and b = 243.04 °C. With the air temperature T in °C and relative humidity RH, it computes γ = ln(RH/100) + a·T/(b + T), and the dew point is b·γ/(a − γ). Going the other way, RH = 100·exp(a·Td/(b + Td) − a·T/(b + T)). Absolute humidity comes from the saturation vapour pressure, 6.112·exp(17.67·T/(T + 243.5)) hPa, times RH × 2.1674 divided by the temperature in kelvin. A surface at or below the dew point gets condensation; for painting, keep it at least 5 °F (3 °C) above.
Dew point comfort chart
| Dew point | Feels | Notes |
|---|---|---|
| Below 40 °F (4 °C) | Very dry | dry skin and static are likely, especially indoors |
| 40–54 °F | Comfortable | pleasant for most people |
| 55–59 °F | Slightly humid | still comfortable for most, a little sticky for some |
| 60–64 °F | Humid | sticky; most people notice the moisture |
| 65–69 °F | Muggy | uncomfortable, sweat evaporates slowly |
| 70–74 °F | Oppressive | very uncomfortable; take care with exercise |
| 75 °F (24 °C) and up | Miserable | dangerous for strenuous activity |
Dew point at 70 °F by humidity
| Relative humidity | Dew point |
|---|---|
| 20% | 27.1 °F |
| 30% | 37.1 °F |
| 40% | 44.6 °F |
| 50% | 50.5 °F |
| 60% | 55.5 °F |
| 70% | 59.8 °F |
| 80% | 63.5 °F |
| 90% | 66.9 °F |
Frequently asked questions
How do I stop condensation on windows?
Lower the indoor humidity with ventilation or a dehumidifier, or warm the glass with better glazing or air flow.
What is the 5 degree rule for painting?
Many coatings specifications require the surface to be at least 5 °F (3 °C) above the dew point so moisture does not form under the coating.
Why do cold-water pipes sweat in summer?
The pipe is colder than the dew point of the humid air, so water condenses on it. Insulation keeps the surface warmer.
Is DewPointCalc free?
Yes, with no sign-up or ads, and your readings stay in your browser.
How accurate is the result?
The formula is accurate to about 0.4 °C; most home hygrometers are only accurate to 2–5% RH, which matters more.