Hygrometer
What the number means
The dew point is a temperature, and it answers one question: how far would this air have to cool before it could not hold its water any more? At that point it starts giving the water back, onto whatever is nearest and coldest.
That makes it two useful things at once. It is a measure of how much water the air is carrying — unlike relative humidity, which only says how close the air is to full — and it is a direct, physical threshold. Any surface at or below the dew point collects moisture. That is not a prediction about your house; it is a property of the air, and it is true wherever that air goes.
The three figures underneath
The calculator shows more than the dew point because the dew point on its own rarely settles anything.
- Water forms on anything below — the dew point again, framed as the threshold it is. Cold glass, a car roof at dawn, a lens brought out of air conditioning, a drink can. If it is colder than this number, it gets wet.
- Margin to saturation — the gap between the air temperature and the dew point, which forecasters call the dew point depression. Under about 1°C the air is effectively saturated and you get mist and fog; a wide gap means condensation needs something genuinely cold.
- Water in the air — grams per cubic metre. This is the figure that makes winter and summer comparable, and it is the one almost nothing else displays.
Worked example
Air at 75°F and 60% relative humidity.
dew point = 243.12 × 1.066 / (17.62 − 1.066) = 15.7°C, or 60°F Temperatures converted to Celsius first, because the coefficients are defined for it.
So the air has to lose 15 degrees Fahrenheit before it saturates. A window at 58°F — perfectly ordinary on a mild night — is already below that, and will be wet.
Where the same calculation shows up
Once you have the dew point, a lot of unrelated-looking problems turn out to be the same problem with different numbers: whether a finish will cure overnight, whether there will be fog on the drive, whether a lens will clear in time, and whether the windows will run.
Accuracy
Magnus-Tetens is an approximation to the Clausius-Clapeyron relation, not the relation itself. Over the range it is fitted for — roughly −45°C to +60°C — it is within about 0.4% of the exact saturation vapour pressure, which is far better than the accuracy of the humidity reading you feed it. In practice the error in your dew point comes almost entirely from the humidity measurement, not from this formula.
Dew Point Human Comfort & Sensory Scale
| Dew Point (°F / °C) | Sensory Comfort Perception | Moisture Content in Air | Recommended Indoor Action |
|---|---|---|---|
| < 50°F (< 10°C) | Dry and crisp | Low absolute humidity | Add humidification if skin or throat feels dry |
| 50–55°F (10–13°C) | Very comfortable | Pleasant, balanced | Ideal indoor target range |
| 56–60°F (13–16°C) | Comfortable | Moderate humidity | Standard comfortable indoor air |
| 61–65°F (16–18°C) | Noticeable humidity | "Sticky" sensation begins | Ventilate or run light dehumidification |
| 66–70°F (19–21°C) | Uncomfortable and muggy | Heavy moisture | Run air conditioning or dehumidifier |
| 71–75°F (22–24°C) | Oppressive and miserable | Tropical moisture | Restrict heavy outdoor exertion |
| > 75°F (> 24°C) | Severely hazardous | Near physical cooling limits | Heat illness precautions required |