The dew point is the temperature to which air must cool for water vapour to start condensing into liquid water. When the air cools to its dew point, dew forms on grass, fog appears, and windows mist up. Unlike relative humidity, which changes whenever the temperature changes, the dew point tracks the actual amount of moisture in the air. That is why weather forecasters trust it: a dew point near 20 degrees feels sticky whatever the temperature, while a dew point near 5 degrees feels dry.
The maths behind this tool is the Magnus approximation, a formula fitted to how water vapour pressure changes with temperature. It uses two constants, 17.27 and 237.7, and the natural logarithm of the relative humidity divided by 100. First it builds an intermediate value from temperature and humidity, then it turns that value back into a temperature. The result is accurate to roughly a few tenths of a degree in normal weather, which is plenty for daily decisions and classroom work.
Here is a worked example. Air at 25 degrees with 60 percent relative humidity has a dew point of about 16.7 degrees. You can check the sense of it: cooling that air by about 8.3 degrees, down to 16.7, would saturate it and start condensation. The spread, the gap between air temperature and dew point, is 8.3 degrees. A large spread means dry air and clear skies overnight, while a small spread means the air is close to saturation and fog or dew is likely.
The spread is a favourite tool of pilots and forecasters. When temperature and dew point are within about 2 degrees, fog, low cloud, or dew become likely, because only a little night cooling is needed to reach saturation. When the spread is above 10 degrees, the air is dry and fog is unlikely. Glider pilots watch the spread too, since a small spread at ground level often means a low cloud base, which limits how high they can climb.
Comfort follows the dew point more than the temperature alone. A dew point below 10 degrees feels dry and comfortable to most people. Between 10 and 16 degrees it feels pleasant, and between 18 and 21 degrees it starts to feel sticky, because sweat evaporates slowly. Above 24 degrees the air feels oppressive, and even shade brings little relief. This is why a 30-degree day with a dew point of 12 feels fine, while a 30-degree day with a dew point of 24 feels exhausting.
Dew point also matters indoors. When warm, humid indoor air meets a cold window or an uninsulated wall, the surface may be below the dew point, so water condenses there. Over time that damp feeds mould. The fix is to lower indoor humidity with ventilation or a dehumidifier, or to warm the cold surface with insulation. If your windows drip every winter morning, compare the indoor dew point with the window temperature, and the cause becomes clear.
There are limits to this calculator. The Magnus formula is an approximation, fitted to measurements, not a law of physics, and it loses accuracy at very low temperatures and very low humidity. The tool also assumes the humidity reading is correct, but cheap home sensors can be off by several percent, which shifts the dew point by a degree or more. For aviation, farming frost warnings, or any safety decision, use calibrated instruments and official forecasts.
To use this tool well, take temperature and humidity at the same place and time, in the shade and away from direct breath or steam. Try your morning and afternoon readings and watch how the dew point stays steadier than relative humidity through the day. If you want to predict overnight fog, enter the evening temperature and humidity, then check the spread: a spread under 3 degrees is your warning sign that dew or fog is on the way.