Dew Point Calculator
Calculate dew point, absolute humidity, and how the air will actually feel from temperature and relative humidity, in Celsius or Fahrenheit.
How to use this calculator
- 1Enter the air temperature and choose Celsius or Fahrenheit.
- 2Enter the relative humidity as a percentage.
- 3Read the dew point and the comfort description — dew point predicts how the air feels far better than relative humidity does.
How it works
Magnus-Tetens dew point formula
γ = ln(RH ÷ 100) + (b × T) ÷ (c + T) Td = (c × γ) ÷ (b − γ) b = 17.62, c = 243.12 °C es = 6.112 × e^((b × T) ÷ (c + T)) saturation vapour pressure AH = 216.7 × e ÷ (T + 273.15) absolute humidity, g/m³
Dew point is the temperature at which air would become saturated if cooled without changing its water content. The Magnus-Tetens formula works by computing how much water vapour the air is actually carrying, then solving for the temperature at which that amount would be 100% relative humidity. Because it depends only on the absolute water content, dew point stays fixed as the day warms — unlike relative humidity, which falls as temperature rises.
Worked example
Air at 28 °C with 65% relative humidity has a saturation vapour pressure of 37.8 hPa and an actual vapour pressure of 24.6 hPa, giving a dew point of about 20.8 °C — firmly in the oppressive range, and the reason that air feels heavy.
Dew Point Calculator: the complete guide
Why dew point beats relative humidity
Relative humidity is a ratio, and its denominator changes with temperature. Air at 60% relative humidity on a 5 °C morning holds far less water than air at 60% on a 30 °C afternoon, yet the number is identical. That is why relative humidity is a poor guide to how the air will feel.
Dew point measures the actual water content and is therefore a stable, absolute figure. Meteorologists and pilots use it for exactly this reason. As a rough guide: below 10 °C is comfortable, 16 °C is where most people start noticing, 18 °C feels humid, and above 21 °C is oppressive regardless of the temperature.
This is also why relative humidity falls through the day even when nothing changes. As the sun warms the air, its capacity to hold water grows while the water content stays constant — so the ratio drops and the dew point does not move.
Condensation, windows, and mould
Any surface cooler than the dew point will collect condensation. That is the whole explanation for a cold drink sweating, for morning dew, and for water running down single-glazed windows in winter.
In buildings it matters more than it sounds. If indoor air has a 12 °C dew point and a wall cavity or window frame sits at 10 °C, moisture will condense there continuously. Sustained surface moisture is what feeds mould, so the fix is either raising the surface temperature with insulation or lowering the indoor dew point with ventilation and dehumidification.
Fog, clouds, and the spread
The gap between temperature and dew point — the spread — tells you how close the air is to saturation. A small spread means fog is likely, particularly overnight as the ground cools. Pilots watch it for exactly this reason, and a spread under about 2.5 °C is a standard fog warning threshold.
The same relationship sets cloud base. Rising air cools at roughly 10 °C per kilometre while its dew point falls at only 2 °C per kilometre, so they converge at about 125 metres for every degree Celsius of spread. A 10 °C spread puts the cloud base near 1,250 metres.
Comfortable indoor humidity
Most guidance recommends keeping indoor relative humidity between 30% and 50%. Below that, static electricity, dry skin, and irritated airways become common. Above 60%, dust mites and mould thrive.
In winter, outdoor air is very dry in absolute terms, so heating it indoors drives relative humidity down and a humidifier helps. In summer the reverse applies, and air conditioning dehumidifies as a side effect of cooling coils sitting below the dew point — which is precisely why an air conditioner produces condensate.
Frequently asked questions
What is a comfortable dew point?
Below about 13 °C (55 °F) feels comfortable to most people. From 16 °C it becomes noticeably sticky, from 18 °C humid, and above 21 °C (70 °F) oppressive. Dew point predicts comfort far more reliably than relative humidity because it does not move as the temperature changes.
Can the dew point be higher than the temperature?
No. At 100% relative humidity the dew point equals the air temperature, and that is the ceiling — the air is already saturated and any further cooling produces condensation. A reported dew point above the temperature indicates a sensor fault.
Why do my windows fog up in winter?
The glass surface is colder than the indoor dew point, so water vapour condenses on it. Either raise the glass temperature with double glazing or insulation, or lower the indoor dew point by ventilating and reducing moisture sources such as unvented drying and long showers.
How is dew point different from wet bulb temperature?
Dew point is the temperature at which air saturates by cooling alone, with no water added. Wet bulb temperature is what a thermometer wrapped in a wet cloth reads, which reflects evaporative cooling. Wet bulb sits between the dew point and the air temperature, and is the figure used for heat stress limits because sweating stops working near a 35 °C wet bulb.