Dew Point Calculator: Humidity, Condensation and Mold Risk
How the dew point calculator works: at 68 °F (20 °C) and 50 % relative humidity the dew point is 48.7 °F (9.3 °C). Any surface colder than that gets wet with condensation. Mold can start earlier – from about 80 % humidity at the surface, here below 54.7 °F (12.6 °C).
Example: Temperature unit: °F (Fahrenheit) · What do you know: Temperature + relative humidity → dew point · Air temperature 68 · Relative humidity 50 % · Surface temperature (wall, window) 57 → Result: Dew point 48.7 °F (at 68 °F and 50 %). Source: WMO-No. 8, WMO-No. 8, Guide to Instruments and Methods of Observation, Vol. I, Ch. 4, Annex 4.B – Magnus formula e_w = 6.112 exp(17.62 t/(243.12 + t)). Updated: .
How it is calculated
What is the dew point?
The dew point is the temperature to which air must cool for its water vapor to start condensing – relative humidity then reaches 100 %. When indoor air cools below its dew point at a cold wall, window or pipe, water condenses there. Outdoors, the US National Weather Service calls dew points of 65 °F and more oppressive.
Dew point formula (Magnus)
The saturation vapor pressure over water follows the Magnus formula with the coefficients published by the World Meteorological Organization (WMO-No. 8):
e_s(t) = 6.112 hPa · exp(17.62 · t / (243.12 + t)), t in °C
- Vapor pressure of the air: e = relative humidity × e_s(t)
- γ = ln(e / 6.112)
- Dew point: τ = 243.12 · γ / (17.62 − γ)
Example 68 °F (20 °C), 50 %: e_s = 23.33 hPa, e = 11.66 hPa, γ = 0.646, τ = 9.3 °C = 48.7 °F.
Dew point chart (°F)
| Air | 40 % | 50 % | 60 % | 70 % |
|---|---|---|---|---|
| 65 °F | 40 °F | 46 °F | 51 °F | 55 °F |
| 68 °F | 43 °F | 49 °F | 54 °F | 58 °F |
| 72 °F | 46 °F | 52 °F | 57 °F | 62 °F |
| 75 °F | 49 °F | 55 °F | 60 °F | 65 °F |
| 80 °F | 54 °F | 60 °F | 65 °F | 69 °F |
Mold on walls: the 80 % rule
Mold doesn’t need liquid water. According to the German Environment Agency’s mold guideline, a surface relative humidity of about 70–80 % over longer periods is enough, while below 70 % mold hardly grows. Because air cools at a cold wall, its relative humidity rises there. The calculator therefore shows the surface temperature at which 80 % is reached – 54.7 °F (12.6 °C) for room air at 68 °F and 50 %.
Limitations
The Magnus formula is an approximation (under 0.5 % deviation from exact tables in the usual range) and applies from about −45 °C to 60 °C. Consumer hygrometers are often only accurate to ±3–5 %. Moisture analysis of whole wall assemblies (vapor retarders, insulation layers) needs the layer build-up and heat flow – that calls for a professional assessment.
Frequently asked questions
How do you calculate the dew point?
With the Magnus formula: get the vapor pressure from temperature and relative humidity, then γ = ln(e/6.112) and τ = 243.12 · γ / (17.62 − γ) in °C. At 68 °F and 60 % the dew point is 53.6 °F (12.0 °C).
What is a comfortable dew point?
The US National Weather Service rates summer dew points up to 55 °F as dry and comfortable, 55–65 °F as sticky with muggy evenings and 65 °F or more as oppressive.
How do I know if my wall will get mold?
Measure the coldest spot (room corner, window reveal) with an infrared thermometer and enter it as surface temperature. If humidity at the surface stays above 80 % for long periods, mold is likely.
What is the difference between relative and absolute humidity?
Absolute humidity is the mass of water per cubic metre of air. Relative humidity is the percentage of the maximum possible at that temperature – so it rises when the same air cools.
Why do windows fog up in winter?
The glass is colder than the dew point of the room air. At 68 °F and 50 %, glass below 48.7 °F fogs up. Ventilating lowers the moisture content and with it the dew point.
Sources and legal basis
- WMO-No. 8: WMO-No. 8, Guide to Instruments and Methods of Observation, Vol. I, Ch. 4, Annex 4.B – Magnus formula e_w = 6.112 exp(17.62 t/(243.12 + t))
- UBA Schimmelleitfaden: Umweltbundesamt – Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Oberflächenfeuchte 70–80 %)
- NWS: US National Weather Service – dew point comfort levels
As of:
Related tools
- Electricity cost calculator for appliancesWork out what an appliance costs to run: enter watts, hours per day, days per week and your rate per kWh to get kWh and cost per day, month and year.
- Density calculator: density, mass and volumeCalculate density, mass or volume with ρ = m/V – step-by-step, with unit conversion from g/cm³ to kg/m³ and lb/ft³ and densities of water, metals and plastics.
- Daily water intake calculatorHow much water should you drink a day? Daily fluid and total water intake by age, sex, pregnancy and breastfeeding, based on EFSA and German DGE values.
- Cups to grams converterConvert cups, tablespoons and teaspoons to grams for flour, sugar, butter, milk, rice and more – US cups, metric cups and USDA weights. Grams to cups too.
- Bar to psi converterConvert bar to psi and back: 1 bar = 14.504 psi. Also pascal, kPa, atm, mmHg and water column – with the working, a chart and a note on psig vs psia.
- Bra size calculator – measure underbust and bustBra size calculator: enter your underbust and bust in inches or cm to get band and cup, converted to US, UK, EU, FR and AU sizes, plus sister sizes. Free.