Dew Point Calculator UK
The dew point is the temperature at which the moisture in the air starts to condense into water. This UK dew point calculator gives you two things: the dew point for any air temperature and humidity, and a condensation risk check for a wall, roof or floor once you know its U-value. It uses the BS EN ISO 13788 method, works in metric throughout, and is not tied to any insulation brand. Use Mode A for a quick dew point, and Mode B when you want to know whether a specific construction will stay dry and mould-free through a UK winter.
Mode
−30 to 50 °C
1 to 100 %
Enter temperature and humidity to calculate
| Room / use | Typical winter RH |
|---|---|
| Living room / bedroom | 50–60% |
| Kitchen (in use) | 60–70% |
| Bathroom (in use) | 70–100% |
| Occupied but unheated | 65–75% |
Bathrooms and kitchens fail first because the air already holds far more moisture before you touch the wall build-up.
How this calculator works
Mode A applies the Magnus formula to your air temperature and relative humidity to return the dew point. Any surface colder than that dew point will collect condensation. Mode B goes a step further: it predicts the temperature of the inside face of your wall, roof or floor from its U-value, then compares that surface temperature against the dew point of the room air. If the surface is colder than the dew point you get liquid condensation. If it is warmer than the dew point but the humidity right at the surface still climbs above 80 percent, you get mould without ever seeing a drop of water.
Takeaway: pick Mode B whenever mould, not just misted glass, is the thing you are trying to prevent.
Formulas used
Saturation vapour pressure (BS EN ISO 13788): pSat = 610.5 × e^(17.269 × T / (237.3 + T)) for T ≥ 0 °C
Dew point (Magnus): γ = ln(RH ÷ 100) + (17.62 × T) ÷ (243.12 + T), then Td = (243.12 × γ) ÷ (17.62 − γ)
Internal surface temperature: Tsi = Ti − U × Rsi × (Ti − Te), with Rsi = 0.25 m²K/W
Temperature factor: fRsi = (Tsi − Te) ÷ (Ti − Te)
Takeaway: the surface resistance for a condensation check is 0.25, not the 0.13 you use for a heat-loss U-value, because you are assessing the coldest realistic surface, not the average.
Reference — the 0.75 benchmark
| Measure | UK dwelling benchmark |
|---|---|
| Temperature factor fRsi (flat) | ≥ 0.75 (BRE IP 1/06) |
| Surface RH for mould | keep below 80% |
| Surface condensation | surface above dew point |
Takeaway: the flat wall usually clears 0.75 easily; the number that matters in practice is the local fRsi at a cold junction.
Worked example — a bathroom wall
A bathroom in use: room at 20 °C and 70 percent relative humidity, outside at 5 °C, on an insulated cavity wall with a U-value of 0.28 W/m²K.
| Step | Result |
|---|---|
| Dew point of the room air | 14.4 °C |
| Internal surface temperature | 20 − 0.28 × 0.25 × 15 = 18.9 °C |
| Surface above dew point? | Yes (18.9 > 14.4) — no liquid condensation |
| Surface relative humidity | ≈ 75% |
| fRsi | 0.93 (passes 0.75) |
| Verdict | Low risk, but close |
The wall stays dry, but surface humidity is already near the 80 percent mould line. Drop the room to 18 °C, push humidity to 75 percent, or move to a colder junction, and the same wall tips into mould territory.
Takeaway: a wall can pass the "no dripping" test and still be one bad extractor fan away from black mould in the corner.
Frequently asked questions
Related tools
- U-Value Calculator UK — build up a wall, roof or floor layer by layer to get the U-value this tool needs
- Loft Conversion Cost Calculator UK — budget a conversion including the insulation upgrades that fix condensation
- Stud Wall Calculator UK — timber and board quantities for an insulated internal wall
- Timber Calculator UK — lengths, volume and board feet for framing and battens
Guides worth reading
- How to Calculate Dew Point and Condensation Risk: UK Guide — the method, the 80 percent mould rule, and a worked solid-wall example
Disclaimer: This calculator is for planning and screening only. It checks surface condensation and mould risk on the flat plane of an element and does not account for interstitial condensation, thermal bridging at junctions, air leakage or ground effects. For Building Regulations submission, condensation risk analysis or any damp diagnosis, use accredited BS EN ISO 13788 software or a qualified thermal or damp surveyor, and confirm the current Approved Document (or Section 6 / Technical Booklet) for your jurisdiction.