Dew Point & Building Moisture Calculator
Calculate exact dew point thresholds, moisture loading, and localized surface water activity for structural assessments.
Atmospheric Readings
Measure the coldest structural surface (e.g. window reveal or corner wall).
Condensation will form on any physical surface at or below this temperature.
Wall is dangerously close to dew point or surface water activity is elevated (aw ≥ 0.75). Mold germination can occur within days.
Social-housing workflow note: Moisture and mould risks at this level need prompt investigation, but this calculator does not classify a statutory hazard. Since 27 October 2025, English social landlords must investigate a potential significant damp and mould hazard within 10 working days, provide a written summary within 3 working days after the investigation concludes, and undertake relevant safety work within 5 working days after that conclusion if a significant hazard is identified. Potential emergency hazards must be investigated and made safe within 24 hours. These Awaab's Law timescales do not currently apply to private rentals. Use our Awaab's Law timeline calculator to map social-housing deadlines.
Frequently asked questions
What does this dew point calculator compute?▾
Enter air temperature, relative humidity and the coldest surface temperature, and the tool calculates the dew point (using the Magnus-Tetens approximation), vapour pressure, absolute humidity and the local surface water activity. From the gap between surface temperature and dew point it indicates a condensation and mould-germination risk band, consistent with the psychrometric approach in BS 5250.
When does condensation form on a wall?▾
Condensation forms on any surface at or below the dew point of the surrounding air. A cold thermal bridge — such as a window reveal or an external corner — is usually the first place to reach the dew point, which is why you measure the coldest structural surface rather than the room average.
Does this replace a moisture meter or calibrated equipment?▾
No. This is a planning and triage aid that models psychrometric thresholds from the values you enter. It does not measure moisture and is not a substitute for calibrated thermo-hygrometers, surface-temperature probes or moisture meters used during a survey. Always verify on-site with your own instruments.
How does DampApp Pro use these calculations?▾
DampApp Pro performs the same psychrometric calculations on-device while you survey, recording temperature and humidity readings against each defect and supporting capture-timestamped photos. You review and edit the findings before exporting a PDF on the device — useful when evidencing a condensation diagnosis under BS 5250, HHSRS or the applicable tenancy duties.
Is this an Alpha Moisture Systems dew point calculator?▾
No. Alpha Moisture Systems calculators are intended for trace moisture and pressure-dew-point conversions in gases. This free tool is for building-survey work: it derives dew point, vapour pressure, absolute humidity and surface water activity from room air and surface-temperature readings to explain condensation risk.
Professional psychrometrics on-site. Automatically.
DampApp Pro™ runs these same psychrometric calculations on-device while you survey. Log air temperature, relative humidity and surface readings against each defect fully offline, then review and export a RICS Home Survey Standard-aligned PDF on the device.
