Dew point calculator
Estimate dew point from air temperature and relative humidity, with optional surface comparison. Change the inputs to update the chart or diagram.
Your inputs
Values shown are editable examples; verify your own measurements and product specifications. Results update as you edit.
Calculations run in your browser. Inputs are not saved or sent to analytics.
How it works
- γ = ln( / 100) + 17.67 × / (243.5 + ); Td = 243.5 × γ / (17.67 − γ)
- Worked example: T = 20°C, RH = 50% ⇒ Td ≈ 9.27008599°C
Input and formula key
Your result
Complete the inputs to see your result.
Assumptions and limitations
Uses the Magnus/Bolton-style saturation-over-water approximation with constants 17.67 and 243.5°C. Air input is −40 to 50°C and relative humidity 0.01–100%; this input range is not a guaranteed accuracy specification. Zero humidity has no finite dew point. Below freezing the result remains an over-water dew point, not frost point. A surface at or below the modeled dew point may condense moisture under the assumed conditions. Excludes pressure correction, sensors, mold prediction and health or building certification.
Sources
- National Weather Service — saturation vapor pressure and dew-point equations Sources reviewed: 2026-10-09
