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Dew Point Calculator
Computes the dew point temperature from air temperature and relative humidity with the Magnus formula (Alduchov & Eskridge coefficients a = 17.625, b = 243.04 °C), plus vapour pressures and a comfort classification.
When to use
You know temperature and relative humidity and need the dew point, the humidity comfort level, or the temperature at which condensation forms on a surface.
Do not use when: You know the dew point and need relative humidity or absolute humidity (use relative-humidity), or the temperature is below −40 °C or above 50 °C where the Magnus fit is not validated.
Formula
γ = ln(RH/100) + a·T/(b + T); Td = b·γ/(a − γ) with a = 17.625, b = 243.04 °C, T in °C; es(T) = 6.1094·exp(a·T/(b + T)) hPa; e = RH/100 · es(T)
Magnus-type formula with the Alduchov & Eskridge (1996) coefficients, accurate to about ±0.35 °C for −40 to +50 °C over liquid water. The dew point does not depend on air pressure as long as the vapour content is unchanged.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
temperature | number | yes | Air (dry-bulb) temperature in temperature_unit; the fit is validated for −40 to +50 °C. Range: ≥ -60, ≤ 140 | |
temperature_unit | enum: c | f | default c | Unit of the temperature input(s). | |
relative_humidity_percent | number | % | yes | Relative humidity in percent (70 means 70 %). Range: ≥ 1, ≤ 100 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
dew_point_c | number | °C | Temperature at which the air would be saturated (100 % RH) at constant pressure. |
dew_point_f | number | °F | Dew point in Fahrenheit. |
dew_point_depression_c | number | °C | Air temperature minus dew point; 0 means saturated air (fog or dew likely). |
vapor_pressure_hpa | number | hPa | Partial pressure of water vapour, e = RH/100 × es(T). |
saturation_vapor_pressure_hpa | number | hPa | es(T) over liquid water from the Magnus formula. |
comfort | string | NWS comfort bands: below 55 °F dry, 55–65 °F sticky, 65 °F and above oppressive. |
Example
25 °C, 70 % RH: {"temperature":25,"temperature_unit":"c","relative_humidity_percent":70} → {"dew_point_c":19.1,"dew_point_f":66.5,"dew_point_depression_c":5.9,"vapor_pressure_hpa":22.13,"saturation_vapor_pressure_hpa":31.62,"comfort":"Oppressive: air heavy with moisture, uncomfortable for most people (65 °F / 18 °C and above)"}
86 °F, 40 % RH: {"temperature":86,"temperature_unit":"f","relative_humidity_percent":40} → {"dew_point_c":14.9,"dew_point_f":58.9,"comfort":"Sticky: noticeably humid, muggy evenings (55–65 °F / 13–18 °C)"}
GET https://tttkmbb.com/api/v1/calculate/dew-point?temperature=25&temperature_unit=c&relative_humidity_percent=70
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/dew-point(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/dew-point · Markdown: https://tttkmbb.com/weather/dew-point.md · JSON definition: https://tttkmbb.com/weather/dew-point.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="dew-point" - OpenAPI operationId:
calculate_dew_point - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
FAQ
Dew point or relative humidity: which describes mugginess?
Dew point. Relative humidity depends on temperature (90 % RH at 5 °C is dry air), whereas a dew point above about 18 °C (65 °F) feels oppressive at any temperature.
What if the dew point is below 0 °C?
Below 0 °C the frost point (saturation over ice) would differ slightly; this calculator always uses the over-water Magnus formula, as most meteorological reports do.
Related calculators
- Relative Humidity Calculator — Inverse problem: relative and absolute humidity from temperature and dew point.
- Heat Index Calculator — Apparent temperature from temperature and humidity.
- Air Density Calculator — Humidity also lowers air density.