Dew Point Calculator
Compute the dew point from temperature & relative humidity with the Magnus formula (Alduchov & Eskridge 1996 constants), back-calculate RH from the dew point, compute the frost point below 0 °C, and read the fog-risk guidance from dew-point spread. 100% runs in your browser — no server, no tracking.
Temperature unit:
Mode 1 — Dew point from temperature & RH
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Arden-Buck (comparison reference)—
Saturation vapor pressure—
Actual vapor pressure—
Dew-point spread (T − Td)—
Mode 2 — RH from temperature & dew point
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Exact inverse of the Magnus formula — results are consistent back and forth with Mode 1.
Mode 3 — Frost point (below 0 °C)
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Dew point (over water, reference)—
Below 0 °C, condensation forms as ice — use the frost point, not the dew point. Computed from actual vapor pressure against ice saturation (Arden-Buck equation for ice).
Mode 4 — Dew-point spread & fog-risk guidance
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Dew point—
Spread (T − Td)—
Methodology & verified examples
Magnus formula (Alduchov & Eskridge, 1996 — Improved Magnus’ form approximation of saturation vapor pressure, J. Appl. Meteor. 35:601–609):
γ = ln(RH/100) + b·T/(c+T), Td = c·γ/(b−γ), with b = 17.625, c = 243.04.
γ = ln(RH/100) + b·T/(c+T), Td = c·γ/(b−γ), with b = 17.625, c = 243.04.
Inverse (RH from T & Td): RH = 100 · exp(b·Td/(c+Td)) / exp(b·T/(c+T)).
Arden-Buck (Buck 1996, reference): over water 6.1121·exp((18.678−T/234.5)·T/(257.14+T)); over ice 6.1115·exp((23.036−T/333.7)·T/(279.82+T)). Frost point solved from actual vapor pressure against ice saturation.
Verified examples (independent recompute, 2 methods):
- 25 °C, 50% RH → dew point 13.86 °C (Magnus); Arden-Buck 13.84 °C.
- 30 °C, 70% RH → dew point 23.93 °C.
- −10 °C, 70% RH → frost point −12.87 °C.
- 18 °C, 96% RH → spread 0.65 °C → high fog risk.
All formula temperatures use °C; °F/K conversion happens before and after computing. Supported range: −90…70 °C, RH 0 (exclusive)–100%.
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Frequently asked questions
What is the dew point?
The dew point is the temperature to which air must be cooled (at constant pressure) to become saturated with water vapor and begin condensation. At this temperature, relative humidity is exactly 100%. Example: 25 °C air at 50% RH gives a dew point of about 13.86 °C.
What formula does this calculator use?
This calculator uses the Magnus formula with the Alduchov and Eskridge (1996) constants: b = 17.625 and c = 243.04. For reference, each mode-1 result also shows the Arden-Buck equation (Buck 1996) version for comparison. Both are empirical approximations, not exact formulas.
When should I use the frost point instead of the dew point?
Use the frost point when air temperature is below 0 °C. Below freezing, the condensation surface is ice, and saturation over ice is reached at a different (slightly lower) temperature than over liquid water. This calculator computes the frost point from actual vapor pressure using the Arden-Buck equation for ice.
What is the dew-point spread and how does it relate to fog?
The dew-point spread is the difference between air temperature and dew point (T minus Td). The smaller the spread, the more saturated the air. Meteorological rule of thumb: a spread of 2.5 °C or less means high fog risk; 2.5 to 5 °C means watch for condensation; above 10 °C the risk is very low.
Is the fog-risk guidance here a weather forecast?
No. This guidance is purely a rule-of-thumb from the instantaneous spread value, not a forecast. Real fog is also influenced by wind, cloud cover, topography, and nighttime radiative cooling. Do not use it for aviation or maritime safety decisions.
How do I compute RH from temperature and dew point?
Mode 2 uses the exact inverse of the Magnus formula: RH = 100 times exp(b times Td divided by (c plus Td)) divided by exp(b times T divided by (c plus T)). Because this is the exact inverse, feeding the result back into mode 1 returns the original dew point.
Why must the dew point not exceed air temperature?
Because it is physically impossible: it would mean relative humidity above 100%, i.e. air holding more water vapor than its saturation capacity. The calculator rejects such input with an honest error message.
How accurate are the results?
The Magnus formula is accurate to a few tenths of a degree for the general meteorological range (about minus 40 to 50 °C). At extreme temperatures or very low RH, differences between coefficient sets can grow. For lab-grade precision measurements, use a calibrated chilled-mirror hygrometer.
How does this tool differ from mixed-air, psychrometric, and cloud-base?
The mixed-air tool #439 is a psychrometric air-mixing suite for HVAC: dew point there is just one property computed inside the mixing process, not the focus. The psychrometric tool #454 computes twelve properties at once from a single state point. The cloud-base tool #1562 uses dew point as an input for cloud-base height. This tool is a pure dew-point calculator for meteorology and everyday needs.
What units are supported?
Temperature in Celsius, Fahrenheit, and Kelvin; relative humidity always in percent (0 to 100). Results follow the selected temperature unit.
Free tool by NRTechStudio