Dew Point Calculator Widget
Add a dew point calculator to a weather, HVAC or building page. Readers enter temperature and relative humidity to get the dew point, or temperature and dew point to get the humidity, along with the dew point depression and vapour pressure.
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Under the widget on your page: Powered by A2Z Tools
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<iframe src="https://a2z.tools/embed/w/dew-point-calculator" title="Dew Point Calculator by A2Z Tools" width="100%" height="600" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.
<div data-a2z-widget="dew-point-calculator" data-height="600"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
Works with
How it works
The calculation is the Magnus formula for saturation vapour pressure over liquid water with the coefficients Alduchov and Eskridge published in 1996 after comparing approximations against reference data: a = 17.625, b = 243.04 °C and 6.1094 hPa. The widget forms gamma = ln(RH/100) + aT / (b + T) and returns the dew point b x gamma / (a - gamma). The inverse mode solves the same relation for humidity: RH = 100 x exp(aTd / (b + Td) - aT / (b + T)). Fahrenheit inputs are converted to Celsius first and the answers converted back. Air temperature is limited to -40 to 50 °C, the range of the coefficient fit, a dew point above the air temperature is refused, and 0% humidity is refused because its logarithm is undefined. At 25 °C and 60% the dew point is 16.7 °C.
Calculation method
- gamma = ln(RH / 100) + a x T / (b + T), with a = 17.625 and b = 243.04 °C
- Dew point Td = b x gamma / (a - gamma)
- Inverse: RH = 100 x exp(a x Td / (b + Td) - a x T / (b + T))
- Vapour pressure e = 6.1094 x exp(a x Td / (b + Td)) hPa; saturation pressure uses T
- °F inputs: T(°C) = (T(°F) - 32) x 5 / 9; valid for -40 to 50 °C
Worked examples
Humid summer afternoon
Inputs: 30 °C, 70% relative humidity
Result: Dew point 23.9 °C; depression 6.1 °C; vapour pressure 29.66 hPa
A dew point above 20 °C is what makes the air feel sticky.
Humidity from a dew point
Inputs: Air 20 °C, dew point 10 °C
Result: Relative humidity 52.5%
Uses the inverse Magnus relation.
Fahrenheit reading
Inputs: 86 °F, 50% relative humidity
Result: Dew point 65.2 °F; depression 20.8 °F
Same maths after conversion to Celsius.
Limitations
- Valid for air temperatures from -40 to 50 °C (-40 to 122 °F), the range of the coefficient fit.
- Gives the dew point over liquid water, not the frost point over ice.
- Assumes sea-level-type atmospheric conditions; the small pressure enhancement factor is ignored.
Where publishers use it
- Local weather sites explaining why a 60% humidity day can feel muggy or comfortable
- Flooring, painting and coating guides that require the surface to stay above the dew point
- Basement and crawl-space damp articles on window and wall condensation
- HVAC technicians checking indoor humidity against coil or duct surface temperatures
- Greenhouse, warehouse and museum climate-control pages
Questions
What is the dew point at 25 °C and 60% humidity?
16.7 °C. Any surface colder than that - a cold-water pipe, an air-conditioned window - will collect condensation from that air.
Why use the Alduchov and Eskridge coefficients?
Their study compared Magnus-type fits against reference vapour-pressure data and recommended b = 243.04 °C and 6.1094 hPa with a = 17.625 (the technical-report version prints 17.62) as the most accurate form for -40 to 50 °C. Between those two values of a the dew point changes by at most 0.015 °C; older textbook constants such as 17.27 and 237.7 differ more.
How do I get relative humidity from a dew point?
Switch Find to relative humidity and enter both temperatures. Air at 20 °C with a 10 °C dew point is at 52.5% relative humidity.
What is dew point depression?
The gap between air temperature and dew point. A small gap means air close to saturation: at 30 °C and 70% the dew point is 23.9 °C, a depression of just 6.1 °C.
Does it work below freezing?
It gives the dew point over liquid water. Below 0 °C frost forms on surfaces at the frost point, which is slightly higher than the dew point, so treat sub-zero results as approximate for frost.
Painting rules say keep the surface 3 °C above the dew point. How do I check?
Enter the air temperature and humidity, read the dew point and add 3 °C. At 15 °C and 80% the dew point is 11.6 °C, so the steel or concrete must be at least 14.6 °C.
Sources
- Improved Magnus Form Approximation of Saturation Vapor Pressure, J. Appl. Meteor. 35(4), 601-609 (1996), doi:10.1175/1520-0450(1996)035<0601:IMFAOS>2.0.CO;2 - Alduchov, O. A. and Eskridge, R. E., American Meteorological Society . Original paper of the coefficients used (a = 17.625, b = 243.04 C, 6.1094 hPa; -40 to 50 C). The DOI resolves to the AMS journal page, which refused our automated fetch on 2026-10-01.
- Improved Magnus form approximation of saturation vapor pressure (technical report, OSTI ID 548871) - US Department of Energy, Office of Scientific and Technical Information . Report version of the same study: recommends e_s(t) = 6.1094 exp(17.62t / (243.04 + t)), maximum relative error under 0.384% from -40 to 50 C. It prints a = 17.62; the widget uses the 17.625 quoted for the journal version - the dew points differ by under 0.02 C.
Cite or recommend this tool
If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.
A2Z Tools Dew Point Calculator https://a2z.tools/embed/dew-point-calculator
<a href="https://a2z.tools/embed/dew-point-calculator">A2Z Tools Dew Point Calculator</a>
[A2Z Tools Dew Point Calculator](https://a2z.tools/embed/dew-point-calculator)
Dew Point Calculator by A2Z Tools - https://a2z.tools/embed/dew-point-calculator
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