Cloud base height calculator.

Worked example

Air at 25°C with a dew point of 15°C has a spread of 10°C. Spread ÷ 2.5 × 1000 = 4000 ft above ground. In metric the rule is spread ÷ 8 × 1000 = 1250 m. In Fahrenheit, 77°F with a 59°F dew point has an 18°F spread, and 18 ÷ 4.4 × 1000 = 4091 ft. The Celsius and Fahrenheit rules are rounded differently, so they differ by about 2%.

How it works

Find the spread between the surface air temperature and the dew point. Divide by 4.4 if in °F or 2.5 if in °C, then multiply by 1000 to get the cloud base in feet above ground level. In metres, divide the °C spread by 8 and multiply by 1000. Add the ground elevation to get height above sea level.

The rule applies to the base of cumulus-type clouds formed by rising surface air, in daytime conditions. It does not describe layer cloud, fog, or the heights an airport reports. It is an estimate.

Sources and basis (all opened 2026-10-08): Wikipedia, Cloud base, which describes this method as one published by the US Federal Aviation Administration. The FAA document itself was not opened for this page.

FAQ

How do you calculate cloud base height?

Subtract the dew point from the air temperature to get the spread, divide by 2.5 if in degrees C or 4.4 if in degrees F, and multiply by 1000 to get feet above ground. In metres, divide the Celsius spread by 8 and multiply by 1000.

How high is the cloud base at 25 degrees C with a 15 degrees C dew point?

About 4000 feet, or 1250 metres, above ground by the rule.

Does this work for any cloud?

No. It estimates the base of cumulus-type convective cloud. It does not apply to fog, layer cloud, or cloud that forms from lifting by fronts or terrain.

Can I use this for flight planning?

No. Use official aviation weather reports and forecasts. This is a rule of thumb for general interest.

Why do the Celsius and Fahrenheit results differ slightly?

The two published rules use rounded divisors, 2.5 and 4.4, which are not exact equivalents. They differ by about 2%.