Station pressure to sea-level pressure
Enter what your barometer reads and how high it is. You get the sea-level pressure, which is the figure weather reports use.
Sea-level pressure
The other way: elevation from two pressures
BaroLog, the barometer app these tools come from, uses this same formula for its altitude estimate. Get it on Google Play.
The formula
Air pressure falls as you go up, because there is less air above you. This calculator uses the international barometric formula, with the same constants as BaroLog. In the app it gives altitude (h, in metres) from station pressure and sea-level pressure:
h = 44330 × (1 − (P_station / P_sea)^(1 / 5.255))
Solved for sea-level pressure, it becomes:
P_sea = P_station / (1 − h / 44330)^5.255
Worked example
A barometer at 250 m reads 985 hPa.
- 250 / 44330 = 0.00564, and 1 − 0.00564 = 0.99436.
- 0.99436 to the power 5.255 is 0.97072.
- 985 / 0.97072 = 1014.7 hPa, which is also 29.96 inHg.
Limits
- It assumes a standard atmosphere. The real air column may be warmer or colder. At low elevations the effect is small. At high elevations, or in very hot or cold weather, the result can differ from an official figure by several hPa or more.
- Elevation errors carry through. Near sea level, 8 m of error is about 1 hPa.
- Weather services have their own methods. They can give a different number from the same reading.
- Not for flying or navigation. Use the official altimeter setting from your briefing or from air traffic control.
How this differs from the app
This page needs you to supply the elevation. BaroLog doesn't ask for one. With "Show sea-level pressure" on, it compares your phone's readings with the sea-level forecast for your area over the last day, and adds that gap to the reading. It then uses the formula above to estimate your altitude. So the app's sea-level figure and this page's can differ slightly.
Frequently asked
What is the difference between station pressure and sea-level pressure? Station pressure is what a barometer measures where it is. Sea-level pressure is an adjusted figure: an estimate of what the pressure would be at that spot if it were at sea level. Weather maps and reports use it so that places at different heights can be compared.
Why doesn't my result match the weather report? A difference of a few hPa is normal. Weather services use the real temperature and their own methods, your elevation may be a little off, the barometer may have a small offset, and the report may come from a station some distance away.
Which floor should I count for elevation? Use the height of the barometer itself, not the ground outside. Near sea level, each 8 m or so of height changes the pressure by about 1 hPa, so a tenth floor makes a visible difference.
Can I use this result as an altimeter setting for flying? No. Use the official altimeter setting from your weather briefing or air traffic control. This page is for learning and general use, not for navigation.