How this calculator works
The standard atmosphere is a fixed reference pressure of exactly 101,325 Pa, roughly the average pressure at sea level. From the exact pound-force and inch definitions, one psi is 4.4482216152605 / 0.0254 squared Pa, approximately 6,894.757293168 Pa, so one atmosphere is 101,325 / 6,894.757293168 = 14.6959487755 psi. Dividing a psi value by that number gives atmospheres. The atmosphere is still common in chemistry, diving and aviation, while psi remains the everyday unit for tyres and compressors in North America.
Formula
Exact factor: atm = psi x (4.4482216152605 / 0.0254^2) / 101325. Approximately, atm = psi / 14.6959487755. psi = atm x 14.6959487755. Equivalently, atm = psi x 6894.757293168 / 101325. The standard atmosphere is exactly 101,325 Pa.
Worked example
14.6959 psi = 0.999997 atm, essentially one atmosphere. 30 psi = 2.04138 atm. 100 psi = 6.80460 atm. 1 atm = 14.6959 psi. 1 psi = 0.0680460 atm. A scuba cylinder filled to 3,000 psi holds about 204.1 atm.
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Frequently asked questions
Why is one atmosphere about 14.7 psi?
One atmosphere is 101,325 Pa and one psi is 6,894.757293168 Pa. The quotient is 14.6959487755 psi, usually rounded to 14.7 psi in tables and tyre discussions.
Is this conversion for absolute or gauge pressure?
Atmospheres here are absolute. A tyre gauge reading of 30 psi is about 2.04 atm above local air, which is about 3.04 atm absolute. Add the local atmospheric pressure to a gauge reading before converting to absolute atmospheres. One standard atmosphere is an approximation to local air pressure.
Why are there both torr and atm on this page?
The torr is defined as exactly 1/760 of a standard atmosphere, so one atmosphere is exactly 760 torr. The millimetre of mercury is a separate conventional unit that is very slightly different, which is why both appear in pressure work.
How accurate is the table?
The atmosphere definition is exact and the psi factor retains about 12 significant digits; the display rounds further. Values near one atmosphere, such as 14.6959 psi, show 0.999997 rather than a hard 1 because 14.6959 psi is marginally below the exact 14.6959487755 psi definition.