How this calculator works
Gauge pressure is the difference between absolute pressure and ambient pressure. If the MPa value is absolute, subtract the ambient pressure before converting to psi. If it is already gauge, only the scale changes. Reverse mode adds ambient pressure when the requested MPa result is absolute. Negative gauge pressure is valid down to a perfect vacuum; a negative absolute result is rejected. The table uses the same reference and ambient pressure as your calculation.
Formula
Pa per psi = 0.45359237 x 9.80665 / 0.0254^2, an exact defining expression (about 6894.757293168 Pa). psig = (MPa absolute x 1000000 - ambient Pa) / (Pa per psi). MPa absolute = (psig x Pa per psi + ambient Pa) / 1000000. For MPa gauge, omit the ambient term.
Worked example
At the default ambient pressure of 101325 Pa, 0.101325 MPa absolute is 0 psig. A pressure of 1 MPa absolute is about 130.341789 psi gauge, while 1 MPa gauge is about 145.037738 psig. At an ambient pressure of 100 kPa, 0.2 MPa absolute is about 14.503774 psig.
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Frequently asked questions
Is psi the same as psig?
Psi names the unit; psig also specifies a gauge reference. Psia specifies an absolute reference. Always establish the reference before comparing readings.
Should I always subtract 14.7 psi?
Only approximately at one standard atmosphere. The local ambient pressure varies, and MPa that is already gauge needs no atmospheric subtraction.
Can the psig result be negative?
Yes. A pressure below ambient has a negative gauge reading. The lowest possible value is the negative of the ambient pressure expressed in psi.
Does the table use local weather automatically?
No. It uses the ambient value you enter, with one standard atmosphere as the starting value.