How this instrument works
Pressure in SI comes out in pascals, and one pascal — a newton distributed over an entire square metre — is minute; a sheet of office paper lying flat presses down with roughly that much. Engineering therefore reaches for the mega prefix, which brings a quiet gift: because a square millimetre is one millionth of a square metre, 1 MPa equals 1 N/mm² exactly. Steel yield strengths, concrete cylinder strengths and injection-moulding pressures all land on tidy two- and three-digit MPa figures for that reason, and anyone reading 355 MPa can picture 355 newtons carried by each square millimetre of cross-section.
Pounds per square inch carries a different pedigree, assembled from three separately frozen numbers. An avoirdupois pound has equalled 0.45359237 kg exactly since 1959; an inch has measured 25.4 mm exactly since that same treaty; and 9.80665 m/s² was adopted as conventional gravitational acceleration by CGPM back in 1901. Chain them together: one pound-force amounts to 4.4482216152605 newtons, bearing on 0.00064516 m² of area. Dividing gives 1 psi = 6894.75729316836… Pa, a decimal with no end to it, because 0.0254 m smuggles a factor of 127 into every square-inch area.
Invert that identity and this page's factor appears: 645.16 ÷ 4.4482216152605 = 145.0377377302092…, held here as 145.03773773. Both units remain in daily service, usually on opposite sides of an ocean. Hydraulic circuits rated 250 bar in Germany read 3625.94 psi on an American gauge; pneumatic fittings stamped 1.0 MPa are 145 psi components; hydrogen dispensers running 70 MPa deliver 10,153 psi; and common-rail diesel injectors at 250 MPa sit above 36,000 psi.
- Type your pressure into the Megapascals (MPa) field — it opens at 1, this page's reference value.
- Read your answer on the PSI (psi) line; it recomputes as you type, so nothing needs pressing.
- Going backwards from psi? Divide that figure by 145.03773773 and enter it as megapascals to confirm.
- Quoting bar instead? Divide bar by 10 first: 250 bar becomes 25 in the Megapascals (MPa) field.
- Confirm that both figures mean gauge, or both mean absolute, before matching any result to a datasheet.
Worked example — rating a 1.0 MPa air fitting
A parts catalogue out of Japan lists a push-in air fitting whose maximum working pressure is 1.0 MPa, while every gauge in your shop reads psi. Type 1 into Megapascals (MPa); PSI (psi) answers 145.03773773 — call it 145 psi, comfortably clear of the 100 psi your compressor delivers.
Keeping those digits shows how little rounding costs. Trimming to flat 145 psi discards 0.038 psi, about 0.026 per cent, well under what any dial gauge can resolve. Rounding up to 150 psi instead would hand you 3.4 per cent of headroom that no fitting on your bench actually has.
Questions
Is 145.03773773 exact, or rounded?
Rounded — though what it rounds is itself exact. Pound-force, inch and pascal all carry zero measurement uncertainty, so their ratio is one specific rational number: 645.16 ÷ 4.4482216152605, written out as 145.0377377302092…. That decimal never terminates, since an inch of 0.0254 m puts 127 as a factor into every square-inch area. Eleven significant figures survive here, far finer than deadweight testers or any transducer you can buy.
Should I enter gauge pressure or absolute pressure?
Either, provided both sides of your comparison agree. Multiplying by 145.03773773 rescales an interval of pressure, so gauge and absolute readings behave identically under it; what no multiplier can do is shift a zero point. Standard atmosphere sits at 0.101325 MPa, or 14.6959 psi, and an absolute figure exceeds its gauge twin by exactly that much. Steam tables and thermodynamic data usually quote absolute; pneumatic and hydraulic catalogues usually quote gauge, sometimes written MPaG to make it plain.
How do megapascals relate to bar and kgf/cm²?
1 MPa is exactly 10 bar, making bar arithmetic trivial: 250 bar becomes 25 MPa, or 3625.94 psi. Older metric-technical practice used kgf/cm², which sits close but not equal — 1 kgf/cm² is 0.0980665 MPa, or 14.2233 psi, roughly 2 per cent under one bar. Legacy Japanese and Korean drawings still carry kgf/cm², so any rating copied across as though it were bar reads about 2 per cent high.
Do imperial and US customary psi differ?
No — they are one and the same. Gallons famously split between those two systems, but pressure never did: both nations adopted the 1959 accord that fixed 0.45359237 kg per pound and 25.4 mm per inch, and both work from identical conventional gravity. So piping rated 145 psi in Sheffield is rated 145 psi in Pittsburgh, and 1 MPa converts the same way on either side of the Atlantic.
How do I turn megapascals into ksi?
Divide by 6.894757293, since one ksi means one thousand psi. American structural drawings lean on ksi: A36 plate at 36 ksi yield equals 248.2 MPa, while European S355 at 355 MPa comes out near 51.5 ksi. Working from this page, take your psi result and shift its decimal point three places left.
Why does that capital M in MPa matter so much?
Because mPa and MPa differ by a factor of one billion. Lower-case m means milli, so 1 mPa is one thousandth of one pascal, around 1.5 × 10⁻⁷ psi — pressure nobody in engineering ever quotes. Upper-case M means mega. SI prefix case is not decoration, and any datasheet typed as 'mpa' is ambiguous enough to be worth querying before somebody pressurises something.