Megapascals to PSI

Checking a Design Strength Written in MPa Against psi

Structural paperwork rarely agrees on units. A European specification names its concrete C30/37, a consultant writes f'c in MPa, and the 1970s American drawing in the same project folder says 4,000 psi. Reinforcement is worse: one bar is stamped Grade 60 in a catalogue and Grade 420 on the mill certificate. Getting a strength figure into the unit the reviewer expects is the small step that keeps a submittal from bouncing back.

Conversion factor: 1 MPa = 145.037738 psi. A C30/37 mix carries a characteristic cylinder strength of 30 MPa, which comes out at 4,351 psi — the strength band an American drawing would call 4,000 psi concrete.

Where the Megapascal Figure Comes From

Concrete class designations

EN 206 writes a class as C(cylinder)/(cube): C25/30 means 25 MPa measured on a cylinder and 30 MPa on a cube of the same mix, both at 28 days.

Rebar yield grades

Metric reinforcement is named for its specified yield: Grade 420 is the metric twin of Grade 60, and Grade 520 stands beside Grade 75 in the same standard.

Structural steel yield points

EN sections are graded S275, S355 and S460 in MPa, while American wide-flange shapes are quoted as 50 ksi or 65 ksi — and one ksi is simply a thousand psi.

Cylinder break reports

A compression machine divides the failure load by the specimen area and prints the answer in MPa or psi, depending on where the laboratory bought its software.

From Break Report to Submittal Number

The same few moves cover a cube result, a bar grade and a mill certificate.

Put the megapascal value in the left field

Type 30, 34.5, 420 — whatever the class or certificate states. The psi figure builds itself as the digits land. A keyboard that produces a comma for the decimal mark is accepted just as well, and stray spaces make no difference to the result.

See which customary band it lands in

25 MPa arrives at 3,626 psi, comfortably inside what a US specifier calls a 3,500 psi mix; 34.47 MPa lands almost exactly on 5,000 psi. Knowing the neighbouring band tells you whether a proposed substitution is generous or marginal.

Lift the plain figure for the paperwork

Each field carries its own copy control, and what reaches the clipboard is the number alone — no unit, no separators — so it drops straight into a spreadsheet column or a bar schedule cell. Holding Ctrl + C with the cursor inside the field does the same job.

Turn it around for a legacy drawing

Old American sheets give everything in psi. The ↔ control flips the pair so psi feeds MPa; worked by hand the multiplier is 0.006894757, which turns 4,000 psi into 27.58 MPa.

A converted number is not a substitution: 30 MPa and 4,351 psi describe the same stress, but exchanging a C30/37 mix for a 4,000 psi mix also changes exposure class, cement content and durability limits.

Concrete Grades and Steel Yield Strengths Side by Side

Characteristic strengths that turn up on drawings and mill certificates, shown as the megapascal value that names the grade and the pound-per-square-inch equivalent.

Material	Grade / class	Strength (MPa)	Strength (psi)
Concrete, cylinder f_ck	C20/25	20 MPa	2,901 psi
Concrete, cylinder f_ck	C25/30	25 MPa	3,626 psi
Concrete, cylinder f_ck	C30/37	30 MPa	4,351 psi
Concrete, cylinder f_ck	C40/50	40 MPa	5,802 psi
Concrete, US f'_c	4,000 psi mix	27.58 MPa	4,000 psi
Concrete, US f'_c	5,000 psi mix	34.47 MPa	5,000 psi
Reinforcing bar	Grade 420 (Grade 60)	420 MPa	60,916 psi
Reinforcing bar	Grade 520 (Grade 75)	520 MPa	75,420 psi
Structural steel	ASTM A992, 50 ksi	345 MPa	50,038 psi
Structural steel	S355	355 MPa	51,488 psi
Prestressing strand	Grade 270	1,860 MPa	269,770 psi

Two patterns fall out of the table. Concrete lives in tens of MPa and thousands of psi, so an entire grade step is only a few hundred psi wide — which is why rounding a converted figure to the nearest hundred can silently move a mix from one class to another. Reinforcement and structural steel sit two orders of magnitude higher, and that is precisely why customary practice abandoned psi there and counts in ksi instead.

What Helps on the Structural Desk

Two live strength fields

Both boxes accept typing and both recalculate, so a whole grade ladder — 25, 30, 35, 40 — can be run through without clearing anything between entries.

Reverse for archive drawings

One press of ↔ turns the page into psi → MPa, the direction a refurbishment file needs when it quotes strengths in customary units only.

N/mm² and kg/cm² within reach

Searchable pickers on both sides carry all 26 pressure units, so the same page reads a legacy kg/cm² note or confirms that N/mm² is MPa under another name.

Figures ready for the checking file

Results run to eight decimal places with the thousands spaced for legibility, while the clipboard receives the bare value a calculation sheet expects.

Strength Grade Questions

C30/37 shows two numbers — which one belongs in the converter?

The first one, 30 MPa, whenever you want the design strength. It is the characteristic cylinder strength that Eurocode 2 calls f_ck and feeds into every capacity equation; 37 MPa is the matching cube figure, kept in the name because many laboratories still cast cubes. Converting 30 MPa gives 4,351 psi, while converting 37 MPa would give 5,366 psi and overstate the design value by roughly a quarter.

Why does one mix break higher as a cube than as a cylinder?

Specimen shape changes how the platens restrain the concrete. A 150 mm cube is squat, so friction at the loaded faces confines it and lifts the failure load; a 150 × 300 mm cylinder is slender enough that its mid-height escapes that confinement. As a working ratio, cylinder strength sits near 80 % of cube strength — which is exactly the 30-to-37 relationship printed in the class name.

Grade 60 rebar is 60,000 psi, so why is the metric bar stamped 420?

420 is a rounded soft conversion. Sixty thousand psi works out at 413.69 MPa, and the standards committees rounded up to a tidy 420 MPa rather than specify an awkward decimal. Read the other way, 420 MPa is 60,916 psi, so the metric designation is marginally the stronger of the two. In practice the bars are the same product with a different mark rolled onto them.

A seven-day cylinder came in under the specified MPa — is the pour rejected?

Not on that result by itself. Grades are defined at 28 days, and an ordinary Portland mix typically reaches only about two thirds of its 28-day strength after a week. A seven-day break of 20 MPa (2,901 psi) on a C30/37 pour is roughly where it should be. Early breaks exist to check the trend and to decide when formwork can be struck, not to accept or condemn the concrete.

An old US drawing calls for 4,000 psi concrete — which metric class should be specified?

Arithmetically 4,000 psi is 27.58 MPa, and no European class matches that exactly. C25/30 at 25 MPa falls short of it while C30/37 at 30 MPa clears it, so specifiers normally step up to C30/37 and note the change. Check the exposure class at the same time: durability requirements, rather than strength alone, often decide which grade goes into the replacement specification.