Concrete psi to MPa conversion chart

A footing schedule reads f'c = 3000 psi, and the ready-mix plant you buy from prices every load in MPa.

Pick the class that sounds closest and the concrete can arrive under the number on the drawing. This post gives the conversion both ways, the classes plants actually sell, and the one rule that keeps a converted figure from turning into a failed cylinder.

psi and MPa measure the same test

Both units report compressive strength: the load a standard cylinder carries before it breaks, usually tested at 28 days. Pounds per square inch is the imperial reading, still common on drawings from American engineers and on sets drawn for US work. Megapascals are the SI unit, and they are what CSA A23.1 and Canadian ready-mix plants use.

The factor is fixed. One psi is 6,894.757 pascals (NIST lists it to that precision), so:

• 1 psi = 0.006894757 MPa
• 1 MPa = 145.0377 psi

For a sanity check in your head, "times 145" and "times 0.0069" are close. For a number you are going to order against, use the full factor, because the answers that matter sit close to the boundary between two classes.

The chart

Converting a psi call-out to MPa:

Converting an MPa call-out to psi:

Look at the right-hand columns. Not one converted figure lands on a class anybody sells. A typical Canadian plant's list runs 20, 25, 30, 32, 35 and 40 MPa; an American plant's runs 3,000, 3,500, 4,000, 4,500 and 5,000 psi. A spec written in one system almost never matches a class in the other, which is why the conversion is only half the job.

Order up to the next class, never the nearest

The converted figure will sit between two classes, and the instinct is to take whichever is closer. That instinct fails in one direction only: down.

Take 3,000 psi. It converts to 20.68 MPa, which is much closer to 20 MPa than to 25 MPa. But 20 MPa concrete is about 2,901 psi, under the 3,000 the drawing asks for. A cylinder from that pour can test below specification even when the plant delivered exactly what was ordered.

So the rule is: convert, then move up to the first class that meets or beats the specified strength. A 3,000 psi call-out is ordered as 25 MPa. Going the other way, 30 MPa converts to 4,351 psi, so a plant selling in psi supplies 4,500, not 4,000.

Moving up a class is a substitution, even when it is a stronger one. Put it in writing: a line on the bid that says the 3,000 psi specified is supplied as 25 MPa, and an RFI if the engineer has not already shown a metric equivalent. The schedule on the drawings governs, and the engineer of record decides what meets it.

Strength is one line of the spec, not the whole spec

Matching the MPa number is necessary and still not enough. CSA A23.1 ties concrete to an exposure class (C-1, C-2, F-1, F-2, N and others) that sets air content and the water-to-cementing-materials ratio separately from strength. Two pours can both be called 32 MPa and still need different mixes if one is an exterior slab exposed to freeze-thaw and de-icing salt and the other is an interior floor.

Converting psi to MPa tells you the strength class. It tells you nothing about exposure, which is a separate line on the schedule and a separate instruction to the plant. Read both before you price the load, and check which edition of CSA A23.1 the contract names, because the classes and their limits have changed between editions.

Worked example: a US-drawn footing, bought in Ontario

A tenant fit-out's structural sheet gives strip footings at f'c = 3000 psi, with no metric figure anywhere. The supplier's list: 20, 25, 30, 32 and 35 MPa.

1. Convert: 3,000 × 0.006894757 = 20.68 MPa.
2. Place it: above 20 MPa, below 25 MPa.
3. Order up: 25 MPa, which is about 3,626 psi, comfortably over 3,000.
4. Record it: "Specified 3,000 psi; supplied as 25 MPa (3,626 psi)" on the bid, and the same line in the RFI log if the engineer has not confirmed it.

Say the footings take 18 m³ (23.5 yd³). Price the 25 MPa mix, not the 20 MPa one, on all 18 m³. The step up costs something per cubic metre; the plant's list tells you how much. It is still cheaper than a low cylinder, a core test and a conversation about why the footing came in under the drawing.

A checklist for the next bid

• List every strength call-out on the structural sheets and note its unit: psi, MPa, or none stated.
• Convert each with the full factor, not "times 145".
• Order up to the first class that meets or beats it, never down to the nearest.
• Read the exposure class on the same line; it is a separate requirement.
• Write any substitution on the bid, and send an RFI when the drawings give no metric figure.
• Where one set mixes psi and MPa between elements, convert each element on its own.

Reading every strength off a long structural set by hand is the kind of flat, repetitive check that slips on the eighth footing. PreConstruct AI reads a concrete set's notes on request and lists each strength it finds, in psi or MPa, with its words and the sheet it came from, for you to check against the schedule. Your first bid is free, on a set of up to 75 sheets.