The number is the yield strength
A rebar grade is the stress at which the steel stops springing back and starts to deform permanently — its yield strength. The number is that figure in thousands of pounds per square inch.
- Grade 40: 40,000 psi, about 280 MPa
- Grade 60: 60,000 psi, about 420 MPa
- Grade 75 and 80: Heavy structural and seismic work
Metric systems name the same property directly. A bar marked 500 has a yield strength of 500 MPa, which sits above Grade 60.
Yield is the design limit, not the breaking point. Steel carries more before it snaps, but a structure whose reinforcement has yielded has permanently sagged and cracked. Design stops at yield for that reason.
What a higher grade buys you
Grade 60 yields at one and a half times the stress of Grade 40. For the same force, you need roughly two thirds of the steel area — a smaller bar, or the same bar at wider spacing.
That is a genuine saving in weight, in delivery and in the labour of placing it. It is why Grade 60 has become the default for structural work in most of the world.
But it is a saving in *strength*, not in *stiffness*. Both grades are the same steel as far as deflection is concerned, because stiffness depends on the elastic modulus, which is essentially identical for all reinforcing steel. Halving the steel area because the grade is higher makes a beam that is strong enough and bendier than the one it replaced.
Why grade 40 still exists
- More ductile: It stretches further before it fails, which gives visible warning rather than a sudden fracture.
- Easier to bend: Lower-grade steel is more forgiving to work on site, particularly for links and stirrups bent by hand.
- Adequate for light work: Domestic slabs and small footings are rarely limited by steel strength.
Some seismic detailing deliberately calls for a grade with guaranteed ductility, because in an earthquake you want the structure to bend and absorb energy rather than shatter.
The substitution trap
This is the practical warning. If a drawing calls for Grade 60 and the yard sends Grade 40, the bars look identical. Same diameter, same ribs, same weight per metre. Nothing about handling them tells you anything is wrong.
But the element now has about two thirds of the strength it was designed for. Nobody will notice until it is loaded.
Substituting the other way — Grade 60 where Grade 40 was specified — is usually harmless structurally, but it is still a change, and on anything designed it should be agreed rather than assumed.
Reading the marks
Every bar is rolled with identification along its length: the mill, the bar size, the steel type, and the grade. Grade 60 is commonly marked with the number 60, or with a single continuous longitudinal line. Grade 40 often carries no grade mark at all — the absence is the mark.
The marking conventions vary by standard and by country, so the certificate that came with the delivery is the authority. If bundles have been mixed on site and nobody can tell which is which, the honest answer is to treat the whole lot as the lower grade or send it back. Guessing at the grade of steel going into a foundation is not a decision worth making to save a phone call.
What this means for ordering
Grade does not change weight per metre — a 12 mm bar weighs the same whatever its grade — so a weight-based estimate is unaffected. What grade changes is how many bars the design needs in the first place.
That is why the grade belongs on the order alongside the size, and why "reinforcing bar, 12 mm, 40 lengths" is an incomplete order.