Stress is force per unit area inside a material, measured in the same units as pressure. Engineers in the UK and Europe use megapascals, MPa, or newtons per square millimetre, N/mm², which are exactly the same: 1 MPa equals 1 N/mm². In the United States, stress is more often given in pounds per square inch, psi, or thousands of psi, ksi. One MPa is about 145 psi, and one ksi is about 6.895 MPa.
What UK material grades mean
Many UK material grades are stress values in disguise. Structural steel grade S355 has a minimum yield strength of 355 MPa, the stress at which it starts to deform permanently. Concrete is classed by compressive strength: C30/37 means a characteristic strength of 30 MPa measured on cylinders and 37 MPa on cubes, the two test shapes used in Europe. Structural timber is graded by bending strength, so C24 timber has a characteristic bending strength of 24 MPa. Knowing this makes product names much easier to compare with American specifications.
Precision and pitfalls
The main pitfall is treating a material's strength as the stress it can safely carry. Design codes apply safety factors, and a beam is never designed to reach its characteristic strength in normal use. A second pitfall is mixing up stress and pressure. They share units, but pressure acts on a surface, while stress acts within a material, and the stress inside a component can be many times the pressure applied to it.
Worked conversions
S355 steel's yield strength of 355 MPa is about 51.5 ksi, which lines up with the American A572 Grade 50 steel at 50 ksi. A C30/37 concrete's cylinder strength of 30 MPa is about 4,350 psi, close to the 4,000 psi concrete common in US specifications. And C24 timber's 24 MPa bending strength is about 3,480 psi. Engineering calculations should always use the values and safety factors from the relevant design code; these conversions are for understanding and comparison.
Stress in everyday products
Stress ratings appear outside construction too. Climbing equipment, lifting slings and ratchet straps are rated by load rather than stress, but the materials inside them are chosen by stress. Bolts are graded by strength class: a common metric bolt of property class 8.8 has a nominal tensile strength of about 800 MPa, with a yield strength around 640 MPa. That's why replacing a high-strength bolt with an unmarked one from a general hardware pack can be dangerous, even when the size matches.
Checking your answer
Typical values give a sense of scale. Structural steel yields at about 250 to 460 MPa. Ordinary concrete is designed around 20 to 50 MPa in compression, but has only about a tenth of that strength in tension. Structural timber's characteristic bending strength is around 14 to 30 MPa for common grades. Rubber and soft plastics fail at a few MPa. If a converted figure puts concrete stronger than steel, or timber in the thousands of MPa, check whether psi and MPa were confused.
One more worked example
A steel tie rod with a cross-section of 300 square millimetres carries a tension of 60 kilonewtons. The stress is the force divided by the area: 60,000 newtons divided by 300 square millimetres is 200 N/mm², or 200 MPa, about 29 ksi. That's comfortably below the 355 MPa yield strength of S355 steel, but a designer would also apply partial safety factors from the design code before confirming the rod is adequate.
Pressure ratings on pipes and fittings
Plumbing fittings are often rated in bar, which is a pressure, but the material inside them is chosen for the stress that pressure creates in the pipe wall. Thinner walls and larger diameters raise that stress, which is why large pipes need thicker walls to carry the same pressure.