Stress Converter

Is N/mm² the same as MPa?

Yes — 1 N/mm² equals exactly 1 MPa, which is 1,000,000 Pa. They look different because N/mm² spells the unit out in Newtons and millimetres, while MPa uses the SI mega- prefix on the Pascal, but the number in front is identical. UK structural engineering, the Eurocodes and British Standards all use N/mm² and MPa interchangeably, so a steel grade quoted as "355 N/mm²" and one quoted as "355 MPa" describe the same yield strength.

Stress itself is internal force per unit area, written σ = F / A, which is why it shares the Pascal with pressure. The difference is context: pressure is a fluid pushing equally in every direction, while stress describes the internal forces inside a solid — and those can be tensile (pulling apart), compressive (squeezing) or shear (sliding). For the fluid side of the same units, see the pressure converter, and for the raw forces behind the stress, the force converter.

How do you convert MPa to psi and ksi?

Multiply MPa by 145.038 to get psi, or by 0.145038 to get ksi (since 1 ksi = 1,000 psi). So 355 MPa — the yield strength of S355 structural steel — is about 51,489 psi or 51.49 ksi. You can verify this against the tool: type 355 into the N/mm² field and it returns 355 MPa, 51488.39908 psi, 51.48839908 ksi, 3550 bar and 3620.199 kgf/cm². This conversion comes up whenever a UK engineer works from N/mm² British Standard values into US design software or AISC and ASTM specifications, which are written in psi and ksi.

What is a ksi?

1 ksi is a kilopound per square inch — 1,000 psi, or 6.89476 MPa. Ksi keeps the numbers compact for the high stresses common in US structural and aerospace engineering, where a steel might be described as "50 ksi" rather than "50,000 psi". It appears in ASTM material grades and AISC steel design, so UK projects using American-sourced sections or software need to move between ksi and N/mm².

What are the yield strengths of common UK materials?

Grade S275 structural steel yields at about 275 N/mm² and S355 at 355 N/mm², the two most common hot-rolled grades in UK construction under BS EN 10025 and Eurocode 3. Grade 8.8 bolts have a yield of around 640 N/mm², and aluminium alloy 6061-T6 about 276 N/mm². Concrete is specified differently — by characteristic compressive strength such as C25/30 (25 N/mm² cylinder / 30 N/mm² cube) — because it is strong in compression but weak in tension, which is why it is reinforced with steel.

What is the difference between stress and strain?

Stress is the internal force per unit area (N/mm² or MPa), while strain is the resulting deformation expressed as a ratio — the change in length divided by the original length — so strain has no units. The two are linked by the material's stiffness, or Young's modulus, in the relationship stress = modulus × strain within the elastic range. Steel's Young's modulus is about 210 GPa, which is why this converter includes gigapascals: material stiffnesses run into the hundreds of GPa even though working stresses stay in the tens or hundreds of MPa.

Understanding which one a figure refers to avoids a common mistake: a "strength" value like 355 N/mm² is a stress limit, whereas an elongation figure like "20%" is a strain. The converter handles the stress side, letting you move a modulus in GPa, a yield strength in N/mm² and a US spec in ksi onto the same scale for comparison.