Start with the units on the source
A ksi is 1,000 pounds-force per square inch. An MPa is one million pascals, equivalent to N/mm². Both can express normal stress, shear stress or pressure; conversion changes the unit, not which physical quantity the value represents. A yield strength in ksi therefore remains a yield strength after conversion.
The NIST SI conversion tables give the psi-to-pascal and inch-to-metre factors. Using the defining relations for pound-force and inch gives the more precise factor below; the displayed decimal is rounded.
Inverse: ksi = MPa ÷ 6.894757293
Worked example: a reported strength of 50 ksi
Multiply 50 × 6.894757293 = 344.73786465 MPa. Report 344.738 MPa if six significant digits suit the task, or round further to match the source measurement. Extra conversion digits do not make the underlying material test more precise.
| Source | Converted value |
|---|---|
| 1 ksi | 6.895 MPa |
| 10 ksi | 68.948 MPa |
| 50 ksi | 344.738 MPa |
| 100 ksi | 689.476 MPa |
Why ksi√in is different
A stress-intensity factor has units of stress times the square root of length. Convert both terms: multiply the stress factor by √0.0254 because one inch is 0.0254 m. This is a dimensional conversion, derived from the same NIST unit relations.
≈ 1.098843494 MPa√m
50 ksi√in ≈ 54.9422 MPa√m
Using the ordinary stress factor alone would leave the square-root length unconverted. In the converter, choose “Pressure / stress” for ksi to MPa, or “Stress intensity / fracture toughness” for ksi√in to MPa√m. Keep K, ΔK and a measured fracture toughness distinct in your notes even though they can share units.
Common questions
Can I convert MPa into MPa√m?
No unit conversion alone can supply the missing length dependence. A fracture-mechanics model needs geometry and loading information. Likewise, energy release rate G or the J integral requires more than rescaling a stress-intensity number.
Is 1 MPa the same as 1 N/mm²?
Yes. A square millimetre is 10⁻⁶ m², so N/mm² equals 10⁶ N/m². This is useful when combining forces in newtons with dimensions in millimetres.
Does conversion validate a material property?
No. Preserve the material grade, temperature, test method and uncertainty separately. The converter performs unit arithmetic; it does not establish whether the source property applies to your design.
Related: keep E and I consistent in a beam calculation or browse all guides.