Stress and Modulus Unit Converter: psi, ksi, MPa, GPa

Stress and elastic modulus values (things like Young’s modulus, unconfined compressive strength, or yield strength) are reported in psi and ksi in most US customary rock and steel mechanics testing, and in MPa or GPa almost everywhere else, including international material data sheets and most published rock mechanics literature. This converter switches instantly between all four units.

Conversion factors

Every unit converts through pascals as a common base. The key relationships are:

• 1 psi = 6,894.757 Pa
• 1 ksi = 1,000 psi = 6,894,757 Pa
• 1 MPa = 1,000,000 Pa
• 1 GPa = 1,000 MPa = 1,000,000,000 Pa

Worked example

Consider the Young’s modulus of steel, approximately 30,000,000 psi, a standard reference value used to sanity check elastic modulus calculations.

1. Value in pascals = 30,000,000 times 6,894.757 = 206,842,710,000 Pa
2. ksi = 30,000,000 / 1,000 = 30,000 ksi
3. MPa = 206,842,710,000 / 1,000,000 = 206,842.71 MPa
4. GPa = 206,842,710,000 / 1,000,000,000 = 206.843 GPa

These are the exact numbers the calculator returns when you click Load Example, matching the commonly cited value of roughly 200 to 210 GPa for structural steel.

Where each unit is used

Psi is the default unit on most US rock mechanics test reports and casing and tubing strength ratings. Ksi is a compact form of psi used when values run into the millions, common in steel and alloy specification sheets. MPa is the international standard for rock strength, concrete strength, and most published geomechanics data. GPa is used almost exclusively for elastic moduli, since Young’s modulus and shear modulus values are large enough that GPa keeps the numbers in a convenient range.

Common mistakes

• Confusing psi and ksi, an easy factor of 1,000 error that is especially common when reading older US steel and drill pipe specification tables
• Mixing up MPa and GPa when comparing an elastic modulus against a strength value, since moduli are usually in GPa while rock strength is usually in MPa
• Rounding too early when converting a high precision equipment rating, which can shift a marginal pass or fail decision on a strength calculation

Keep in mind that ksi shows up almost exclusively in American steel and casing specifications, while the rest of the world reports the same properties in MPa or GPa. If you are comparing a casing grade’s yield strength against a rock mechanics modulus from a different report, make sure you are not accidentally comparing psi scale numbers to giga scale numbers. It is an easy mistake since both use similar looking abbreviations, and it can throw a calculation off by a factor of a thousand.

Related Calculator and Reference

See also the Bearing Pressure Converter for the geotechnical side, where bearing pressure is quoted in tsf, ksf and kg per square centimetre.

For converting between elastic moduli themselves once you have consistent units, see the Elastic Moduli Interconversion Calculator. For the official SI definition of the pascal, see the NIST SI Units: Pressure reference.