How this instrument works
One kip is a thousand pounds-force, so a kilopound per square inch is that load spread across one square inch. This unit exists because American strength tables would otherwise run to walls of five-digit numbers: A36 mild steel yields at 36 ksi rather than 36,000 psi, and structural wide-flange to ASTM A992 is specified 50 ksi minimum yield, 65 ksi tensile. Its metric counterpart is friendlier still — one megapascal is exactly one newton per square millimetre, which is why European detailers read 345 MPa straight off a drawing as newtons carried by every square millimetre of steel.
Bridging both is pure definition, settled long before anyone needed it here. 1959's international yard and pound agreement fixed one avoirdupois pound at 0.45359237 kg and one inch at 0.0254 m, both exactly; standard gravity had been pinned at 9.80665 m/s² by delegates to CGPM back in 1901. Multiply pound by gravity: one pound-force is 4.4482216152605 N precisely. Divide that by 0.00064516 m², one square inch, and 1 psi is 6894.75729316836… pascals. Those digits run on forever, though not from uncertainty — an inch is 127/5000 of a metre, so squaring it buries 127² downstairs, and no such decimal can ever close.
Which unit you meet depends on who drew your part. AISC's Steel Construction Manual, SAE bolt grades and API line-pipe designations all speak in ksi — grade X65 pipe is simply 65 ksi of specified minimum yield. Eurocodes, ISO and JIS speak in megapascals, where that same pipe appears as L450. Aerospace splits identically: American mills certify titanium bar at 128 ksi, and an equivalent bar leaves its European supplier stamped 880 MPa. Converting between them is arithmetic rather than measurement, so your only judgement is how far to round.
- Enter your figure in the KSI (ksi) field. It opens at 5 ksi, and negative entries are refused — strength below zero has no meaning.
- Read your answer on the Megapascals (MPa) line below. It recomputes on every keystroke, with no intermediate rounding anywhere in that chain.
- Going backwards, divide by this same constant: one megapascal is 0.14503773773 ksi, that relationship read in reverse.
- Check yourself mentally first — multiplying by 7 lands within 1.5% of your answer, enough to catch a psi-for-ksi slip.
Worked example — 5 ksi crossing to Rotterdam
An American fabricator rates one fatigue detail to a stress range of 5 ksi, while his Dutch checker reviewing that connection works only in megapascals. Type 5 into the KSI (ksi) field and this sheet returns 34.4737864658 MPa.
On his calculation sheet that becomes 34.5 MPa, and nobody would argue. Carry full precision only where it feeds something else: across 12,000 mm² of weld throat, 34.4737864658 N/mm² gives 413.69 kN, while tidy 34.5 reports 414.00 kN — drift of roughly 315 newtons, invisible in one fatigue check, worth noticing in a load path you are summing forty times.
Questions
Is one ksi exactly 6.89475729317 megapascals?
Not quite — that is an exact value, rounded. Definitions behind it carry no uncertainty whatever: one pound-force is 0.45359237 kg × 9.80665 m/s² = 4.4482216152605 N precisely, and one square inch is 0.00064516 m² precisely. Dividing gives 6894.75729316836… Pa per psi, a fraction whose decimal never terminates, because an inch is 127/5000 of a metre. Rounding at twelve significant figures, as this sheet does, costs roughly two parts in ten trillion — some ten orders of magnitude finer than any testing machine can resolve.
Does imperial ksi differ from its US customary namesake?
No. Gallons and tons diverge across Atlantic waters; pound and inch do not. That 1959 agreement was signed by standards bodies in America, Britain, Canada, Australia, New Zealand and South Africa together, so one ksi means exactly what it says on a Sheffield mill certificate and on a Pittsburgh one alike. Your genuine impostor is technical atmosphere, kgf/cm², still printed on older continental and Japanese gauges: 0.0980665 MPa, close enough to one bar to be mistaken for it, and unrelated to this factor.
Why is 50 ksi steel labelled 345 MPa and not 344.7?
Because grade names are chosen, not calculated. Fifty ksi works out at 344.7379 MPa, and ASTM rounds metric designations of A992 and A572 to an even 345. Reinforcing bar goes further: Grade 60 is 413.69 MPa, yet its metric standard names it Grade 420 — deliberate respecification rather than rounding. That asymmetry repays remembering, because back-converting any designation misleads. Divide 420 MPa by 6.894757 and you get 60.9 ksi, which is not a grade anybody rolls.
Can I use this factor for fracture toughness in ksi√in?
No, and getting that wrong is expensive. Stress intensity carries a square root of length as well as a stress, so its conversion runs 1.0988 MPa√m per ksi√in: 6.894757 handles stress, √0.0254 handles what remains. Toughness of 100 ksi√in is about 110 MPa√m, not 689. Elastic modulus stays safe, being an ordinary stress — 29,000 ksi of steel stiffness converts to 199.95 GPa, which everybody then writes as 200.
How large is one kilopound per square inch in everyday terms?
Large enough that you rarely meet it outside a materials table. A car tyre at 32 psi holds 0.032 ksi, or 0.22 MPa; sea-level atmospheric pressure sits at 0.0147 ksi, that familiar 101.325 kPa. This unit earns its living further up: waterjet cutters run at 60 ksi (414 MPa), A36 mild steel yields at 36 ksi (248 MPa), and Grade 8 bolts are proof-loaded near 120 ksi (827 MPa).
Is this converted figure gauge or absolute pressure?
Neither, because scaling a number cannot shift a datum. Material strengths are stress differences, so that question never arises for them. If you are converting fluid pressure quoted as ksig, convert its magnitude here and keep your label: the answer is megapascals gauge. To reach absolute, add one atmosphere afterwards — 0.101325 MPa, or 14.696 psi — never before, or this factor will scale that offset too.