How this instrument works
One psi is one pound-force bearing on one square inch. Three exact definitions fix its size: an avoirdupois pound of 0.45359237 kg, agreed internationally in 1959; standard gravity of 9.80665 m/s², adopted by the General Conference on Weights and Measures in 1901; and an inch of 25.4 mm. Multiply those through and 1 psi resolves to 6894.757293 pascals, a figure carrying no experimental uncertainty at all, because every input feeding it is a definition rather than a reading.
Kip is American structural shorthand, contracting kilo with pound, current in engineering offices for roughly a century, and denoting 1000 pounds-force. Stack a kip onto a square inch and you have 1 ksi. Steel work lives in these numbers: A36 plate yields at 36 ksi, A992 wide-flange shapes at 50, Grade 60 rebar at 60, F3125 Grade A490 bolts reach 150 in tension, and steel's elastic modulus sits near 29,000. Spelling all of that in psi would put five- and eight-digit strings on every schedule, which is precisely why the shorthand stuck.
Fluid pressure keeps psi — tyre placards, hydraulic circuits running at 3000 psi, boiler ratings — while stress inside a solid takes ksi. Both units read identically in imperial and US customary practice, since 1959 harmonised pound and inch alike, though geography still matters: British and European design runs on N/mm², which is simply a megapascal, so ksi turns up mainly across AISC, ACI, and AASHTO documents. For crossing over, 1 ksi is 6.894757 MPa.
- Type your figure into the KSI (ksi) field — it opens at 5 ksi, matching common high-strength concrete specifications.
- Read PSI (psi) directly beneath it; results recompute on every keystroke.
- Decimals resolve cleanly, because that field steps in thousandths — 0.075 ksi returns 75 psi.
- Working backwards from psi, divide by 1000 and enter that quotient: 65,000 psi becomes 65 in KSI (ksi), which should hand back your original figure.
Worked example — 5 ksi on a column schedule
A column schedule calls for f'c = 5 ksi in cast-in-place cores, while your ready-mix supplier quotes and batches everything in psi. Enter 5 into KSI (ksi) and PSI (psi) returns 5000.0 — a 5000 psi mix, which any plant will recognise on delivery tickets.
Nothing rounds along that path, so an identical shift scales in either direction. A992 framing rated 50 ksi yields at 50,000 psi, an A490 bolt rated 150 holds 150,000 psi, and a bearing allowable of 0.4 becomes 400 psi. Only a decimal point moves.
Questions
Is 1 ksi exactly 1000 psi?
Yes, exactly — by definition rather than by measurement. That leading k stands for kip, a contraction of kilo and pound meaning 1000 pounds-force precisely. Since both units divide force by an identical square inch, their ratio reduces to a pure decimal 1000, with none of a kilobyte's binary ambiguity and no experimental uncertainty whatsoever. Converting at any precision you like therefore shifts your decimal point three places.
What is a kip, and why not simply write kilopound?
A kip is 1000 pounds-force. American structural practice coined that contraction early last century and kept it because building loads then land at readable magnitudes: a 40-kip beam reaction beats 40,000 lbf in hand calculations, and stresses in tens rather than tens of thousands are easier to sanity-check. Kip also sidesteps confusion with kilogram-force, an entirely different quantity still met in older European literature.
Why do structural drawings mix ksi and psi on one sheet?
Convention splits by material. Steel strengths are quoted in ksi — 36, 50, 60, 120, 150 — because those integers are memorable. Concrete compressive strength has traditionally been written in psi, so mixes are ordered as 3000, 4000, or 5000 psi, even though ACI design equations increasingly carry f'c in ksi. Fluid and gas pressures stay in psi always. One sheet can legitimately carry all three habits.
Does psi here mean gauge or absolute pressure?
Neither distinction applies. Gauge (psig) versus absolute (psia) matters for fluid pressure measured against atmosphere, whereas ksi describes stress within a solid — force spread over a cross-section — which has no atmospheric datum and no such thing as ksig. If instead you are scaling a hydraulic reading of 3000 psig, keep that g attached: the arithmetic is identical, but the reference point travels with your number.
Is a psi the same in imperial and US customary units?
Yes. Since 1959, an inch has been 25.4 mm and an avoirdupois pound 0.45359237 kg on both sides of the Atlantic, and standard gravity was pinned at 9.80665 m/s² back in 1901, so 1 psi is 6894.757293 Pa under either system, making 1 ksi roughly 6.894757 MPa. British engineers rarely use either symbol day to day, preferring N/mm², so ksi is in practice North American notation rather than British.
What goes wrong most often when converting ksi to psi?
Losing or gaining a factor of 1000 — reading 50 ksi as 50 psi, or recording steel yield as 50,000 ksi. Magnitude gives you a free check: structural steels yield between roughly 36 and 65 ksi, high-strength bolts sit in low hundreds, and steel's elastic modulus is 29,000, sometimes printed instead as 29 Mpsi. Anything beneath 1 ksi for a metal's strength, or past 1000 for a working stress, almost always signals a slipped decimal somewhere upstream.