PSI to Pascals for SI Reporting and FEA Model Input

Bringing psi Test Data into an SI Simulation

A test report arrives with every pressure written in psi, and the model waiting for it runs in SI. Before that number can become a boundary condition — or a line in a manuscript a reviewer will accept — it has to be restated in pascals. The arithmetic is trivial; what matters is doing it once, cleanly, and carrying a defensible number of digits into everything downstream.

Conversion factor: 1 psi = 6 894.757 293 Pa exactly, because the pound-force and the inch are themselves defined exactly in SI. A rig running at 3 000 psi is therefore 20 684 272 Pa, which almost every SI write-up would print as 20.7 MPa.

Why the Restatement Is Not Optional

Solvers carry no units of their own

A finite-element code simply multiplies the numbers handed to it. Mix a psi load with a modulus in pascals and a mesh in millimetres and you get a converged, confident, wrong answer.

A millimetre model wants MPa

Build in millimetres and newtons and every stress comes back in N/mm². That is the megapascal, so an imperial figure has to land as MPa rather than as raw pascals.

Legacy data outlives its units

Decades of US drawings, qualification reports and vendor curves were recorded in psi. Reusing that evidence in a modern study means restating it, not merely repeating it.

Journals expect SI throughout

Instructions to authors normally ask for SI in the running text, the tables and the figure axes, with the customary value kept in parentheses when it is what the instrument actually showed.

From a Report Line to a Load Card

Getting a printed test value into a form a solver or a manuscript table can hold takes about four moves.

Enter the figure exactly as reported

Put the psi value in the left field — 14.7, 150, 3000, whatever the report prints — and the pascal figure appears while you type. A decimal comma is read just as happily as a point, and spaces inside the number are ignored.

Set the right-hand unit to the prefix you will publish

Each side has a searchable list of all twenty-six pressure units, so one entry yields kPa for a ventilation-scale load or MPa for a structural one. Choosing the prefix here beats shifting a decimal point by hand afterwards.

Take the bare number across to the model

The copy button on a field places the plain digits on the clipboard — no unit string, no thousands spacing — which is exactly what a load definition box, an input deck or a spreadsheet column will accept. Ctrl + C inside the field behaves the same way.

Turn it around to sanity-check a result

The swap arrows (↔) run the page as Pa → psi, the direction wanted when a peak stress of 3.4e8 Pa has to be weighed against an allowable printed in psi. Done by hand it is a division: pascals ÷ 6 894.757 293.

Do not inherit precision you never had: the factor is exact, the source value is not. A specification reading "3,000 psi" almost certainly carries two significant figures, so quoting 20 684 271.88 Pa dresses the measurement in six digits it never earned. Convert with the full factor, then round the answer back to the precision of the input.

Reported Test Pressures Restated in SI

A spread of pressures of the kind that turns up in US test documentation, each shown as the pascal value the conversion produces and as the rounded SI form you would actually publish or type into a model.

Reported quantity	As reported (psi)	Converted (Pa)	As published
Blast overpressure, glass-breakage threshold	1 psi	6 894.76 Pa	6.89 kPa
Standard sea-level atmosphere	14.6959 psi	101 325 Pa	101.325 kPa
Low-pressure proof test on an enclosure	15 psi	103 421.36 Pa	103 kPa
Autoclave cure pressure quoted in a materials paper	100 psi	689 475.73 Pa	0.689 MPa
Design pressure on a legacy vessel drawing	150 psi	1 034 213.59 Pa	1.03 MPa
Hydrostatic proof at 1.3 × design	195 psi	1 344 477.67 Pa	1.34 MPa
Component burst test	3 000 psi	20 684 271.88 Pa	20.7 MPa
Peak internal pressure on a fatigue rig	10 000 psi	68 947 572.93 Pa	68.9 MPa

Notice how fast the middle column stops being readable. That is the practical reason SI writing leans on prefixes: past a few hundred kilopascals nearly everyone moves to MPa, and it is the last column — not the exact one — that belongs in a table caption or a load field.

What Helps When a Whole Data Set Has to Move

Retyping is the whole workflow

Both boxes stay editable and recompute on every keystroke, so a column of reported values can be walked through one entry after another with nothing to clear and nothing to press.

N/mm² listed under its own name

The unit list carries N/mm² next to MPa, handy when the pre-processor labels its stress output the newton-based way instead of the pascal way.

Both directions on one page

After a swap the same page reads a solver output back into psi, so a reviewer's question about the original customary figure is settled without opening anything else.

Digits where the work needs them

Answers hold up to eight decimals, group thousands with a space for legibility, and move into exponent form once a value passes 1e10 or drops below 1e-6 — which happens the moment you ask for GPa.

Questions from the Modelling Bench

Which consistent unit system should I set up before entering a psi-derived load?

Pick one set and convert everything into it. Full SI means metres, kilograms, seconds, newtons and pascals: a steel modulus becomes 2.1e11 and density 7 850. The usual CAD-scale alternative is millimetres, tonnes, seconds and newtons, where stress is in MPa, that same modulus is written 210 000 and density becomes 7.85e-9. A 150 psi load enters the first system as 1 034 214 and the second as 1.034.

My post-processor labels stress N/mm² — does my pascal figure need rescaling?

One newton spread over a square millimetre is one newton over 1e-6 m², which is 1e6 Pa, so N/mm² and MPa are the same unit written two ways — no rescaling between those two labels. What does matter is the model's own length unit: a millimetre mesh genuinely returns MPa, so typing 1 034 214 into it because that was the pascal value inflates the load a millionfold.

I applied 100 psi to a face, so why is the reported stress far above 0.69 MPa?

They are different quantities. The applied pressure is a traction acting normal to a surface; the stress plotted afterwards is the internal state equilibrium produces, and geometry concentrates it. A thin-walled cylinder on its own converts internal pressure into a hoop stress of roughly pr/t, so a moderate radius-to-thickness ratio multiplies 0.689 MPa many times before any fillet or bolt hole is considered. Comparing a peak stress with the pressure that caused it is not a check on the model.

How many digits should survive when a rounded psi value is restated in pascals?

As many as the original carried, and no more. Convert with the exact factor so nothing is lost mid-calculation, then round at the end: a two-figure 3 000 psi specification becomes 21 MPa, while a calibrated reading of 3 002.4 psi justifies 20.701 MPa. Spare digits belong in intermediate steps only — rounding early is what actually causes drift by the end of a chain.

The journal wants SI — should the psi figures come out of the manuscript?

Usually not. The convention most author guidelines describe is SI first with the instrument's own reading in parentheses: "20.7 MPa (3,000 psi)". That keeps the result traceable to the rig while staying readable to every audience, and it stops a later reader back-converting your rounded SI value and recovering a psi figure the test never produced. Confirm the target journal's house style first — a few do ask for the customary unit to be dropped entirely.

PSI to Pascals