Pascals to PSI

Ground Bearing Pressure, Written Both Ways

A geotechnical report speaks in pascals — usually gathered into kilopascals, because a footing that presses on the ground with 150 000 Pa is easier to talk about as 150 kPa. The structural drawing coming back from a US office may quote the same limit in pounds per square inch or, more often, pounds per square foot. Anyone reconciling a soil investigation with an imperial foundation schedule ends up making this conversion several times a day.

The factor: 1 Pa = 0.000145 037 74 psi. A 2 m × 2 m pad carrying 600 kN presses on the soil at 600 ÷ 4 = 150 kPa, or 150 000 Pa, and 150 000 × 0.000145 037 74 = 21.76 psi — the same bearing pressure, stated for an imperial reviewer.

Four Pressures a Foundation Note Keeps Apart

Contact pressure

What the footing actually delivers: the load divided by the plan area of its base. Eccentric loading makes it uneven, so a pad with a moment on it is checked at its most heavily pressed edge.

Ultimate bearing capacity

The pressure at which the ground shears and the footing punches in. It comes out of a bearing-capacity expression using shear strength, footing width, depth and shape factors.

Allowable bearing pressure

The number written on the drawing. It is the ultimate value divided by a factor of safety, typically between 2.5 and 3, and it is often reduced further so settlement stays inside tolerance.

Effective vs total stress

Pore water carries part of the vertical stress. Subtracting pore pressure from total stress leaves the effective stress that the soil skeleton feels, and it is the one that governs strength.

Carrying a Report Value Across to Imperial Practice

A site investigation is written once and read by several offices. These four steps cover the round trip between a metric report and an imperial foundation schedule.

Put the pascal figure in first

Reports quote kilopascals, so multiply by a thousand before typing: 250 kPa becomes 250000. The psi value builds up on the right as the digits go in, and a decimal comma is accepted just like a dot.

Compare against the borehole log

Check the converted figure against the stratum the footing actually sits in, not the surface material. A pad founded 1.5 m down in stiff clay is judged on that layer's presumed value, not on the topsoil above it.

Go the other way for a US drawing

When an imperial schedule states an allowable pressure in psi, the swap arrow (↔) reverses the page to psi → Pa. The reverse multiplier is 6 894.757, so 20 psi is 137 895 Pa, near enough 138 kPa.

Lift the number cleanly into the calculation sheet

Copy on either field puts the bare digits on the clipboard, without a unit tag or thousands spacing, so it drops straight into a settlement spreadsheet. The same happens with Ctrl + C from inside the field.

Presumed values are a starting point, not a design. Published bearing pressures assume a described ground condition and a modest footing width. A real foundation still needs a site investigation, and on soft or variable ground the settlement check usually decides the answer.

Presumed Bearing Values by Ground Description

Foundation codes publish indicative bearing pressures for described ground so that a preliminary sizing can be done before laboratory results arrive. The three columns hold the same quantity: the value as codes state it, the same value in pascals, and its imperial equivalent.

Ground description	Presumed bearing value	In pascals	In psi
Soft clay or silt	75 kPa	75 000 Pa	10.88 psi
Loose sand	100 kPa	100 000 Pa	14.50 psi
Firm clay	150 kPa	150 000 Pa	21.76 psi
Medium dense sand	250 kPa	250 000 Pa	36.26 psi
Stiff clay	300 kPa	300 000 Pa	43.51 psi
Dense sand	400 kPa	400 000 Pa	58.02 psi
Very stiff or hard clay	600 kPa	600 000 Pa	87.02 psi
Dense sand and gravel	800 kPa	800 000 Pa	116.03 psi
Weak mudstone or shale	2 000 kPa	2 000 000 Pa	290.08 psi
Strong limestone or sandstone	4 000 kPa	4 000 000 Pa	580.15 psi
Strong igneous or gneissic rock	10 000 kPa	10 000 000 Pa	1 450.38 psi

Two things stand out. The usable range for real soils spans little more than one order of magnitude, from about 11 psi on soft clay to 116 psi on dense gravel, which is why footing sizes change so quickly with ground quality. And sound rock is in another league altogether — over a hundred times a soft clay — which is why founding on rock removes the settlement argument almost entirely.

What Helps When Reconciling Two Unit Systems

Two live boxes, no submit

Enter the metric value or the imperial one — whichever the document in front of you carries — and the other side follows keystroke by keystroke while you read down a borehole log.

Pounds per square foot on tap

US foundation tables are usually written in psf rather than psi. Both dropdowns search across all 26 pressure units, so psf, kN/m² and kg/cm² are one keystroke away on either side.

One control for the return trip

Reviewing a US-issued schedule means going psi → Pa instead. The swap arrow turns the page around without retyping anything you already entered.

Wide numbers stay readable

Bearing pressures on rock run into the millions of pascals, and thousands are separated by a space so a seven-digit value can be checked at a glance before it is copied.

Foundation Pressure Questions

What separates ultimate bearing capacity from the allowable value on the drawing?

Ultimate capacity is the pressure at which the ground fails in shear and the footing punches downward — a calculated collapse value. The allowable pressure is that figure divided by a factor of safety, conventionally 2.5 to 3 for shallow foundations, and then trimmed again if the resulting settlement would be too large. A soil with an ultimate capacity of 450 kPa might therefore be given an allowable value of 150 kPa, which is 21.76 psi.

Why do soil reports use kPa while American foundation tables are printed in psf?

Both traditions picked the unit that keeps the numbers convenient. Bearing pressures land in the hundreds when written in kPa, and in the thousands when written in psf — 2 000 psf, for instance, is 95 760 Pa or about 95.8 kPa. Pounds per square inch make the same values awkwardly small, so US codes reserve psi mostly for material strengths and use psf for ground pressure. If a drawing gives psf, divide by 144 to reach psi before comparing it with anything on this page.

How do I work out the pressure a pad footing actually applies?

Take the total vertical load at founding level, including the weight of the footing and any soil sitting on it, and divide by the base area. A 600 kN column load on a 2 m × 2 m pad gives 600/4 = 150 kN/m², which is 150 kPa or 21.76 psi. When a moment also acts, pressure is no longer uniform: add the bending term so the edge value is checked, and confirm the resultant stays inside the middle third so no part of the base tries to lift.

Does shear failure or settlement usually decide the footing size?

On most sites settlement wins. Shear failure is a dramatic but rare limit, while total and differential movement upsets cladding, drainage and services long before the ground is anywhere near collapse. That is why an allowable value quoted for a wide raft is often lower than for a narrow strip on the same soil — a bigger footing stresses a deeper block of ground and settles more, even at the same contact pressure. On clay the movement also continues for years as consolidation runs its course.

What does a plate load test tell you, and what does it miss?

A rigid plate, typically 300 to 750 mm across, is jacked into the base of a trial pit in increments while settlement is recorded, producing a pressure–settlement curve for the ground immediately beneath it. What it misses is depth: a 300 mm plate only stresses a shallow bulb of soil, so a soft layer sitting a couple of metres down is invisible to it even though a real footing several metres wide would feel that layer strongly. Results are read alongside borehole logs rather than instead of them.