Where 0.433 comes from
Every plumber and every engineer quotes 0.433 psi per foot of water, and it is worth deriving once so it stops being a magic number. Fresh water weighs about 62.4 pounds per cubic foot. Stand a one foot cube of it on a square foot of floor and that square foot carries 62.4 pounds. A square foot is 144 square inches, so 62.4 ÷ 144 = 0.4333 pounds on each square inch. That is the whole derivation, and it is why the field is on the form rather than hard coded: a different fluid, a different temperature or a geotechnical report with a different figure changes every number below it.
The second thing water does is push sideways as hard as it pushes down. That is what makes it different from soil, which arches and has friction and pushes with only a fraction of its weight. So a wall standing in water gets the full 62.4 pounds per foot of depth as a horizontal pressure, growing linearly from zero at the water surface to its maximum at the base.
| Line | Default wall | Arithmetic |
|---|---|---|
| Wall below grade | 8.00 ft | |
| Water table depth | 3.00 ft | |
| Head on the wall | 5.00 ft | 8 − 3 |
| Pressure at the base | 312 psf | 62.4 × 5 |
| Same figure in psi | 2.167 psi | 312 ÷ 144 |
| Resultant per linear foot | 780 lb | ½ × 62.4 × 5² |
| Acting above the base | 1.67 ft | 5 ÷ 3 |
| Moment per foot | 1,300 lb-ft | 780 × 1.67 |
| Over 10 ft of wall | 7,800 lb | 3.9 tons |
| Uplift under a 1,200 sq ft slab | 374,400 lb | 312 psf × 1,200 |
The last two rows are the ones that make people sit up. Seven thousand eight hundred pounds against ten feet of wall is a lot. A hundred and eighty seven tons trying to lift a basement floor is a different order of thing entirely, and it is acting all at once over the whole underside rather than at one point. That is the load that floats empty swimming pools and septic tanks out of the ground.
Why the pressure grows as the square
The pressure at any depth is proportional to the depth, so the total force is the area of a triangle: half the base pressure times the height, which works out to half of the unit weight times the head squared. Doubling the head does not double the force, it quadruples it. Five feet of head gives 780 pounds per foot of wall; ten feet gives 3,120. That non-linearity is the reason a basement that has coped for twenty years can fail in the one season the water table comes up an extra three feet.
The resultant also acts a third of the way up from the base rather than half way, because the pressure diagram is a triangle and its centroid is at one third. That is what turns the force into a specific overturning moment on a specific wall, and it is one of several reasons this page reports numbers and stops there.
What this page is not
It is not a design load and it is not a check on anything. Water pressure is one component; saturated soil pressure is another and it has its own coefficient depending on soil type, wall restraint and how much the wall can move; surcharge from a driveway, a vehicle or a slope above is another again. Combining them correctly is engineering work that starts from a geotechnical investigation of your site, and no arithmetic on a web page substitutes for it. Nothing here says whether any wall is adequate, and nothing here should be read as saying so.
What the page is good for is scale. If you are deciding whether a damp basement is a drainage job or an engineering job, the head is the number that tells you which conversation to have. On the material side, the below-grade membrane take-off prices the barrier, a drain and grading address the water that arrives from above, and the water entry organizer helps sort out which of those you actually have.
Questions people ask
How much pressure does groundwater put on a basement wall?
62.4 pounds per square foot for every foot of head, or 0.433 psi per foot. With the water table 3 feet down and the wall 8 feet deep, the head is 5 feet and the pressure at the base is 312 pounds per square foot, which is 2.167 psi. The total push is 780 pounds on every linear foot of wall, acting 1.67 feet above the base. Those are the water figures only — saturated soil adds its own pressure on top.
Where does 0.433 psi per foot come from?
From the density of water and the number of square inches in a square foot. Water weighs about 62.4 pounds per cubic foot, and a cubic foot sits on 144 square inches, so 62.4 divided by 144 is 0.4333 pounds on each square inch per foot of depth. Because a fluid pushes equally in all directions, that same figure applies sideways against a wall as well as downward on a floor.
Why does doubling the water depth more than double the force?
Because the pressure grows with depth and the force is the area under that pressure diagram. The diagram is a triangle, so the force is half the unit weight times the head squared. Five feet of head gives 780 pounds per foot of wall and ten feet gives 3,120 — four times as much, not twice. It is also why a wall that has been fine for two decades can fail in the season the water table comes up a few feet.
How much uplift does water put under a basement slab?
The same 62.4 pounds per square foot per foot of head, applied upward over the entire underside. On the defaults here, a slab 8 feet below grade with the water table 3 feet down has 5 feet of head under it, which is 312 pounds per square foot, and over 1,200 square feet that is 374,400 pounds or about 187 tons. Whether a structure resists that is a design question and this page does not answer it, which is why the resist field only ever produces a subtraction.
Can I use this to decide whether my basement wall is strong enough?
No, and it is not built to. It gives water pressure only, and a design load also has to include saturated soil pressure with the right coefficient, any surcharge above the wall, and the way the wall is restrained top and bottom. Assessing a wall against that load needs its thickness, reinforcement, span and condition. That is structural engineering work informed by a geotechnical investigation, and the honest role of this page is to tell you the size of the number so you know to make the call.