Basement Wall Calculator
Check a concrete basement wall: enter its height, the soil against it and the floor resting on it, and see the vertical bars it needs and whether it's thick enough. Pro adds what the floor and slab must hold, the footing width and a PDF report.
The wall works with #4 at 18 in vertical bars, using 68% of what it can take (limited by the floor load with the bending).
The wall in section, to scale: the soil pressure on it, the floor and slab holding it, and its vertical bars near the inside face.
The wall
#4 at 18 in vertical bars 1.5 in from the inside face, where the soil bends the wall, and #4 horizontal bars at 12 in.
| Bending29.0 kip-in per ft (factored), 3′ 2″ above the slab | 66% |
| Shear1.67 kips per ft (factored) at the slab; the wall takes 2.99 | 56% |
| The floor load with the bending1.66 kips per ft of floor load (factored) on 88.5, with the bending | 68% |
The soil
At-rest pressure coefficient K0 = 1 − sin 30° = 0.500, the same as 60 pcf of fluid pressure. The largest moment, 1,509 lb-ft per ft without an earthquake, is 3′ 2″ above the slab.
The floor, slab and footing Pro
What the floor framing and the slab must hold to keep the wall in place, the footing width, and a PDF report of the whole wall with every calculation.
- In Seismic Design Category D, E or F, a wall holding back more than 6 ft of soil also needs the earthquake soil pressure (IBC 1803.5.12): enter SDS to add it.
- Backfill only after the floor and the slab are in place: until then the wall is a cantilever and can crack or tip.
- Drain the soil against the wall with a footing drain, so water can't build up behind it, and dampproof the outside face.
3,000 psi concrete and grade 60 bars to ACI 318-19, the wall spanning 8′ from the slab to the floor, with the soil and water factored by 1.6 and an earthquake by 1.0.
Basement wall bars by soil height
Vertical #4 to #7 bars for drained soil weighing 120 pcf with a 30° friction angle (at rest, the same as 60 pcf of fluid pressure), no surcharge, 500 lb/ft dead and 300 lb/ft live load from the floor, and 3,000 psi concrete.
| Soil height | 8 in wall, 8′ tall | 8 in wall, 9′ tall | 10 in wall, 9′ tall | 10 in wall, 10′ tall |
|---|---|---|---|---|
| 4′ | #4 at 18 in | #4 at 18 in | #4 at 16 in | #4 at 16 in |
| 5′ | #4 at 18 in | #4 at 18 in | #4 at 16 in | #4 at 16 in |
| 6′ | #4 at 18 in | #4 at 18 in | #4 at 16 in | #4 at 16 in |
| 7′ | #4 at 18 in | #4 at 18 in | #4 at 16 in | #4 at 16 in |
| 8′ | #4 at 16 in | #4 at 15 in | #4 at 16 in | #4 at 16 in |
| 9′ | – | #4 at 14 in | #4 at 16 in | #4 at 15 in |
| 10′ | – | – | – | #4 at 13 in |
Bars 1.5 in from the inside face. Check your own wall above; heavier soil, water or a surcharge takes more.
How the calculator checks a basement wall
A basement wall is held at the bottom by the slab and at the top by the floor framing, so it spans between them like a beam standing on end, with the soil pushing on its outside face. Per foot of wall:
- Soil pressure: at rest, K0 = 1 − sin φ, since a wall held at the top can't lean away from the soil the way a retaining wall does; or the at-rest fluid pressure from a soils report. Water below its level pushes with its full weight while the soil weighs 62.4 pcf less. A surcharge adds K0 q over the soil's height.
- Earthquakes: with SDS entered, the soil pushes harder by 3/8 kh γ H², kh = SDS / 2.5, acting at 0.6 of the soil's height (Seed and Whitman).
- Bending and shear: the largest moment between the slab and the floor, with the soil and water times 1.6 and the earthquake times 1.0 (ASCE 7), against the vertical bars near the inside face by the American Concrete Institute's ACI 318-19, with crack control and at least 0.0012 of the wall (ACI 11.6.1).
- The floor load: 1.2 times the dead load and 1.6 times the live load, against the simplified wall strength of ACI 11.5.3 for a wall pinned at the top and bottom, added to the bending.
- The floor, slab and footing (Pro): the reactions where the wall pushes on the floor and the slab, and the footing width for the gravity load on the allowable soil pressure.
Not checked: the floor's connection to the wall and the floor as a diaphragm, walls the slab doesn't brace (like a walkout), walls with large openings, and settlement.
Basement wall questions
Why not use the retaining wall pressure?
A retaining wall leans away from the soil a little, which lets the soil settle into its lower, active pressure. A basement wall can't, because the floor holds its top, so the soil stays at rest and pushes about half again as hard: 60 pcf of fluid pressure instead of 40 for typical sand.
Where do the bars go?
Near the inside face. The soil pushes the middle of the wall inward, so the inside face stretches and the outside face squeezes. Bars near the outside face do little for the bending.
When can I backfill?
After the slab is poured and the floor is framed and nailed to the sill. Before then the wall stands free, and backfill can crack it or push it over. If you must backfill sooner, brace the wall.
What about the building code's basement wall tables?
The International Residential Code (IRC) has prescriptive tables for plain and reinforced basement walls by height, soil and thickness. This calculator works the wall out directly, so it can take water, a surcharge, an earthquake or a soils report's pressure that the tables don't.