Poured Concrete Wall Calculator
A {length} ft wall, {height} ft tall and {thickness} in thick, needs about {bags} {bag}-lb bags of concrete — {cubicYards} cubic yards if you order ready-mix.
Concrete needed for common wall sizes
| Length | Height | Thickness | Cubic yards | 50-lb bags | 60-lb bags | 80-lb bags |
|---|---|---|---|---|---|---|
| 10 ft | 4 ft | 8 in | 1.09 | 79 | 66 | 49 |
| 20 ft | 4 ft | 8 in | 2.17 | 157 | 131 | 98 |
| 30 ft | 4 ft | 8 in | 3.26 | 235 | 196 | 147 |
| 20 ft | 6 ft | 8 in | 3.26 | 235 | 196 | 147 |
| 20 ft | 8 ft | 10 in | 5.43 | 392 | 326 | 245 |
| 40 ft | 4 ft | 8 in | 4.35 | 313 | 261 | 196 |
Includes 10% extra. Past a couple of cubic yards, ready-mix into forms is almost always faster than bags.
How the math works
A poured wall is a flat rectangular prism standing on edge — the same volume math as a slab, just oriented vertically. Length and height are typically given in feet, thickness in inches.
cubic yards = cubic feet ÷ 27
bags = (cubic feet × 1.10) ÷ bag yield
Bag yields used here are the manufacturer figures for placed concrete: 0.375 cubic feet from a 50-lb bag, 0.45 from a 60-lb bag, 0.60 from an 80-lb bag.
Stem wall or full foundation wall — the distinction that decides everything else
A stem wall is short — it runs from the footing to just above grade and mostly carries load straight down, like a column. A full foundation wall is taller — basement or crawlspace height — and does that same job plus retains soil over its full height. That second job changes the physics: a stem wall is primarily an axial member, while a full foundation wall has to resist lateral soil pressure as well, which is a bending problem, not just a compression one.
That difference is why a stem wall's reinforcement is lighter, why drainage is far more critical on a full foundation wall, and why the two fail differently — a stem wall tends to settle or crack at corners, while a full foundation wall under too much lateral pressure bows inward and cracks horizontally near mid-height, or shears at the base.
Wall reinforcement is genuinely engineered, and prescriptive rebar tables are conditional on wall height, soil class, unbalanced backfill height, and wall thickness — they run out quickly outside common cases. Rather than publish a simplified "rebar grid" that could get applied outside the conditions it assumes, it's more useful (and more accurate) to know what actually drives the steel:
- Unbalanced backfill height — soil pushing on one side and not the other is the single biggest driver of bending load.
- Wall height — a longer span between supports means more bending moment.
- Wall thickness — more thickness gives the steel more effective depth to work against, so thicker walls generally need less steel for the same load.
- Soil class — granular soils push less than expansive clays.
- Support conditions — whether the top of the wall is braced by floor framing changes it from a cantilever to a spanning member.
- Surcharge — a driveway, slope, or structure above the backfill adds load that standard tables don't cover.
Vertical steel resists the bending from soil pressure and typically goes toward the interior face on a basement wall — the tension side — which is counterintuitive and a common field mistake. Horizontal steel mainly controls shrinkage cracking. If your wall falls outside a straightforward, well-covered case, it needs an engineer, not a rule of thumb.
Backfilling before the wall is ready is one of the costliest sequencing errors on a typical job, for two separate reasons. First, fresh concrete needs time to reach strength before it can resist lateral soil pressure at all. Second — and less obvious — a basement wall is usually designed assuming the floor framing braces the top of the wall. Backfilling before that floor is on turns a braced wall into an unsupported cantilever, which is a structurally different wall than the one that was designed. The visible result later is a horizontal crack near mid-height or an inward bow, and both are expensive to fix. Heavy equipment working close to a fresh wall creates the same kind of problem from unplanned surcharge.
The rule: don't backfill until the concrete has reached adequate strength and the top of the wall is actually braced — either by the floor system or by temporary bracing designed for the purpose.
Drainage is load relief, not leak prevention
Water sitting against a foundation wall is a load, not just a moisture problem — hydrostatic pressure is exactly what pushes walls inward. A footing drain at the base relieves that pressure and is the single most important drainage element. Free-draining backfill (gravel or crushed stone against the wall) gets water to that drain instead of letting it load the wall directly, and filter fabric keeps fines from silting up the drainage layer over time. Dampproofing and waterproofing aren't the same thing — dampproofing resists moisture, waterproofing resists water under actual pressure — and grading the surrounding soil away from the wall is free and removes a lot of water before it ever becomes the wall's problem.
Before you buy
- Build and brace forms on both faces before ordering concrete — a poured wall needs full-height forms, unlike a slab.
- Rebar is standard practice for any structural poured wall; place it on a grid per your local code before the pour, not after.
- Past roughly 2 cubic yards, a ready-mix pump truck is usually far more practical than filling tall forms with bagged concrete — see concrete cost per yard.
- Plan for a footing under the wall if it doesn't already have one — see the footing calculator.
- Waterproof and provide drainage on the backfill side for any below-grade foundation or retaining wall.
Common questions
How much concrete does a poured wall need?
Multiply the wall's length by its height (both in feet) by the thickness in inches, converting thickness to feet first. A 20 ft wall, 4 ft tall and 8 inches thick, needs about 2.17 cubic yards, or 98 80-lb bags.
How thick should a poured concrete foundation wall be?
8 inches is a common standard for residential foundation walls. Taller walls, walls holding back significant soil, or walls in areas with expansive soil often need to be thicker — check local code and an engineer's recommendation for anything load-bearing.
Is a poured wall the same as a block wall?
No. A poured wall is cast as one continuous piece using forms, while a block wall is built up from individual CMU units and mortar. Poured walls are generally stronger for the same thickness but need forms and, usually, ready-mix delivery rather than bags for anything sizeable.
Do poured walls need rebar?
Yes, for any structural wall. Foundation and retaining walls typically use horizontal and vertical rebar on a grid, sized and spaced according to local code and the wall's height and loading.
Can I use bags for a full foundation wall, or do I need ready-mix?
Technically yes, but past roughly 2 cubic yards, ready-mix pumped into forms is dramatically faster and usually cheaper than mixing dozens of bags by hand. Bags make more sense for a short wall section or a repair.