Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Wall length 20 ft; Wall height 4 ft; Wall thickness 8 in = 0.6667 ft
- Wall face = 20 × 4 = 80 ft²
- Volume = 80 × 0.6667 = 53.3333 ft³
- Net volume = 53.3333 ft³ ÷ 27 = 1.9753 yd³ (× 0.0283168 = 1.5102 m³)
- Order quantity = 1.9753 yd³ × (1 + 10%) = 2.1728 yd³ = 58.6667 ft³
- 80 lb bags = 58.6667 ft³ ÷ 0.6 ft³ per bag = 97.78 → 98 bags (always round up)
- Weight = 58.6667 ft³ × 150 lb/ft³ = 8,800 lb
How to measure
Measure the length along the wall, the height from the top of the footing to the top of the wall, and the thickness of the forms. Wall thickness is the distance between the two form faces. For a foundation wall that steps, measure each section separately and add them.
For each opening, measure the width and height of the hole in the wall. Enter the count of identical openings and the calculator subtracts them.
The wall formula
A wall is a slab stood on its edge. Its face area is length × height, you subtract the area of every opening, and you multiply what is left by the thickness. Work in feet, convert thickness from inches by dividing by 12, and divide by 27 for yards.
Subtracting openings matters more than people expect. A standard door is about 3 × 6.67 ft, so each one removes 20 ft² of face. On an 8 in wall that is 13 ft³, or half a yard, per door.
Formula: V (ft³) = (L × H − Σ openings) × (T ÷ 12).
Worked example: a 40 × 8 ft wall with two doors
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Wall length 40 ft; Wall height 8 ft; Wall thickness 8 in = 0.6667 ft; Opening width (door or window) 3 ft; Opening height 6.67 ft
- Wall face = 40 × 8 = 320 ft²
- Openings = 3 × 6.67 × 2 = 40.02 ft²
- Net face = 320 − 40.02 = 279.98 ft²
- Volume = 279.98 × 0.6667 = 186.6533 ft³
- Net volume = 186.6533 ft³ ÷ 27 = 6.9131 yd³ (× 0.0283168 = 5.2854 m³)
- Order quantity = 6.9131 yd³ × (1 + 10%) = 7.6044 yd³ = 205.3187 ft³
- 80 lb bags = 205.3187 ft³ ÷ 0.6 ft³ per bag = 342.2 → 343 bags (always round up)
- Weight = 205.3187 ft³ × 150 lb/ft³ = 30,798 lb
| Quantity | Result |
|---|---|
| Net volume | 6.913 yd³ (186.65 ft³, 5.285 m³) |
| Order with 10% waste | 7.60 yd³ (5.81 m³) |
| 80 lb bags | 343 |
| 60 lb bags | 457 |
| Weight at 150 lb/ft³ | 30,798 lb |
The same wall with no openings needs 8.69 yd³, so the two doors save 1.09 yd³. Always subtract openings from the finished plan, but remember that bucks and form work around openings make the pour harder and add a little waste.
How thick is a poured wall?
Thickness comes from the design, not from a rule of thumb. Residential foundation walls are often 8 in or 10 in thick, retaining walls may be thicker at the base, and garden walls can be 6 in. Check the plan or engineer. A single inch changes the volume by 12.5% on an 8 in wall, which is the reason to confirm it before you order.
Placing concrete in walls
Wall forms are tall and narrow, so concrete is usually placed in lifts of about 2 ft and vibrated to remove voids. That is slow compared with a slab, which means the truck waiting time can run long. Ask the plant about slump and about a pump, because a chute alone seldom reaches a full-height wall form.
Concrete pressure on forms increases with the rate of rise and height. Bracing and tie spacing come from the form designer. Underestimating formwork is a bigger risk than underestimating concrete.
Common wall mistakes
- Mixing feet and inches without converting: a wall 8 in thick is 0.667 ft, not 8.
- Subtracting openings but forgetting the lintel or sill that the opening needs.
- Forgetting the footing under the wall. That is calculated separately with the footing calculator.
- Counting the height of the footing as wall height.
- Not allowing for the extra volume where the wall thickens at pilasters or corners.
Frequently asked questions
How do I calculate concrete for a wall?
Multiply the wall length by its height to get the face area in square feet, subtract the area of openings, multiply by the thickness in feet, and divide by 27. A 40 × 8 ft wall at 8 in is 213.3 ft³ or 7.90 yd³ before waste.
Do I subtract door and window openings?
Yes, subtract the opening area from the face before multiplying by thickness. Keep a little waste in the order because forming around openings is fiddly.
How much concrete is in a 4 ft high wall?
A wall 20 ft long, 4 ft high and 8 in thick holds 53.3 ft³ or 1.98 yd³ before waste.
How many bags for a small garden wall?
A 10 ft wall, 2 ft high and 6 in thick holds 10 ft³, about 17 bags of 80 lb mix with no waste.
How thick should a concrete wall be?
It depends on height, loads and whether it retains soil. Use the thickness on your plans or from an engineer.
Do I include the footing in the wall volume?
No. Calculate the footing separately because it has different dimensions.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.