Concrete Wall Calculator
Calculate a foundation or retaining wall pour from length, height and thickness — including tapered walls using the average thickness.
Your concrete pour
Concrete to order (waste included)
4.35 yd³
- 80 lb bags
- 196 bags
- Volume before waste
- 106.67 ft³
- Weight
- 17,383 lb
- Bag yields vary by brand and mix — confirm the yield printed on the bag.
How this is calculated
Raw volume
length × height × (thickness ÷ 12)
= 20 × 8 × (8 ÷ 12)
106.67 ft³
Add waste
raw volume × (1 + waste% ÷ 100)
= 106.67 × (1 + 10 ÷ 100)
117.33 ft³ = 4.35 yd³
80 lb bag count
ceil(volume ÷ bag yield)
= ceil(117.33 ÷ 0.6)
196 bags
Weight
yd³ × 4000 lb/yd³
= 4.35 × 4000
17,383 lb
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab poured 4 inches thick contains 10 × 10 × (4 ÷ 12) = 33.3 ft³. Divide by 27 to get 1.23 cubic yards. At 0.6 ft³ per 80 lb bag that is 56 bags. Add the standard 10% slab waste and order 1.36 yd³ of ready-mix. The same math runs for every shape above — the calculator just substitutes your numbers.
Concrete bags by slab size (4 inch thick, before waste)
| Slab size | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 8 × 8 ft | 0.79 | 36 | 48 | 72 |
| 10 × 10 ft | 1.23 | 56 | 75 | 112 |
| 10 × 12 ft | 1.48 | 67 | 89 | 134 |
| 12 × 12 ft | 1.78 | 80 | 107 | 160 |
| 12 × 16 ft | 2.37 | 107 | 143 | 214 |
| 20 × 20 ft | 4.94 | 223 | 297 | 445 |
Concrete coverage — area one cubic yard covers
| Thickness | Coverage per yd³ |
|---|---|
| 4 inches | 81 ft² |
| 5 inches | 65 ft² |
| 6 inches | 54 ft² |
| 8 inches | 40 ft² |
Standard slab thickness by application
| Application | Typical thickness |
|---|---|
| Patio / walkway | 4 inches |
| Shed / garden slab | 4 inches |
| Driveway (cars) | 4 inches |
| Driveway (heavy vehicles) | 5–6 inches |
| Garage floor | 4–6 inches |
| Footings | 6–12 inches (per design) |
Common concrete estimating mistakes
- •Forgetting waste. Spillage, uneven subgrade and over-excavation mean you should order 5% (formed footings) to 10% (slabs) extra.
- •Rounding bags down. Always round up to whole bags — running short mid-pour ruins a monolithic finish.
- •Using nominal pi (3.14) for columns. Small errors compound across many piers; this calculator uses full-precision π.
- •Ignoring the short-load fee. Ready-mix orders under about 4 yd³ usually carry a surcharge — bagging may be cheaper for small pours.
Frequently asked questions
How do I calculate concrete for a tapered wall?+
Use the average of the top and bottom thickness. A wall 6 inches at the top and 10 inches at the base is calculated at an 8-inch effective thickness.
How much concrete do I need for a 10×10 slab?+
A 10×10 ft slab at 4 inches thick is 33.3 ft³ = 1.23 cubic yards before waste, or about 56 bags of 80 lb concrete mix. Add 10% waste and order 1.36 yd³.
How many 80 lb bags of concrete are in a cubic yard?+
There are 45 bags of 80 lb concrete mix in a cubic yard (27 ft³ ÷ 0.6 ft³ per bag). A cubic yard is 60 × 60 lb bags or 90 × 40 lb bags.
How much does a cubic yard of concrete weigh?+
Standard ready-mix concrete weighs about 4,000 lb (roughly 2 tons) per cubic yard.
How much waste should I add when ordering concrete?+
Add about 10% for slabs and 5% for formed footings. Increase it on rough or sloped ground where over-excavation fills with concrete.
Methodology & sources
Volume uses the standard geometric formula for this pour shape with full precision and exact π. Bag counts divide the waste-adjusted volume by published per-bag yields and round up. Confirm the yield on the bag.
- Quikrete published bag yield data (Concrete Mix No. 1101) — verified June 28, 2026
Last reviewed June 28, 2026. Estimates are indicative — verify against current product specs and local requirements before ordering.
We're committed to keeping these tools accurate and improving them over time. If you'd like to contribute to their accuracy, or you run into any issues or errors, please email us at info@tradesppl.com.