How Much Concrete Do I Need?
Concrete estimating comes down to volume, waste, and delivery format. Here's the math and the decision rule between bags and ready-mix.
Concrete is unforgiving. A short pour means a cold joint that never bonds properly. An over-order means paying for material you can't return. Getting the volume right the first time is the whole ballgame.
This guide covers slabs, footings, and columns — the three shapes that account for most residential concrete work — plus how to decide between bags and ready-mix.
The universal formula
Every concrete estimate is volume in cubic feet, divided by 27 to get cubic yards. The trick is measuring the right dimensions for the shape.
- Slab: length × width × thickness.
- Column: radius × radius × 3.14 × height.
- Footing (rectangular): length × width × depth.
- Stairs: sum of tread and riser volumes.
Slab worked example
A 12 ft × 12 ft patio at 4 inches thick:
- Thickness in feet = 4 ÷ 12 = 0.33 ft.
- Cubic feet = 12 × 12 × 0.33 = 47.5 cu ft.
- Cubic yards = 47.5 ÷ 27 = 1.76 cu yd.
- Add 10% waste: 1.76 × 1.10 = 1.94 cu yd.
- Order 2 cubic yards.
Bags vs ready-mix
| Bag size | Yield (cu ft) | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
| 90 lb | 0.675 | 40 |
Thickness recommendations
| Application | Minimum thickness |
|---|---|
| Sidewalks and patios | 4 inches |
| Driveways (car) | 4 inches with rebar or WWM |
| Driveways (truck / RV) | 5–6 inches |
| Garage slabs | 4–6 inches |
| Shed foundations | 4 inches |
| Structural footings | 8 inches (verify with local code) |
Waste factor
Always order 5–10% more than the calculated volume. Reasons:
- Forms flex during pour and add capacity.
- Subgrade is rarely perfectly flat.
- Some material sticks to the truck and chute.
- A slightly over-poured slab is fine; a short pour is a rebuild.
Common mistakes
- Forgetting to convert thickness from inches to feet.
- Ordering ready-mix without confirming truck access.
- Not accounting for short-load fees on small orders.
- Ignoring the subgrade — poor compaction cracks the best concrete within a season.
- Skipping control joints on slabs over 100 sq ft.
Frequently asked questions
- How many 80 lb bags of concrete make a cubic yard?
- 45 bags. That's why anything over about 15 bags (0.33 cu yd) is worth considering ready-mix.
- How much does a cubic yard of concrete weigh?
- About 4,050 lb, or roughly 2 tons.
- How thick should a concrete driveway be?
- 4 inches for passenger vehicles with 6x6 W2.9 welded wire mesh. Bump to 5 inches for regular truck use.
- Can I mix bagged and ready-mix concrete on the same slab?
- Not in a single pour. If you have to top up ready-mix with bags, do it before the first pour flashes off — usually within 30 minutes.
Summary
Concrete volume is straightforward: area × thickness ÷ 27. Add 10% waste, and switch from bags to ready-mix around one cubic yard. The subgrade and reinforcement matter more than the volume itself.
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These results are estimates only. Confirm quantities, compaction, waste, and delivery requirements with your supplier or project professional before ordering materials.