Concrete Calculator
Work out how much concrete a slab or footing needs — in cubic yards, cubic feet, and bags of pre-mix — with a waste allowance built in.
The concrete formula
Cubic yards = cubic feet ÷ 27 · thickness in ft = inches ÷ 12
A 10 × 10 ft slab, 4 inches thick: 10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cubic yards (≈ 1.36 with 10% waste). That's about 62 × 80 lb bags — well into ready-mix territory.
Six holes, each 10 inches diameter and 24 inches deep: volume per hole = π × (5 in)² × 24 in ≈ 1,885 cu in ≈ 1.09 cu ft; six holes = 6.5 cu ft, plus 10% waste ≈ 7.2 cu ft — exactly 12 bags of 80 lb mix (16 bags of 60 lb).
Bags vs. ready-mix
Bagged concrete makes sense for small jobs — post holes, small pads, repairs. But it adds up fast: an 80 lb bag yields only ~0.6 cu ft, so a modest slab can need dozens of bags and a lot of mixing. Above roughly one cubic yard, ready-mix delivery is usually cheaper, faster, and gives a stronger, more uniform pour. Always order 5–10% extra — running short mid-pour creates a cold joint you can't undo. For area-only calculations use the square footage calculator, and for garden beds the mulch calculator uses the same cubic-yard math.
Concrete needed by common slab size (4" thick)
| Slab size | Cubic yards |
|---|---|
| 8×8 ft (64 sq ft) | 0.79 |
| 10×10 ft (100 sq ft) | 1.23 |
| 12×12 ft (144 sq ft) | 1.78 |
| 10×20 ft driveway pad (200 sq ft) | 2.47 |
| 20×20 ft garage slab (400 sq ft) | 4.94 |
Common concrete estimating mistakes
- Confusing nominal bag weight with yield. An "80 lb bag" doesn't yield 80 lb of finished volume — it yields about 0.6 cu ft after mixing with water, which is the figure that actually matters for coverage math.
- Forgetting a thickened edge or footing along a slab's perimeter. Many slabs need extra depth at the edges for strength, which this flat-thickness calculator doesn't add automatically — run the edge strip as a separate calculation if your plan calls for one.
- Pouring on a dry, unprepared subgrade. Even with exactly the right volume of concrete, pouring over loose or dry soil without proper compaction and moistening can crack or settle the slab regardless of how accurately the mix was measured.
Frequently asked questions
How do I calculate concrete for a slab?
L × W × thickness (all in feet) ÷ 27 for cubic yards. A 10×10 ft, 4-inch slab ≈ 1.23 cu yd.
How many bags are in a cubic yard?
~45 × 80 lb or ~60 × 60 lb bags. Above 1 cu yd, ready-mix is usually better.
Should I order extra?
Yes — 5–10% for spillage and uneven subgrade.
How much concrete do I need for fence post footings?
Treat each hole as a cylinder (π × radius² × depth) and multiply by the count. Six 10"-wide, 24"-deep holes need about 6.5 cu ft before waste.
Does a thicker slab need proportionally more concrete?
Yes, linearly — doubling the thickness doubles the volume. A 10×10 ft slab is ~1.23 cu yd at 4" but ~2.47 cu yd at 8".
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Note: Estimates only; subgrade, reinforcement, and mix design affect real quantities. Confirm with your supplier for large pours. Last reviewed: September 2026.