How much extra concrete should I order?

Add 5–10% to the calculated volume. Subgrades are never perfectly level, forms flex slightly, and spillage happens. Running short mid-pour creates a weak cold joint, so a small surplus is cheap insurance.

Concrete Calculator — Cubic Yards & Bags (80, 60, 40 lb)

Q: How many bags of concrete are in a cubic yard?

One cubic yard is 27 cubic feet, so you need about 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. An 80 lb bag yields roughly 0.60 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30.

Q: When should I order ready-mix instead of bags?

Around one cubic yard (roughly 45 × 80 lb bags) is the practical crossover. Above that, mixing bags by hand becomes slow and physically demanding, and cold joints between batches can weaken the pour. Ready-mix trucks deliver a consistent mix all at once.

Q: How thick should a concrete slab be?

Common practice is 4 inches (100 mm) for patios, walkways and shed bases, and 5–6 inches (125–150 mm) for driveways or slabs that carry vehicles. Local codes and soil conditions can require more — check before pouring structural work.

How to calculate concrete volume

Concrete is ordered by volume. Multiply the dimensions of your pour, keeping units consistent:

Slab: length × width × thickness. A 10 × 10 ft patio at 4 in thick is 10 × 10 × 0.333 = 33.3 cu ft ≈ 1.24 cubic yards.
Footing: length × width × depth — same math, long and narrow.
Column / post hole: π × (diameter ÷ 2)² × height for round forms.

1 cubic yard = 27 cubic feet. 1 m³ = 1000 liters. Add 5–10% for waste before ordering.

How many bags of concrete?

Bagged concrete mix yields much less volume than its weight suggests, because you add water and the mix compacts:

80 lb bag → ~0.60 cu ft · ~45 bags per cubic yard
60 lb bag → ~0.45 cu ft · ~60 bags per cubic yard
40 lb bag → ~0.30 cu ft · ~90 bags per cubic yard
25 kg bag → ~12.5 L · ~80 bags per m³ · 20 kg bag → ~10 L, ~100 bags per m³

Those bag counts add up fast — which is why anything much bigger than a small pad is usually a ready-mix order, not a bag job.

Common slab thicknesses

4 in (100 mm) — patios, walkways, shed bases.
5–6 in (125–150 mm) — driveways and slabs carrying vehicles.
8 in+ (200 mm+) — footings and structural elements, per local code.

Frequently asked questions

How many bags of concrete are in a cubic yard?

About 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags — one cubic yard is 27 cubic feet, and the bags yield roughly 0.60, 0.45 and 0.30 cu ft respectively.

When should I order ready-mix instead of bags?

Around one cubic yard is the practical crossover. Beyond that, hand-mixing is slow and heavy, and pauses between batches create weak cold joints. A ready-mix truck delivers a consistent mix in one pour.

How thick should a concrete slab be?

4 inches (100 mm) for patios and walkways; 5–6 inches (125–150 mm) where vehicles drive. Local codes and soil conditions can require more for structural work.

How much extra should I order?

5–10%. Subgrades are never perfectly level and forms flex; running short mid-pour forces a cold joint, so a small surplus is cheap insurance.