Concrete Slab Calculator
Updated 2026-10-12
Enter the length, width and thickness of your slab. You'll get the volume of concrete, with a safety margin, and how many bags of premixed concrete to buy.
How the concrete calculator works
Concrete is ordered by volume. For a slab, volume is simply length × width × thickness. The only trick is keeping the units consistent: thickness is usually measured in inches, so it's divided by 12 to turn it into feet. Ready-mix suppliers in the US sell by the cubic yard, and one cubic yard is 27 cubic feet.
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag
Worked example
A 10 × 10 ft patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Add 10% for spillage and uneven ground and you need about 36.7 cubic feet (1.36 cubic yards). With 80 lb bags yielding about 0.6 cubic feet each, that is 62 bags.
How thick should a concrete slab be?
| Project | Typical thickness |
|---|---|
| Garden path, shed base, patio | 4 in (10 cm) |
| Car driveway | 4–5 in (10–12 cm) |
| Driveway for heavy vehicles, RVs | 5–6 in (12–15 cm) |
| Garage floor | 4–6 in (10–15 cm) |
Local building codes and soil conditions can require more. For anything structural, such as a house foundation, follow your engineer's or local authority's specification.
Bags or ready-mix?
Bagged premix is convenient for small jobs: fence posts, steps, a small pad. Once a project needs more than about one cubic yard (roughly 45 bags of 80 lb), mixing by hand becomes heavy, slow work, and it's hard to keep the whole pour wet enough to finish evenly. At that point, a ready-mix truck or a short-load delivery is usually the better choice. Many suppliers have a minimum order, so call ahead.
Why add extra?
Ground is rarely perfectly flat, forms bow slightly under the weight of wet concrete, and some always ends up spilled or left in the mixer. Running out mid-pour creates a weak "cold joint", so it's far better to have a little left over. 5–10% extra is standard; use more for uneven ground.
Quick reference: concrete per 100 sq ft
These figures are before any extra for spillage. Add 5–10% when you order.
| Slab thickness | Cubic feet | Cubic yards | 80 lb bags (0.6 cu ft) |
|---|---|---|---|
| 3 in | 25 | 0.93 | 42 |
| 4 in | 33.3 | 1.23 | 56 |
| 5 in | 41.7 | 1.54 | 70 |
| 6 in | 50 | 1.85 | 84 |
In metric, every 10 m² of slab needs 1 m³ of concrete at 10 cm thick, 1.2 m³ at 12 cm and 1.5 m³ at 15 cm.
Planning the pour
- Prepare the base. Most slabs sit on a layer of compacted gravel, often around 4 in (10 cm), which drains water and gives an even surface. Our gravel calculator works out how much to order.
- Reinforcement. Wire mesh or rebar helps hold cracks together. Driveways and anything carrying vehicles usually need it; check local requirements.
- Control joints. Concrete shrinks as it cures and will crack somewhere. Cutting or tooling joints guides the cracks into straight lines. A common rule of thumb is to space joints no more than 2–3 times the slab thickness in inches, in feet: a 4 in slab gets joints every 8–12 ft.
- Weather. Avoid pouring when frost is expected or in very hot, windy conditions, which dry the surface too fast.
- Curing. Keep the slab damp, or covered with plastic sheeting, for the first several days. Slow curing makes stronger concrete.
Frequently asked questions
- How many 80 lb bags make a cubic yard?
- About 45. One cubic yard is 27 cubic feet, and an 80 lb bag yields roughly 0.6 cubic feet.
- How many 60 lb bags make a cubic yard?
- About 60, at roughly 0.45 cubic feet per bag.
- How much does a cubic yard of concrete weigh?
- Roughly 4,000 lb (about 1.8 tonnes). A cubic meter weighs around 2,400 kg.