Concrete Slab Calculator
Estimate the cubic yards of concrete for a slab and how many 40, 60, or 80 lb bags it takes — perfect for patios, walkways, shed pads, and footings.
This calculator estimates the volume of concrete for a rectangular slab and converts it to cubic yards and the number of 40, 60, and 80 lb bags. Enter the length and width in feet and the thickness in inches. Great for patios, walkways, shed pads, and small footings.
Enter slab dimensions to see cubic yards and the number of 40, 60, and 80 lb bags.
How the concrete calculator works
- Convert thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Volume in cubic feet = length × width × thickness. A 10 × 10 ft, 4 in slab = 10 × 10 × 0.333 = 33.3 cu ft.
- Cubic yards = cubic feet ÷ 27 = about 1.23 cu yd.
- Bags = cubic feet ÷ bag yield (0.6 for 80 lb, 0.45 for 60 lb, 0.3 for 40 lb), then add waste and round up.
Before you pour
- Prepare the base. A compacted gravel sub-base and proper forms matter as much as the concrete itself.
- Check local code for thickness, reinforcement (rebar or wire mesh), and footing depth in freeze-prone areas.
- Have help ready. Concrete sets on its own schedule — line up tools and hands before you start mixing.
- Round up your order. Running short mid-pour creates a cold joint; a little extra is cheap insurance.
Two worked examples
Example 1 — a small shed pad. A 10 × 12 ft pad at 4 inches thick is 10 × 12 × (4 ÷ 12) = 40 cubic feet, or 40 ÷ 27 = about 1.5 cubic yards. At roughly 0.6 cubic feet per 80 lb bag, that is 40 ÷ 0.6 = 67 bags before waste — already past the point where a ready-mix delivery is easier and usually cheaper.
Example 2 — a small walkway. A 3 × 20 ft walkway at 4 inches is 3 × 20 × 0.333 = 20 cubic feet, about 0.74 cubic yards. That is 20 ÷ 0.6 = 34 bags of 80 lb, or 20 ÷ 0.45 = 45 bags of 60 lb. Add 5–10% so an uneven subgrade does not leave you short, and you can reasonably mix this by hand over a session.
Cost and planning factors
The concrete itself is only part of the spend. A durable slab needs excavation, a compacted gravel sub-base for drainage, forms to hold the shape, and often rebar or wire mesh for strength — steps that are easy to leave out of a quick budget but not optional for a slab that lasts. In freeze-prone regions, footings must reach below the frost line, and thicker, reinforced slabs are required wherever vehicles will park. The finish matters too: a basic broom finish is economical, while stamped, colored, or polished concrete costs more. For the full bags-versus-ready-mix breakdown and the prep costs people miss, see our concrete cost guide.
Concrete costs in 2026
The concrete itself is only part of the bill — the base and finishing work drive much of the cost.
| Cost item | Why it matters |
|---|---|
| Concrete (bags or ready-mix) | Bags for small jobs; ready-mix past ~1 cu yd |
| Gravel sub-base | Drainage and a stable, crack-resistant base |
| Forms & reinforcement | Rebar or wire mesh, often required by code |
| Excavation & grading | More for slopes or hard-to-reach areas |
| Finish | Broom finish (basic) → stamped/colored (premium) |
Concrete and labor prices vary by region, finish, and site conditions — planning factors, not a quote.
Common mistakes to avoid
- Skimping on the compacted gravel base — the top cause of cracking.
- Under-ordering; running short creates a weak cold joint mid-pour.
- Ignoring local code on thickness, rebar, and frost-line footings.
- Hand-mixing far more than ~1 cubic yard instead of ordering ready-mix.
- Pouring without enough help — concrete sets on its own schedule.