Concrete Slab Calculator
Planning a patio, walkway, shed pad, or driveway? Our free concrete slab calculator tells you exactly how much concrete to buy: enter your slab's length, width, and thickness to get the cubic yards and cubic feet needed, plus how many 80-lb, 60-lb, and 40-lb bags that works out to — with a waste allowance built in so you don't run short.
Enter your numbers
Your results
| Bag size | Bags needed |
|---|---|
| 80-lb bags | — |
| 60-lb bags | — |
| 40-lb bags | — |
How to use this calculator
Measure your slab's length and width in feet and the pour thickness in inches, then type them in. Pick a waste allowance — 10% is the default and covers spillage, form leakage, and uneven subgrade — and your results update instantly. Planning multiple slabs or footings? Run each section separately and add the totals together.
If the bag counts look huge (or the ready-mix hint appears), that's your cue to price a ready-mix delivery instead of buying pallets of bags. Bring your cubic-yard number to the concrete plant when you call.
How it works
The slab volume is cubic feet = length (ft) × width (ft) × thickness (ft), with inches converted to feet by dividing by 12. That volume divided by 27 gives cubic yards, the unit concrete is sold in. A waste multiplier is applied on top: withWaste = volume × (1 + waste ÷ 100).
Bag counts assume standard yields per bag: an 80-lb bag yields about 0.60 cu ft, a 60-lb bag about 0.45 cu ft, and a 40-lb bag about 0.30 cu ft. Bags = ceil(cubic feet with waste ÷ yield per bag), always rounded up to whole bags. Results are planning estimates — actual yield varies slightly by mix brand and how wet you mix it.
Frequently asked questions
How many yards of concrete do I need for a slab?
Multiply length by width by thickness, with thickness converted to feet (inches ÷ 12), then divide by 27: cubic yards = (L × W × T ÷ 12) ÷ 27. For example, a 20 ft × 10 ft slab at 4 inches thick needs (20 × 10 × 4 ÷ 12) ÷ 27 = 2.47 cubic yards before adding a waste allowance.
How many 80 lb bags of concrete are in a cubic yard?
45 bags. One 80-lb bag of concrete mix yields about 0.6 cubic feet, and one cubic yard contains 27 cubic feet, so 27 ÷ 0.6 = 45. For reference: 60-lb bags come out to 60 per yard and 40-lb bags to 90 per yard.
Should I order extra concrete?
Yes — add a 5–10% waste allowance on top of your calculated volume. It covers spillage during the pour, slight form leakage, and spots where the subgrade dips deeper than your form. Running short mid-pour is far worse than having a little extra, and this calculator defaults to 10% extra.
How thick should a concrete slab be?
4 inches is the standard for patios, walkways, and sidewalks. Use 5–6 inches for driveways or slabs carrying heavy loads (trucks, hot tubs, sheds full of equipment). Thicker slabs cost more concrete but resist cracking under load.
Ready-mix or bags — which is cheaper?
Bags win for small pours under about 1 cubic yard (roughly 45 eighty-pound bags) — no delivery minimums and you mix on your own schedule. For anything bigger, a ready-mix truck is usually cheaper per yard and saves hours of mixing and hauling. The calculator shows a hint when your pour exceeds 1 cubic yard.