About this calculator
This calculator works out how much concrete a rectangular slab needs — a patio, shed base, walkway, hot-tub pad or garage floor — and converts that volume into the number of pre-mixed bags to buy. It also totals the cost and the weight you’ll need to haul.
Slabs are simple boxes, so the math is just length × width × thickness. What trips people up is the units: slabs are measured in feet, thickness in inches, bags in cubic feet and ready-mix trucks in cubic yards. The calculator handles every conversion for you.
How to use it
- Measure the length and width of the form (inside edge to inside edge).
- Enter the thickness. Use 4 in for patios and walkways, 5–6 in for driveways or heavy loads.
- Keep the default 10% extra unless your subgrade is perfectly level; use 15% on rough ground.
- Pick the bag size you plan to buy and enter your local price.
The formula
Volume = length × width × thickness × (1 + extra%)
Bags = Volume (cu ft) ÷ bag yield (cu ft), rounded up
Cubic yards = cubic feet ÷ 27
Bag yields come from the manufacturer’s data sheets: an 80 lb bag of standard concrete mix makes about 0.60 cubic feet, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.30 cu ft. That means one cubic yard takes roughly 45 bags of 80 lb mix.
Worked example
These are the calculator’s starting values and the results they produce:
| Slab length | 10 ft |
|---|---|
| Slab width | 10 ft |
| Thickness | 4 in |
| Extra for waste and uneven ground | 10 % |
| Bag size | 80 lb bag (0.60 cu ft) |
| Price per bag | $6.50 |
Tips before you buy
- Over about 1 cubic yard, price out ready-mix. Mixing 45+ bags by hand is a long day; many suppliers deliver short loads, though a short-load fee may apply.
- Compact 4 in of gravel under the slab first — see the gravel calculator for the base.
- Measure thickness in several spots. A slab that is ½ in thicker than planned over 100 sq ft uses about 4 extra cubic feet (7 more bags).
- Buy a couple of extra bags. Unopened bags can usually be returned; running short mid-pour cannot be fixed.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags it’s 60 bags, and with 40 lb bags about 90.
How much concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 in thick is 33.3 cubic feet, or 1.23 cubic yards, before waste. With 10% extra that is about 1.36 cubic yards — 62 bags of 80 lb mix.
How thick should a concrete slab be?
4 in is the usual minimum for patios, sidewalks and shed floors. Driveways and slabs that carry vehicles are typically 5–6 in. Check local codes and consider reinforcement (wire mesh or rebar) for anything structural.
Is it cheaper to use bags or ready-mix?
For small jobs under about one cubic yard, bags are usually cheaper and easier. Above that, ready-mix delivered by truck generally costs less per yard and saves hours of mixing, even with a short-load fee.