Concrete Calculator — Bags, Yards and Slabs
Enter your slab dimensions and get the number of bags to buy — plus the cubic yards, in case a ready-mix truck works out cheaper.
About the Concrete Calculator
Concrete is sold two completely different ways and the crossover matters. Below about a cubic yard, bags mixed on site are usually cheaper and far less stressful. Above it, a ready-mix truck wins on both price and on the fact that a large slab poured in sections rarely ends up monolithic. This calculator gives you both numbers so you can decide which side of that line your job falls on.
How to use it
- Measure the length and width of the slab in feet.
- Enter the thickness in inches — 4" is standard for a patio or shed base, 6" for a driveway.
- Pick the bag size sold at your store.
- Leave waste at 10% unless your ground is very uneven, in which case use 15%.
The formula
Cubic feet = length × width × (thickness ÷ 12)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag
Thickness is divided by 12 because it is measured in inches while length and width are in feet. The divide-by-27 converts cubic feet to cubic yards, since a yard is 3 ft in each of three dimensions. Bag yields are the manufacturer's own figures for mixed volume: an 80 lb bag yields about 0.60 cu ft, 60 lb about 0.45, 50 lb about 0.375 and 40 lb about 0.30.
Worked examples
| Slab | Thickness | Cubic feet (bare) | Cubic yards (+10%) | 80 lb bags |
|---|---|---|---|---|
| 10 ft × 10 ft | 4" | 33.3 | 1.36 | 62 |
| 12 ft × 12 ft | 4" | 48.0 | 1.96 | 88 |
| 20 ft × 20 ft | 6" | 200.0 | 8.15 | 367 |
| 4 ft × 4 ft | 4" | 5.3 | 0.22 | 10 |
Cubic feet is the bare slab; the yardage and bag count both include 10% waste, because that is what you order. Look at the third row: 367 bags is roughly seven pallets and two solid days of mixing. That is the job where you stop calculating bags and phone a ready-mix supplier.
Common mistakes
- Measuring thickness in feet. A 4-inch slab is 0.33 ft, not 4. Entering 4 gives twelve times too much concrete — the single most common error on this calculation.
- Forgetting waste entirely. Ground is never perfectly level and some concrete always stays in the mixer. Ten percent is the standard allowance and it is not padding.
- Ordering exactly what the maths says. Running out mid-pour means a cold joint — a permanent weak seam where the first batch set before the second arrived. One spare bag is cheap insurance.
- Ignoring the base. This calculates concrete only. A slab also needs 4 inches of compacted gravel underneath, which is a separate order.
Terms explained
- Cubic yard
- 27 cubic feet. The unit ready-mix is sold in. One yard covers about 80 sq ft at 4 inches thick.
- Yield
- How much mixed concrete one bag produces. Always less than you would guess, because the dry powder compacts when water is added.
- Cold joint
- The weak seam formed when fresh concrete is poured against a batch that has already started to set.
- Slump
- How wet the mix is. Wetter is easier to place and weaker when cured — resist the temptation to add extra water.
Common questions
- How many bags of concrete for a 10x10 slab?
- At 4 inches thick, 62 bags of 80 lb concrete including 10% waste — about 1.23 cubic yards. At 6 inches it rises to 92 bags.
- How many 80 lb bags make a yard?
- Forty-five. An 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet.
- Is it cheaper to mix bags or order ready-mix?
- Bags win below about one cubic yard. Past that, ready-mix is usually cheaper per yard and enormously less work — though most suppliers charge a short-load fee under 3 yards.
- How thick should a concrete slab be?
- Four inches for patios, walkways and shed bases. Six inches where vehicles park. Anything structural should be specified by an engineer, not a calculator.
- Do I need gravel under a slab?
- Almost always. Four inches of compacted gravel gives drainage and a stable base. Concrete poured straight onto soil cracks as the ground moves.
- How long does concrete take to cure?
- It is walkable in 24 to 48 hours and takes vehicles after a week, but full strength is 28 days. Keep it damp for the first few days — concrete cures by chemical reaction, not by drying out.
- How much does a yard of concrete weigh?
- About 4,000 lb, or two tons. That is why a 20 ft × 20 ft driveway is a delivery, not a project you carry.