Concrete for common slab sizes
At 4 in thick. Select a size for 5 and 6 in, and the bag counts for every size.
| Slab | Exact yd³ | Order yd³ | 80-lb bags | 60-lb bags |
|---|---|---|---|---|
| 4 × 4 ft at 4 in | 0.2 | 0.25 | 10 | 14 |
| 8 × 8 ft at 4 in | 0.79 | 1 | 40 | 53 |
| 10 × 10 ft at 4 in | 1.23 | 1.5 | 62 | 82 |
| 10 × 12 ft at 4 in | 1.48 | 1.75 | 74 | 98 |
| 12 × 12 ft at 4 in | 1.78 | 2 | 88 | 118 |
| 12 × 16 ft at 4 in | 2.37 | 2.75 | 118 | 157 |
| 16 × 16 ft at 4 in | 3.16 | 3.5 | 157 | 209 |
| 20 × 20 ft at 4 in | 4.94 | 5.5 | 245 | 326 |
| 24 × 24 ft at 4 in | 7.11 | 8 | 352 | 470 |
How to measure
- Measure the length and width of the area at the inside faces of the forms, where the concrete will actually go.
- Measure the thickness of the concrete only. Don't include the gravel base underneath.
- If the ground isn't level, take the depth in several places and use the average, then keep the waste allowance at 10% or more.
- Break odd shapes into rectangles, circles and triangles, and add each as a part. Subtract anything that won't be concrete.
How the calculation works
Volume is length × width × thickness, with every dimension converted to feet first. Divide cubic feet by 27 to get cubic yards. Then the calculator adds your waste allowance, rounds the order up to the supplier's increment, and divides by each bag's published yield, rounding up to whole bags.
Here is the default example worked through, exactly as the calculator does it. A 10 × 10 ft slab at 4 in is 1.23 cubic yards, so you'd order 1.5 yd³ or buy 62 80-lb bags.
Slab
- Length: 10 ft
- Width: 10 ft
- Thickness: 4 in ÷ 12 = 0.3333 ft
- Area: 10 ft × 10 ft = 100 ft²
- Volume: 100 ft² × 0.3333 ft = 33.3333 ft³
Total and order
- In cubic yards: 33.3333 ft³ ÷ 27 = 1.2346 yd³
- With 10% extra: 1.2346 yd³ × 1.1 = 1.358 yd³
- Order, rounded up to the next 0.25 yd³: 1.5 yd³
Bags (with extra)
- 40 lb bags: 36.6667 ft³ ÷ 0.3 ft³ per bag = 122.2222, rounded up to 123
- 50 lb bags: 36.6667 ft³ ÷ 0.375 ft³ per bag = 97.7778, rounded up to 98
- 60 lb bags: 36.6667 ft³ ÷ 0.45 ft³ per bag = 81.4815, rounded up to 82
- 80 lb bags: 36.6667 ft³ ÷ 0.6 ft³ per bag = 61.1111, rounded up to 62
Trucks and weight
- Trucks: 1.5 yd³ ÷ 10 yd³ per truck = 0.15, rounded up to 1
- Weight of the poured concrete: 1.1468 m³ × 2,400 kg/m³ = 2,752 kg = 6,068 lb
Open "Show the math" under any result to see the same steps for your own numbers.
Every shape in one project
Pick a shape for each part of the pour and add as many parts as you need. A patio with steps and a cut-out around a tree is three parts: two that add concrete and one that subtracts it. The result is one order for the whole job.
- Slabs: patios, driveways, sidewalks, shed bases, round and triangular slabs, and slabs with a thickened edge.
- Footings: strip, pad and bell footings.
- Columns and tubes: round tube forms, square columns and hollow rings.
- Post holes: with the post and gravel base subtracted.
- Walls: with door and window openings subtracted.
- Steps: solid stairs with an optional landing.
- Curb and gutter, and any slab you already know the area of.
You can type dimensions however you measured them: 10' 6", 10.5, 3 1/2 or90 cm. The line under each field shows how the calculator read it, so you can catch a typo before you order.
Tips before you order
- Order a little extra. The National Ready Mixed Concrete Association suggests ordering 4% to 10% more than you measured. Running short mid-pour is worse than having a little left over.
- Count bags before you commit. One cubic yard is 45 80-lb bags, each mixed by hand or in a mixer. Use the cost comparison to weigh that against a ready-mix delivery.
- Check your vehicle's payload. 45 80-lb bags weigh 3,600 lb. Enter your payload under "Bag weight and trips" to see how many trips that is.
- Ask the supplier about truck size and fees. Trucks carry 8 to 12 cubic yards (NRMCA), and small orders can come with extra charges. Enter any fees to compare.
- Use the order message. "Copy order message" gives you a ready-to-send note with the quantity, so nothing gets lost on the phone.
Questions people ask
How much concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab 4 in thick is 1.23 cubic yards. With 10% extra, order 1.5 cubic yards from a ready-mix supplier, or buy about 62 80-lb bags (82 60-lb bags).
How many bags of concrete make a cubic yard?
It takes 45 80-lb bags, 60 60-lb bags, 72 50-lb bags or 90 40-lb bags to make one cubic yard, based on the yields printed on QUIKRETE's data sheet (0.60 ft³ per 80-lb bag). Add your waste allowance on top.
How much extra concrete should I order?
The National Ready Mixed Concrete Association suggests ordering 4% to 10% more than the volume you measured, to cover uneven ground, form bulges and spillage. The calculator defaults to 10% for slabs, footings and post holes, and 5% for formed shapes like columns and walls. You can change it with the slider.
Should I use bags or a ready-mix truck?
It depends mostly on volume. One cubic yard is 45 80-lb bags, which is a lot of mixing. Ready-mix trucks carry 8 to 12 cubic yards, according to the NRMCA. Enter your local prices in the calculator to compare the two costs for your project.
How thick should a concrete slab be?
The International Residential Code sets 3½ in as the minimum for slab-on-ground floors, and bagged-mix instructions for slabs commonly call for at least 4 in. Check local code and your project's loads, and ask a professional for anything structural.
How much does a cubic yard of concrete weigh?
About 4,045 lb (2.02 tons) at 2,400 kg/m³ (about 150 lb/ft³), the upper end of the 140 to 150 lb/ft³ range the NRMCA gives for normal-weight concrete. QUIKRETE gives about 140 lb/ft³ for its bagged mix, which works out to 3,780 lb per cubic yard.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft. For example, a 10 × 10 ft slab at 4 in is 33.33 ft³, and 33.33 ÷ 27 = 1.23 yd³.
Related calculators
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- SAKRETE High-Strength Concrete Mix technical data sheet, retrieved October 2, 2026
- Easy Mix Super Strength Concrete technical data sheet, retrieved October 2, 2026
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete, retrieved October 2, 2026
- International Residential Code 2021, Section R506.1, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.