Concrete Calculator
Enter the size of a slab, round column, wall or footing to get the concrete you need in cubic yards (or cubic meters), with an allowance you choose and a count of whole bags.
Quick answer: A 12 ft × 10 ft slab, 4 in thick, needs about 1.48 cubic yards of concrete, or 1.63 yd³ with a 10% allowance, which is 74 bags of 80 lb mix.
Concrete volume is length × width × thickness. For a slab, convert the thickness to feet (inches ÷ 12), multiply the three numbers, then divide by 27 to get cubic yards.
Example: a 12 ft × 10 ft slab, 4 in thick, is 12 × 10 × (4 ÷ 12) = 40 ft³, or 1.48 yd³. With a 10% allowance that is 44 ft³, or 1.63 yd³, which rounds up to 1.75 yd³ for a ready-mix order. In 80 lb QUIKRETE Concrete Mix #1101 bags (0.60 ft³ each), 44 ÷ 0.60 = 73.3, so 74 bags.
Method
How this calculator works
Every result comes from exact geometry and exact unit factors. Rounding happens only when numbers are displayed, and bag counts always round up.
Volume of the pour
Pick the shape that matches your forms. Thickness and widths in inches are divided by 12 to get feet; the round column uses the inside diameter of the form.
Slab: Volume (ft³) = L (ft) × W (ft) × T (in) ÷ 12Round: Volume (ft³) = π × (D (in) ÷ 24)² × H (ft)Wall or footing: Volume (ft³) = L (ft) × W (in) ÷ 12 × H (ft)Cubic feet to cubic yards
One cubic yard is 27 cubic feet, and one cubic foot is 0.028316846592 cubic meters. Both are exact definitions in NIST Handbook 44.
Cubic yards = Volume (ft³) ÷ 27Cubic meters = Volume (ft³) × 0.028316846592Your allowance
Extra volume covers spillage, an uneven subgrade and bowed forms. The calculator starts at 10% and lets you pick 0 to 25%. That starting point is a common rule of thumb, not a standard, so adjust it to your job. Ready-mix is shown rounded up to the next quarter yard; suppliers set their own increments.
Order volume = Net yd³ × (1 + Allowance ÷ 100)Bags
Yield is specific to each product. The bag counts use QUIKRETE Concrete Mix #1101: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³. Other mixes can differ, so use the yield printed on your own bag.
Bags = round up (Volume with allowance (ft³) ÷ Yield per bag (ft³))Worked example: 12 ft × 10 ft slab, 4 in thick, 10% allowance
Arithmetic checked| Step | Math | Result |
|---|---|---|
| Thickness in feet | 4 in ÷ 12 | 0.3333 ft |
| Volume in cubic feet | 12 × 10 × 0.3333 | 40 ft³ |
| Convert to cubic yards | 40 ÷ 27 | 1.4815 yd³ |
| Add a 10% allowance | 1.4815 × 1.10 | 1.6296 yd³ |
| Round up to the next 0.25 yd³ (ready-mix) | 1.6296 → 1.75 | 1.75 yd³ |
| Volume with allowance in cubic feet | 40 × 1.10 | 44 ft³ |
| 80 lb bags (0.60 ft³ each) | 44 ÷ 0.60 = 73.33, round up | 74 bags |
| 60 lb bags (0.45 ft³ each) | 44 ÷ 0.45 = 97.78, round up | 98 bags |
| You would order | 1.75 yd³ ready-mix, or 74 bags (80 lb) | |
Disclaimer
Results are planning estimates based on the dimensions and assumptions you enter. They are not engineering, structural or building-code advice. Product yields, densities and coverage vary by manufacturer, batch and site conditions. Check product labels, your supplier and your local building department before you order materials or start work. Read the full calculation disclaimer.
Accuracy
What affects your estimate
The formula is exact. The pour is not, so these are the places real jobs drift from the number on the screen.
Subgrade and forms
A slab is only as thick as the ground under it is flat. Dig 1/2 in deeper across a 12 ft × 10 ft slab and you need 120 × (0.5 ÷ 12) = 5 ft³ more, about 0.19 yd³. That is what the allowance is for.
Bag yield differs by product
The standard QUIKRETE mix yields about 0.0075 ft³ per pound of bag, but that ratio should not be carried over to specialty mixes such as fast-setting, high-early, countertop or grout. Read the yield on your own bag.
Bags or a truck
At a few hundred bags the job is mostly mixing. Ask a ready-mix supplier for a quote, and ask about minimum loads and short-load charges on small orders.
Round forms
Concrete fills the inside of a tube form, so enter its inside diameter. Measuring the outside of the cardboard adds volume that is not there, and the rounded-up bag count can change.
Slab thickness, reinforcement, sub-base and footing size depend on local code, soil, load and frost depth. Ask your local building department or a licensed engineer for dimensions. Wet cement is caustic: wear gloves and eye protection.
Common mistakes
Mixing feet and inches
Entering 4 as feet instead of inches multiplies the answer by 12. The thickness field takes inches (centimeters in metric); type 1 ft if you prefer.
Using one bag yield for every mix
An 80 lb bag of one product is 0.60 ft³. A different mix can yield less, and then you run out before the last corner.
Ordering the exact volume
With a 0% allowance, one low spot in the subgrade leaves the last stretch short. Running short mid-pour means a second delivery and a cold joint, which is a bigger problem than a slightly high order.
Questions people ask
How many bags of concrete are in a yard?
It depends on the bag. With QUIKRETE Concrete Mix #1101, one cubic yard (27 ft³) is 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb. The guide on bags of concrete has the full table. For post holes, see how much concrete a fence post needs. A pallet of that mix holds 42 bags of 80 lb, as how many bags of concrete are on a pallet explains.
How many yards of concrete for a 20×20 slab, 4 inches thick?
20 × 20 × (4 ÷ 12) = 133.33 ft³, which is 4.94 yd³. With a 10% allowance that is 5.43 yd³, so ask for 5.5 yd³.
How many bags of concrete for a 10×10 slab at 4 inches?
The slab is 33.33 ft³. At 0.60 ft³ per 80 lb bag that is 55.6, so 56 bags with no allowance and 62 bags with a 10% allowance.
What allowance should I use?
Many people start around 10%. A tight, well-prepared form needs less and an uneven excavation needs more. The slider runs from 0 to 25%.
Does this design my slab?
No. It estimates volume from the sizes you enter. Thickness, strength and reinforcement come from local code and your designer or inspector. Walls laid in block or brick are counted separately, with the concrete block calculator and the brick calculator, which also estimate mortar.