Concrete Calculator
Estimate the concrete for a slab, footing, or post hole. Get cubic yards for a ready-mix order and the number of 40, 50, 60, 80, or 90 lb bags for a mix-it-yourself job.
Concrete calculator
Sidewalks: at least 4 in. Small slabs: 4 to 6 in. Footings and holes: use the size your plans or local code call for.
The sizes below are an example. Type over them with your own.
Allowances
Leave blank to use 10%. NRMCA suggests 4 to 10% over plan dimensions for waste and over-excavation. DIY pours usually need the top of that range.
Cost (optional)
The cost is your price times the rounded amount in the result, plus delivery. Tax and minimum-order charges are not included.
- Area
- 100 sq ft
- Volume
- 33.3 cu ft1.23 cu yd, before the extra
- Extra
- 10%Adds 3.3 cu ft
- Order quantity
- 1.36 cu yd36.7 cu ft
- Bags
- 62 bags80 lb bag (0.60 cu ft), rounded up
- Other bag sizes
- 40 lb bags: 123, 50 lb bags: 98, 60 lb bags: 82, 90 lb bags: 55
How to use the concrete calculator
- For a slab, enter the length and width, and the thickness as the depth (4 inches is typical for a sidewalk). For a round footing, hole, or column, choose Circle, enter the diameter, and enter the depth in feet.
- Use Quantity for identical shapes, such as a row of fence-post holes, and Add another area for different ones.
- Choose your bag size. The 80 lb bag is selected by default.
- Read the cubic yards (for ready-mix) and the bag count (for mixing it yourself).
Volume only. This calculator does not size structural work. Footings, foundations, and anything that carries a load should follow your plans, local building code, and a licensed professional.
The concrete formula
Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27
Cubic feet = 3.1416 × (diameter ÷ 2)² × depth, all in feet
Order = volume × (1 + extra % ÷ 100)
Bags = cubic feet ÷ yield per bag, rounded up to a whole bag
Worked example: a 10 ft by 10 ft slab, 4 inches thick
- Volume: 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, or 1.23 cubic yards.
- Add 10%: 33.33 cubic feet becomes 36.67. Divided by 27, that is 1.358, shown as 1.36 cubic yards.
- 80 lb bags yield 0.6 cubic foot: 36.67 ÷ 0.6 = 61.11, rounded up to 62 bags.
- With 60 lb bags (0.45 cubic foot) the same slab takes 82 bags.
Exact fits. A 12 ft by 12 ft slab 4 inches thick is exactly 48 cubic feet, which is exactly 80 bags of 80 lb mix when the extra is set to 0. The calculator only rounds up when a real fraction of a bag is left over.
A round hole. A hole 12 inches across and 4 feet deep holds 3.1416 × 0.5 × 0.5 × 4 = 3.14 cubic feet. With 10% extra that is 3.46 cubic feet, or 5.76 bags, rounded up to 6 bags of 80 lb mix.
How much concrete is in a bag?
Bag yields come from the manufacturer. Quikrete lists the yield of its Concrete Mix as follows.
| Bag size | Yield (cu ft) | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
| 90 lb | 0.675 | 40 |
Yields are the manufacturer's approximate figures from the Quikrete Concrete Mix data sheet. The 90 lb bag is available only in some regions. Other brands differ slightly, so check the bag.
Slab sizes at a glance
These figures include the default 10% extra. Bags are rounded up to whole bags, and cubic yards are shown to two decimals.
| Slab size | Cubic yards | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 4 × 4 ft | 0.22 | 14 | 10 |
| 5 × 10 ft | 0.68 | 41 | 31 |
| 10 × 10 ft | 1.36 | 82 | 62 |
| 12 × 12 ft | 1.96 | 118 | 88 |
| 20 × 20 ft | 5.43 | 326 | 245 |
Thicker slabs scale in proportion. A 6-inch slab needs 1.5 times as much as a 4-inch slab.
Post holes, tubes, and columns
For round shapes, bags per foot of depth depend only on the diameter and the bag size.
| Bag size | 6 in hole | 8 in hole | 10 in hole | 12 in hole |
|---|---|---|---|---|
| 40 lb | 0.7 | 1.2 | 1.8 | 2.6 |
| 50 lb | 0.5 | 0.9 | 1.5 | 2.1 |
| 60 lb | 0.4 | 0.8 | 1.2 | 1.7 |
| 80 lb | 0.3 | 0.6 | 0.9 | 1.3 |
No extra added. Multiply by the depth in feet and round up. These figures match the table in Quikrete's Quik-Tube guide.
How thick should a slab be?
Quikrete's guide to sidewalks and small slabs says a sidewalk should be at least 4 inches thick, and that simple slabs for small foundations and bases are normally 4 to 6 inches thick, depending on the load. For larger jobs or poor drainage it adds several inches of gravel or crushed stone under the slab. Its Concrete Mix data sheet describes the product for pours 2 inches thick or more.
A slab has low tensile strength and can crack unless control joints are cut, so plan the joints and the curing before you pour. Quikrete's guide covers both, and the instructions on the mix you buy come first.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
It takes 45 bags of 80 lb mix to make one cubic yard, because each bag yields about 0.6 cubic foot and a cubic yard is 27 cubic feet. For 60 lb bags the count is 60, and for 40 lb bags it is 90. Round up and add a margin for waste.
How do I calculate concrete for a slab?
Multiply length by width (in feet) by thickness (in inches), divide by 12 for cubic feet, and divide by 27 for cubic yards. A 10 ft by 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra the order is 1.36 cubic yards, or 62 bags of 80 lb mix.
How much extra concrete should I order?
The National Ready Mixed Concrete Association (NRMCA) recommends an allowance of 4 to 10% over plan dimensions for waste, over-excavation, and similar causes. Small errors add up: a ⅛-inch error in a 4-inch slab is a shortage of about 3%. NRMCA adds that one-off jobs need a bigger allowance than repetitive ones, so the calculator defaults to 10%, the top of the range. Lower it if your forms and subgrade are accurate.
How many bags of concrete do I need for a post hole?
Use the table above. A 12-inch hole needs about 1.3 bags of 80 lb mix per foot of depth, so a hole 4 feet deep calculates to 3.1 cubic feet, or 6 bags once you add 10% extra. For several identical holes, enter the number under Quantity: six 12-inch holes 3 feet deep come to 14.1 cubic feet, or 26 bags with the extra.
Should I buy bags or order ready-mix?
Ready-mixed concrete is sold by the cubic yard, as NRMCA explains, and a truck delivers it mixed. With bags, one cubic yard means 45 bags of 80 lb mix, or 3,600 lb to lift and mix. Compare the cost of bags against the ready-mix quote including delivery, and ask the supplier about minimum orders and small-load charges.
How much does a cubic yard of concrete weigh?
Quikrete lists a unit weight of about 140 lb per cubic foot for its mixed Concrete Mix. A cubic yard is therefore about 3,780 lb, or 1.89 tons. Check this against what your truck or trailer can carry.