Concrete Calculator
A concrete calculator multiplies length × width × depth (ft) and divides by 27 for cubic yards. A 10 × 10 ft pour 4 in thick needs 1.23 yd³, or 1.36 with 10%.
How much concrete you need
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard (27 cu ft) takes about 45 bags. Divide the pour’s volume in cubic feet by the yield printed on your bag.
A concrete calculator finds the volume of a pour in cubic yards, the unit ready-mix suppliers sell by. Multiply length × width × depth, all in feet, then divide by 27 because one cubic yard holds 27 cubic feet. Cubic yards = L × W × (depth in ÷ 12) ÷ 27. A 10 × 10 ft slab 4 inches thick needs 1.23 yd³, or 1.36 yd³ with the 10% allowance.
The cost line multiplies cubic yards by your price per yard. Tick the 10% box and the allowance is included in both figures.
One formula, three kinds of pour
The three inputs are just the three sides of a box, so they work for more than slabs:
- Slab, patio, or pad: length and width of the surface, depth = thickness.
- Continuous footing: length of the run, width of the trench in feet, depth = footing height in inches.
- Wall: length of the wall, wall height entered as “width” in feet, depth = wall thickness in inches.
Run each part of a job separately and add the yards. A garage might be a slab entry, a footing entry, and a thickened-edge entry.
Worked examples
Example 1: Patio slab, 10 × 10 ft, 4 in, $150 per yd³
- Volume: 10 × 10 × 0.333 = 33.3 ft³ ÷ 27 = 1.23 yd³
- With 10% extra: 1.36 yd³
- Cost: 1.358 × $150 = $203.70 ($185.19 without the allowance)
Example 2: Two-car driveway, 24 × 20 ft, 5 in, $150 per yd³
- Volume: 24 × 20 × (5 ÷ 12) = 200 ft³ ÷ 27 = 7.41 yd³
- With 10% extra: 8.15 yd³, costing $1,222.22
Example 3: Basement wall, 30 ft long, 8 ft tall, 8 in thick
- Enter length 30, width 8, depth 8
- Volume: 30 × 8 × 0.667 = 160 ft³ ÷ 27 = 5.93 yd³
- With 10% extra: 6.52 yd³
Most suppliers sell in quarter- or half-yard steps. Round the result up to their increment.
Cubic yards per 100 sq ft
| Thickness | yd³ per 100 sq ft | 80-lb bags per 100 sq ft | Typical use |
|---|---|---|---|
| 4 in | 1.23 | 56 | Patios, walks, shed floors |
| 5 in | 1.54 | 70 | Driveways for cars |
| 6 in | 1.85 | 84 | Heavier driveways, garage floors |
| 8 in | 2.47 | 112 | Thick pads, light equipment |
| 12 in | 3.70 | 167 | Footings, heavy pads |
Bag counts use about 0.6 ft³ per 80-lb bag and are rounded up. Net figures, before any allowance.
How to measure
- Measure inside the forms. The forms set the true size of the pour.
- Check depth in several places. Subgrades are rarely flat. If one corner is 5 inches and another 3, use the average, then keep the allowance on.
- Use the concrete thickness, not the dig depth. The excavation also holds the gravel base.
- Add thickened edges separately. A slab with a 12-inch-deep perimeter is two entries: the slab, plus the edge strip.
Common mistakes
- Entering depth in feet. The depth box takes inches; 4 means 4 inches.
- Ordering the exact volume. Running out mid-pour leaves a cold joint. A small leftover is the cheaper mistake.
- Ignoring form bulge. Tall wall forms can bow outward under wet concrete and take more than the drawing says.
- Mixing up bags and yards. A 1.36 yd³ job is about 62 bags of 80 lb, which is a lot of hand mixing. Compare both with the concrete bag calculator.
Related calculators
For a single shape with extra outputs, use the concrete slab calculator, the concrete footing calculator, or the sonotube calculator for round piers. Reinforcement is sized with the rebar calculator. For footings, walls, and other load-bearing pours, have the design checked by a licensed engineer or your building department.
Frequently asked questions
How much concrete do I need for my project?
Multiply length × width × depth, all in feet, and divide by 27 to get cubic yards. A 10 × 10 ft slab at 4 inches is 100 × 0.333 = 33.3 ft³, or 1.23 yd³. Add 5 to 10% for an uneven subgrade and spillage, which brings it to about 1.36 yd³.
Can I use this calculator for walls and footings?
Yes, for any rectangular block of concrete. For a wall, enter the length, the wall height as the width in feet, and the wall thickness as the depth in inches. A 30 ft wall 8 ft tall and 8 inches thick is 5.93 yd³. For round columns or post holes, a cylinder calculator is more accurate.
How many bags of concrete do I need?
Divide the pour's volume in cubic feet by the bag's yield. An 80 lb bag yields about 0.6 cu ft, so a 10 × 10 ft slab 4 inches thick (33.3 cu ft) needs about 56 bags. A 60 lb bag yields about 0.45 cu ft, so the same slab needs about 74.
How many bags of concrete equal a cubic yard?
A cubic yard is 27 cubic feet. An 80-lb bag yields about 0.6 ft³, so you need 45 bags per yard; a 60-lb bag yields about 0.45 ft³, so you need 60. Beyond about 1 cubic yard, ready-mix is usually easier than mixing bags.
Does the cost estimate include labor, forms, rebar, or a gravel base?
No. The cost is cubic yards × the price per yard you enter, which is the concrete only. Forming, finishing labor, reinforcement, the compacted base, delivery fees, and pump hire are separate.
What depth should I use for a typical concrete pour?
Four inches is common for patios, walks, and shed floors, and 5 to 6 inches for driveways that carry cars. Footings and foundation walls are sized by local code or an engineer based on the load and soil, so do not guess on structural pours.
Estimates only. Verify quantities with your supplier before purchasing.