Concrete Bag Calculator
Work out cubic yards and the number of 40, 50, 60, or 80 lb bags for a slab, footing, post hole, or column. Add a waste allowance before you head to the store.
How to calculate concrete volume
Concrete is sold by volume. Measure everything in feet, convert inches to feet by dividing by 12, then use the formula for your shape. Divide cubic feet by 27 to get cubic yards.
Length × width × thickness in feet = cubic feet.
Length × (width ÷ 12) × (depth ÷ 12) = cubic feet.
π × (diameter ÷ 24)² × depth in feet × number of holes = cubic feet.
Bag yields and coverage
| Bag size | Yield | Bags per cubic yard | Typical use |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | Small repairs, post holes |
| 50 lb | 0.375 cu ft | 72 | Fence posts, small pads |
| 60 lb | 0.45 cu ft | 60 | Walkways, small slabs |
| 80 lb | 0.60 cu ft | 45 | Slabs, footings, larger pours |
Yields vary slightly by mix. Always check the bag, especially for fast-setting or high-strength concrete.
Common slab thicknesses
| Project | Typical thickness | Reinforcement |
|---|---|---|
| Garden walkway | 4 in | Mesh optional |
| Patio | 4 in | Mesh recommended |
| Driveway | 5–6 in | Rebar or mesh |
| Shed base | 4–6 in | Mesh recommended |
| Garage floor | 4–6 in | Rebar recommended |
Thicker is not always better if the base is poor. Compact the subbase, provide drainage, and use control joints on slabs larger than about 10 feet.
Bags vs. ready-mix
| Project | Volume | 80 lb bags (with 10% waste) | Best option |
|---|---|---|---|
| 10 × 10 slab, 4 in | 1.23 cu yd | 62 | Bags or small ready-mix load |
| 20 × 20 slab, 4 in | 4.94 cu yd | 245 | Ready-mix |
| 12 in post hole, 30 in | 0.07 cu yd | 4 per hole | Bags |
| 16 in footing, 12 in deep, 40 ft | 1.98 cu yd | 98 | Compare quotes |
Ready-mix typically costs $140–$190 per cubic yard, plus delivery and short-load fees. Bags cost more per yard but let you work in stages and avoid truck access problems.
How much does concrete cost?
Bag prices vary by mix and store. Expect roughly $3.50–$4.50 for a 40 lb bag, $4.50–$5.50 for 60 lb, and $5.50–$6.50 for 80 lb. The calculator uses mid-range 2026 prices, then rounds the result.
Pouring a fence? Thefence material calculatorestimates concrete bags per post along with pickets, rails, and posts. For slab costs, see concrete slab cost.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 slab at 4 inches thick needs about 1.23 cubic yards, or 56 bags of 80 lb concrete before waste. With a 10% waste allowance, plan for 62 bags of 80 lb, 82 bags of 60 lb, or 123 bags of 40 lb.
How many 80 lb bags of concrete are in a cubic yard?
One 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so you need about 45 bags of 80 lb concrete per cubic yard. A 60 lb bag needs about 60 bags per yard, and a 40 lb bag needs about 90.
How much concrete does a 12-inch post hole need?
A 12-inch diameter hole, 30 inches deep, holds about 1.96 cubic feet. That is roughly 4 bags of 80 lb concrete with a 10% waste allowance. A 10-inch by 30-inch hole needs about 3 bags of 80 lb.
Is it cheaper to mix bags or order ready-mix concrete?
Bags are usually cheaper for small pours under about 1 cubic yard. Above 1 to 2 cubic yards, ready-mix is often faster and can be cheaper per yard, but watch for short-load fees, delivery minimums, and limited access for the truck.
How much water do I add to a bag of concrete?
Most 80 lb bags need about 2.5 to 3 quarts of water, but always follow the bag instructions. Add water gradually and aim for a workable, not soupy, mix. Too much water weakens the concrete.
How long does concrete take to cure?
You can usually walk on a new slab after 24 to 48 hours, but keep heavy loads off it for about 7 days. Full design strength develops over roughly 28 days. Cold or wet weather slows curing.