Concrete Bag Calculator – How Many Bags Do I Need?
Find out exactly how many bags of premixed concrete you need for a slab, footing, post holes or a column. Works with 40, 50, 60 and 80 lb bags or 20–40 kg bags and adds a waste allowance.
Formula checked 23 Oct 2026Free · no sign-up
Was this calculator helpful?
Found a bug or have a suggestion?
Leave it here and we will fix it and reply to you.
Embed this calculator on your website (free)
Copy this code into your page. It stays free and updates automatically.
How to use the concrete bag calculator
Choose Slab / footing / wall for anything rectangular, or Round post hole / column.
Enter the size. Thickness for a patio or shed base is usually 4 inches (10 cm); footings are often 8–12 inches.
Enter how many identical pours you have — for example 8 fence post holes.
Pick your bag size. The calculator knows the yield of standard 40, 50, 60 and 80 lb bags and 20–40 kg bags.
Keep the 10% extra unless your forms are perfectly level. Add a price per bag for the total cost.
Make sure all three measurements are in the same unit before multiplying. A common mistake is multiplying feet by inches: a 4-inch slab is 0.333 ft thick. The calculator converts everything for you.
Concrete bag yields
Bag size
Yield per bag
Bags per cubic yard
Bags per cubic metre
40 lb
0.30 ft³ (8.5 L)
90
118
50 lb
0.375 ft³ (10.6 L)
72
94
60 lb
0.45 ft³ (12.7 L)
60
79
80 lb
0.60 ft³ (17.0 L)
45
59
25 kg
≈ 0.41 ft³ (11.7 L)
≈ 65
≈ 85
Figures are for standard premixed concrete (about 0.0075 ft³ of concrete per pound of dry mix) before waste. Always check the yield printed on your bag.
Total weight ≈ 4,960 lb — about two and a half tons, so plan your transport.
Tips for mixing bagged concrete
Use the right water. An 80 lb bag typically needs about 3 quarts (2.8 L) of water. Too much water makes weak, crack-prone concrete.
Rent a mixer for more than about 20 bags. Hand-mixing is slow and the first batch starts setting before the last is placed.
Compact the base. 4 inches of compacted gravel under a slab reduces cracking and stops concrete disappearing into soft ground.
Cure slowly. Keep new concrete damp for 5–7 days; it gains most of its strength in the first month.
Post holes: make the hole about three times the post width and set the post one-third of its length deep, or follow local frost-depth rules.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag of premixed concrete yields about 0.60 cubic feet. A cubic yard is 27 cubic feet, so you need 27 ÷ 0.60 = 45 bags per cubic yard before adding any waste.
How many 60 lb bags are in a yard of concrete?
A 60 lb bag yields about 0.45 cubic feet, so a cubic yard needs 27 ÷ 0.45 = 60 bags. A 50 lb bag (0.375 ft³) needs 72 bags and a 40 lb bag (0.30 ft³) needs 90 bags.
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet. That is 56 bags of 80 lb mix, or 62 bags with a 10% allowance for waste and uneven ground.
How much concrete do I need for a fence post?
A 12-inch-wide hole 36 inches deep holds about 2.36 cubic feet, which is four 80 lb bags or about five 60 lb bags. Subtract the volume of the post if you want to be exact, but most people keep it as a safety margin.
Should I add extra concrete for waste?
Yes. Add 5–10% for spillage, uneven sub-base and form flex. Running short in the middle of a pour causes a weak cold joint, while a spare bag costs little.
When is ready-mix cheaper than bags?
Above roughly 1 to 1.5 cubic yards, a ready-mix truck is usually cheaper and much less work than mixing 45+ bags per yard. Use our Concrete Slab Calculator to plan a truck delivery.
Does the yield depend on the brand?
Standard premixed concrete from the major brands yields about 0.60 ft³ per 80 lb (0.0075 ft³ per lb). Fast-setting, high-strength and lightweight mixes can differ — choose “Custom yield” and enter the number printed on your bag.
How we checked this calculator
We test this tool against worked examples before every release (last check: 23 Oct 2026, version 1.0.0). Results are estimates for planning — confirm critical figures with your supplier or a qualified professional.
