Free · runs on this device · planning numbers
Concrete Bag Calculator
Enter the pour size (or a known volume) to see how many 80, 60, and 40 lb bags of premix to buy, with waste. Also searched as "how many bags of concrete" or the 80lb concrete bag calculator.
Formula
How the Math Works
Bags = ⌈ volume ft³ × (1 + waste %) ÷ bag yield ft³ ⌉
Worked example: a 10 ft × 10 ft slab, 4 in thick, with 10% waste.
- 10 ft × 10 ft × (4 ÷ 12) ft = 33.3 ft³ (1.23 yd³)
- Add 10% waste: 33.3 × 1.10 = 36.7 ft³
- 80 lb: 36.7 ÷ 0.60 = 61.1, so 62 bags
- 60 lb: 36.7 ÷ 0.45 = 81.5, so 82 bags. 40 lb: 36.7 ÷ 0.30 = 122.2, so 123 bags
Units & assumptions
Bag yield assumption: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, and 40 lb ≈ 0.30 ft³ of mixed concrete (standard premix yields). Yield varies by brand and mix, so check the bag. Counts are always rounded up to whole bags. 1 yd³ = 27 ft³. Enter depth in inches.
Planning tool, not a PE stamp. Your spec, drawings, and supplier still win.
Field Questions
How many 80 lb bags of concrete make a yard?
About 45. One cubic yard is 27 ft³, and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45 bags before waste.
How many 60 lb and 40 lb bags are in a cubic yard?
At 0.45 ft³ per 60 lb bag, 27 ÷ 0.45 = 60 bags. At 0.30 ft³ per 40 lb bag, 27 ÷ 0.30 = 90 bags, before waste.
How many bags of concrete for a 10 × 10 slab 4 inches thick?
The slab is 33.3 ft³. That's 56 × 80 lb bags with no waste, or 62 × 80 lb bags with 10% waste (82 × 60 lb or 123 × 40 lb).
When should I order ready-mix instead of bags?
Around a cubic yard (about 45 × 80 lb bags), hand mixing gets slow and it's hard to keep the pour consistent. Many crews call a ready-mix plant or use a short-load supplier at that point. Use the cubic yard or slab calculator to size the order.