Concrete
Concrete Bag Calculator
For small pours it is often easier to mix bagged concrete than to order ready-mix. This calculator converts your slab volume into a count of 40, 60 and 80 lb bags using typical manufacturer yields.
Formula
Volume (ft³) = length × width × thickness (in) ÷ 12 × (1 + waste%). Bags = volume ÷ bag yield, rounded up. Typical yields printed on premixed concrete bags: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
Worked example
A 10 ft by 4 ft walkway, 4 in thick: 10 × 4 × 4 ÷ 12 = 13.33 ft³, plus 10% = 14.67 ft³. That is 25 bags of 80 lb (14.67 ÷ 0.60 = 24.4), 33 bags of 60 lb, or 49 bags of 40 lb.
Assumptions and limits
- Yields are typical published values for standard premixed concrete; check your product's bag.
- One cubic yard (27 ft³) takes about 45 bags of 80 lb mix, so ready-mix usually becomes more practical above about 1 yd³.
Frequently asked questions
How many 80 lb bags make a yard of concrete?
About 45. One cubic yard is 27 ft³ and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45 bags.
Should I use bags or ready-mix?
Bags make sense for fence posts, small pads and repairs. Above roughly 1 yd³ the mixing labor and the risk of cold joints usually favor ready-mix.
Can I mix bags with less water to make it stronger?
Follow the bag's water quantity. Adding extra water weakens concrete because strength depends heavily on the water-cement ratio.