Concrete bag calculator

How this calculator works

It works out the volume of the pour (length × width × depth, times the number of identical pours), adds a waste allowance and divides by how much concrete one bag makes. The result is rounded up to whole bags, and the line also shows the count for the other bag sizes.

Yields are the standard published figures for regular concrete mix: about 0.60 cu ft per 80 lb bag, 0.45 per 60 lb and 0.30 per 40 lb. Metric bags make about 12 L per 25 kg. High-strength, fast-setting and fiber mixes can differ, so check the yield printed on the bag.

Worked example

A 4′ × 3′ landing pad 4″ deep holds 4 × 3 × 4 ÷ 12 = 4 cu ft. With 10% waste that is 4.4 cu ft. At 0.60 cu ft per 80 lb bag that is 7.33, so the list buys 8 bags. The same pour takes 10 bags of 60 lb or 15 bags of 40 lb.

Tips before you buy

• Buy one extra bag for jobs over about ten bags. Returning an unopened bag is easier than a second trip mid-pour.
• Keep bags off the ground and covered. Concrete mix that has absorbed moisture goes lumpy and loses strength.
• Add water a little at a time and follow the amount printed on the bag. Too much water makes concrete easier to pour but weaker.
• For more than about 1 cubic yard (45 bags of 80 lb), compare the cost and effort of a ready-mix delivery.

Questions

How much concrete does an 80 lb bag make?

About 0.60 cubic feet, the standard published yield for regular concrete mix. A 60 lb bag makes about 0.45 cubic feet and a 40 lb bag about 0.30. That is roughly 45, 60 and 90 bags per cubic yard.

Which bag size should I buy?

Larger bags cost less per cubic foot but are heavy to carry and mix. The 60 lb and 40 lb bags are easier to handle on your own and useful for small repairs. The amount of concrete you end up with is the same either way.

How much does a 25 kg bag of concrete make?

Pre-mixed concrete makes about 12 litres (0.012 m³) per 25 kg bag, so a cubic metre takes around 84 bags. Check the bag, because yields vary a little between mixes.