What the concrete bag calculator does
Bagged concrete mix is sold by weight, but what matters is how much finished concrete each bag makes. That figure is its yield. Yield depends on the mix and the manufacturer, which is why this calculator lets you enter it directly. It divides your project volume by the yield and rounds up, because you cannot buy part of a bag.
The bag presets use yields of the kind printed on common products (for example around 0.6 ft³ for an 80 lb bag). They are illustrative only: always check the yield on the bag you are buying, and switch to Custom yield if it differs.
How to use it
- Enter the volume of one unit (one post hole, one footing or the whole pour) in cubic feet, cubic yards, cubic metres or litres.
- Enter the number of identical units.
- Pick a bag size, or type the yield from the bag label.
- Optionally add an extra allowance and the price per bag.
Concrete bag formula
Total volume = Volume per unit × Number of units × (1 + Extra % ÷ 100)
Bags = Total volume ÷ Yield per bag, rounded up
Left over = Bags × Yield − Total volume
Volume and yield are converted to the same unit before dividing. 1 cubic yard = 27 cubic feet; 1 cubic foot = 28.32 litres.
Worked examples
Eight fence post holes, 0.5 ft³ of concrete each, using 80 lb bags (≈ 0.6 ft³ yield):
- Total volume: 0.5 × 8 = 4.0 ft³
- Bags: 4.0 ÷ 0.6 = 6.67 → 7 bags
- Supplied: 7 × 0.6 = 4.2 ft³, leaving 0.2 ft³ spare
One cubic yard needs 27 ÷ 0.6 = 45 bags at 0.6 ft³ per bag, or 27 ÷ 0.45 = 60 bags at 0.45 ft³.
Metric: 0.25 m³ (250 litres) using 20 kg bags at an assumed 9.5 L yield needs 250 ÷ 9.5 = 26.3 → 27 bags.
Understanding the results
- Bags needed – always rounded up to a whole bag.
- Concrete needed – your total volume including any extra allowance.
- Supplied and left over – how much concrete the bags make in total, and the surplus after rounding.
- Total weight – for presets, bag weight × number of bags; useful for planning transport.
Tips for working with bagged concrete
- Add water as directed on the bag; extra water weakens concrete.
- Check your vehicle’s load limit. 45 bags of 80 lb weigh 3,600 lb (about 1,630 kg).
- Buy one or two extra bags; unopened bags can usually be returned or kept dry for later use.
- Post-setting and fast-setting mixes can have different yields from general-purpose mixes.
Bags or ready-mix?
Bags suit small jobs: post holes, steps, small pads and repairs. As the volume grows, mixing many bags by hand becomes slow and makes it hard to place the concrete before it starts to set. For larger pours, compare the bag cost with a ready-mix delivery, including any short-load charge.
Work out the volume first with the concrete calculator (round holes and footings) or the concrete slab calculator. For fence projects, the fence calculator gives the number of posts.