Tools for working out how much concrete a job needs — slabs, footings, columns, stairs and post holes — plus the reference numbers worth having in your head before you call the supplier.
Every calculator takes feet and inches the way you'd read them off a tape, adds a waste factor, and gives you a printable ticket in bags or cubic yards.
Patios, shed floors, driveways and garage pads. The one most jobs start with.
Continuous strip footings under a wall, by trench length and cross-section.
Fence and deck posts. Subtracts the post itself from the hole volume.
Round piers and square columns, including sonotube sizes.
Poured steps from rise, run, tread count and width.
Already know your volume? Convert straight to 40, 60 and 80 lb bags.
Grid spacing, bar length and tie count for a reinforced slab.
| Thickness | Cubic ft | Cubic yd | 80 lb bags |
|---|---|---|---|
| 3 in | 25.0 | 0.93 | 42 |
| 4 in | 33.3 | 1.23 | 56 |
| 5 in | 41.7 | 1.54 | 70 |
| 6 in | 50.0 | 1.85 | 84 |
| 8 in | 66.7 | 2.47 | 112 |
Before waste. Add 5–10% for an uneven subgrade.
Bag yields: 0.60, 0.45 and 0.30 ft³. Check your own bag — yields differ by product.
Concrete is sold by volume, not weight, and in the US that means the cubic yard — 27 cubic feet. Suppliers deliver in quarter-yard increments, so a calculated 3.09 yd³ becomes an order of 3.25 yd³.
The number people get wrong is thickness. A 4 inch slab is 0.333 ft thick, and dropping that conversion inflates the volume twelvefold.
One cubic yard is 45 eighty-pound bags. That's around 3,600 lb of material to haul, open and mix. Most people switch to ready-mix somewhere around 1 yd³, though the short-load fee below 3–4 yd³ narrows the gap.
A subgrade is never perfectly level, and the low spots drink concrete. Running short mid-pour gives you a cold joint — a permanent weak seam — because the first section has already begun to set.