Concrete Calculator
Cubic yards, cubic feet, weight, and bag count for slabs, footings, columns, and post holes. Built-in waste factor and order-rounding for ready-mix.
How to calculate concrete
Concrete is sold by the cubic yard. There are 27 cubic feet in one yard, so once you have volume in ft³ you just divide by 27. The trick is just remembering to convert your thickness from inches to feet first.
Slabs (and footings, walls — any rectangular pour)
Example: A 20 ft × 12 ft × 4" slab → (20 × 12 × 4) / 324 = 2.96 yd³. Order 3.25 yd³ to allow for waste.
Columns (round pours, sonotubes, drilled piers)
Example: A 12" diameter × 8 ft column → π × 0.5² × 8 / 27 = 0.23 yd³.
Post holes
For a fence post or mailbox post, calculate the hole volume and subtract the wood post volume so you don't over-order. A 6"-wide hole, 30" deep, with a 4×4 post takes about 0.30 ft³ of concrete — roughly half a 60-lb bag per hole.
Bag yields (manufacturer typical values)
- 80-lb bag (Quikrete / Sakrete): 0.60 ft³ (about 45 bags = 1 yd³)
- 60-lb bag: 0.45 ft³ (about 60 bags = 1 yd³)
- 40-lb bag: 0.30 ft³ (about 90 bags = 1 yd³)
Anything past about 1 cubic yard is usually cheaper to order from a ready-mix truck. Short-load fees kick in for orders under 3 yd³ — typically $80–$150 — but you still come out ahead of the bag math.
Waste — why you add it
Forms move. Subgrade isn't flat. The slab thickness varies. Some of every load sticks to the chute, the wheelbarrow, the trowel, and the rebar. Add a waste factor or you'll be stretching the last 10 minutes of pour like a magic trick.
- 5% — flat slab, set forms, level subgrade, you measured twice
- 10% — typical pour with footings, curbs, or any shape change
- 15% — irregular site, sloped grade, anything you haven't done before
Quick reference — common slab quantities
| Slab size | 4" thick | 5" thick | 6" thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 10 × 20 ft | 2.47 yd³ | 3.09 yd³ | 3.70 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 12 × 20 ft | 2.96 yd³ | 3.70 yd³ | 4.44 yd³ |
| 16 × 20 ft | 3.95 yd³ | 4.94 yd³ | 5.93 yd³ |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 20 × 24 ft (1-car garage) | 5.93 yd³ | 7.41 yd³ | 8.89 yd³ |
| 24 × 24 ft (2-car garage) | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ |
Values rounded. Add 5–10% on top for waste before ordering.