1. Measure what you’ll actually fill
- Measure inside the forms, not the outside of the boards. On a 10 × 10 ft slab, measuring outside 2× forms adds 3 in each way — about 5% more concrete than you need.
- Measure depth in several places. Check corners and the middle, then average. A base that dips an inch in the middle of a 4 in slab adds a quarter to the volume over that area.
- Break odd shapes into simple ones. An L-shaped patio is two rectangles; a rounded end is half a circle. Work each out and add them.
2. Convert to feet and multiply
Turn every measurement into feet first — inches ÷ 12 — then use the shape’s formula:
The answer is in cubic feet. Divide by 27 for cubic yards, which is how ready-mix is sold.
Quick reference: cubic yards per 100 square feet
| Thickness | Cubic yards per 100 ft² | 80 lb bags per 100 ft² |
|---|---|---|
| 3½ in | 1.08 | 49 |
| 4 in | 1.23 | 56 |
| 5 in | 1.54 | 70 |
| 6 in | 1.85 | 84 |
Before any extra. Bag counts use 0.60 ft³ per 80 lb bag and are rounded up.
3. Add a margin
The National Ready Mixed Concrete Association recommends ordering 4–10% more than the calculated volume. Its reasons: spillage, over-excavation, an uneven subgrade, forms that spread or bow, and the roughly 2% that concrete shrinks between fresh and hardened. Sakrete suggests 10% for bagged jobs.
Use the low end for a small, well-formed job on a compacted base; the high end for rough ground or a trench.
4. Turn it into an order
For bags, divide the volume by the yield on the bag — 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb — and round up. For ready-mix, give the supplier the cubic yards and ask how they round partial yards and what the short-load fee is. The bagged vs ready-mix guide covers where one becomes cheaper than the other.
Worked example: an L-shaped patio
- Split it into a 12 × 10 ft rectangle and a 6 × 4 ft rectangle: 120 + 24 = 144 ft².
- At 4 in (0.333 ft): 144 × 0.333 = 48 ft³, or 1.78 cubic yards.
- Add 10%: 52.8 ft³, or 1.96 cubic yards.
- In 80 lb bags: 52.8 ÷ 0.60 = 88 bags. That much hand mixing is a strong case for ready-mix.
The expensive mistake is running short
Extra concrete fills a post hole or a step. Missing concrete means a second trip or truck while the first batch starts to set — and a cold joint where the two meet.
Sources for the numbers on this page
- National Ready Mixed Concrete Association — CIP 8 — Discrepancies in yield (ordering 4–10% extra) · checked October 1, 2026
- Quikrete — Concrete Mix No. 1101 — technical data sheet (bag yields, strength gain, curing) · checked October 1, 2026
- Sakrete — High-Strength Concrete Mix — technical data sheet · checked October 1, 2026
- Sakrete — Estimating bagged concrete jobs · checked October 1, 2026
Corrections and updates are logged on the sources page.