How to use it
- Measure inside the forms. Length and width are the inside faces of the boards, not the outside.
- Measure thickness in several places. Ground is rarely flat. Check the depth from the top of the form to the base at the corners and the middle, and use the average — a low middle can add more concrete than you’d guess.
- Keep or change the extra. The default is 10%. Lower it for a tight, well-compacted form; raise it for rough ground.
- Add prices if you want a cost. The defaults are a real 80 lb bag price and the US average for ready-mix — replace them with your local quote.
Every input is kept in the page address, so you can bookmark a result or send it to whoever is quoting the job.
The formula
Thickness is usually measured in inches, so divide by 12 first: 4 in is 0.333 ft. For a round slab, use π × radius² × thickness instead of length × width.
Worked example: a 12 × 10 ft patio, 4 in thick
- 12 × 10 × 0.333 = 40 ft³, which is 1.48 cubic yards.
- With 10% extra: 44 ft³, or 1.63 yd³.
- In 80 lb bags (0.60 ft³ each): 44 ÷ 0.60 = 73.3, so 74 bags. In 60 lb bags: 98.
- At $6.47 a bag that’s about $479 of bagged mix; 1.63 yards of ready-mix at $180 is about $293 before delivery and short-load fees.
Why the extra matters
The National Ready Mixed Concrete Association lists the usual reasons a pour comes up short: spillage, over-excavation, an uneven subgrade, and forms that spread or bow under the weight of wet concrete. Concrete also loses about 2% of its volume between fresh and hardened. Its advice is to order 4–10% more than the calculated volume. Sakrete suggests 10% for bagged jobs, which is this calculator’s default.
How thick should it be?
The residential code (IRC R506.1) sets 3½ in as the minimum for concrete floors on ground, garages included, and calls for a 4 in base course under slabs that sit below grade. Beyond that, guidance depends on use:
| Use | Typical thickness | Source |
|---|---|---|
| Patios and walks | 3½–4 in | Ohio Ready Mixed Concrete Assoc. |
| Driveway, cars only | 4 in minimum | Michigan Concrete Assoc. |
| Driveway with delivery vehicles | 5 in minimum | Michigan Concrete Assoc. |
| Slab floors on ground | 3½ in minimum | IRC R506.1 |
The two associations disagree slightly on driveways — Ohio treats 3½ in as acceptable, Michigan asks for 4 — so check what your local code and supplier expect. More detail is in the slab thickness guide.
Bags or a truck?
At one store’s October 2026 prices, a cubic yard of 80 lb bags (45 bags) costs about $291 — against a 2025 US ready-mix average of about $180 a yard. Bags win on small jobs because ready-mix suppliers add fees to short loads; a truck wins once mixing by hand becomes the bottleneck. A pallet of 42 × 80 lb bags is 25.2 ft³, a little under a yard, and every bag is mixed by hand. The full comparison is in bagged vs ready-mix.
Before you order
Ask your supplier for their minimum load, short-load fee and how they round partial yards. Those three numbers decide the real cost far more than the price per yard.
Questions people ask
How many bags of concrete for a 10 × 10 slab?
At 4 in thick a 10 × 10 ft slab is 33.3 cubic feet. With 10% extra that’s 36.7 ft³ — 62 bags of 80 lb mix or 82 bags of 60 lb mix. With no extra at all it’s 56 bags of 80 lb, which leaves nothing for spills or a low spot in the base.
How many cubic yards is a 20 × 20 slab?
At 4 in thick: 20 × 20 × 0.333 = 133.3 ft³, which is 4.94 cubic yards. Add 10% and order about 5.4 yards. At 6 in thick the same slab is 7.4 yards before the extra.
How much area does one cubic yard of concrete cover?
A cubic yard is 27 cubic feet, so it covers 81 ft² at 4 in thick, about 93 ft² at 3½ in and 54 ft² at 6 in. Divide 27 by the thickness in feet to get the area for any other thickness.
Does rebar or wire mesh reduce the concrete I need?
Not by enough to matter. #4 bar at 12 in each way in a 4 in slab takes up under 1% of the volume. Order as if the slab were solid concrete.
Should I round my ready-mix order up or down?
Up. Running short mid-pour is the expensive mistake: you wait for a second truck while the first load starts to set, and the two meet at a cold joint. A few spare cubic feet can always fill a post hole or a step.
Sources for the numbers on this page
- 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
- National Ready Mixed Concrete Association — CIP 8 — Discrepancies in yield (ordering 4–10% extra) · checked October 1, 2026
- National Ready Mixed Concrete Association — CIP 36 — Structural lightweight concrete (unit weights) · checked October 1, 2026
- ConcreteNetwork.com — Concrete prices — national ready-mix average ($179.95 per cubic yard, 2025) · checked October 1, 2026
- The Home Depot — Quikrete Concrete Mix, 80 lb — shelf price at one US store · checked October 1, 2026
- State of Ohio (adopted IRC text) — Residential Code of Ohio 4101:8-5-01 — R506.1 concrete floors on ground (3½ in minimum) · checked October 1, 2026
- Ohio Ready Mixed Concrete Association — Guidelines for durable driveways, patios and walks · checked October 1, 2026
- Michigan Concrete Association — Driveway and sidewalk guidelines · checked October 1, 2026
Corrections and updates are logged on the sources page.