Concrete Calculator
Work out exactly how much concrete you need for a slab, footing, column, or fence posts — in cubic yards and bags — before you buy a pound more than the job takes.
4″ is standard for patios and walkways; 4–6″ for driveways.
Add up all runs of footing for the project.
Sonotube sizes: 8″, 10″, 12″ are most common.
Rule of thumb: 3× the post width.
Typically ⅓ of the above-ground post height, below frost line.
A “4×4” post actually measures 3.5″.
You need
Enter dimensions
| Bagged mix | Bags needed |
|---|---|
| 80 lb bags | — |
| 60 lb bags | — |
| 50 lb bags | — |
| 40 lb bags | — |
Ready-mix commonly runs about $125–$165 per yard delivered; call local suppliers for real quotes.
How to Calculate Concrete
Concrete is ordered by volume — in the US, by the cubic yard (27 cubic feet). Every shape on this page comes down to the same three steps:
- Measure in one unit. Convert everything to feet (divide inches by 12).
- Find the volume. Multiply the dimensions for your shape (formulas below).
- Convert to yards and add waste. Divide cubic feet by 27, then add 5–10% so you don't run short mid-pour.
Concrete volume formulas
Slab or footing: volume = length × width × thickness
Round slab or column: volume = π × (diameter ÷ 2)² × depth
Post hole: volume = hole volume − post volume, × number of posts
Worked example — 10 × 10 ft patio, 4″ thick: 10 × 10 × 0.333 = 33.3 cu ft. Divide by 27 → 1.23 cubic yards. With 5% waste → 1.30 yards, or about 59 bags of 80 lb mix.
How Many Bags of Concrete Do You Need?
Bagged concrete mix yields a fixed volume per bag once mixed with water. These yields come from manufacturer specifications (Quikrete/Sakrete standard mix):
| Bag size | Yield per bag | Bags per cubic yard | Bags per cu ft |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3.3 |
| 50 lb | 0.375 cu ft | 72 | 2.7 |
| 60 lb | 0.45 cu ft | 60 | 2.2 |
| 80 lb | 0.60 cu ft | 45 | 1.7 |
Always round up to whole bags. An unopened spare bag returns to the store; a half-poured slab doesn't.
Bags vs. Ready-Mix: Which Should You Order?
The break-even point is around one cubic yard:
- Under 1 yard — bagged mix wins. A yard takes ~45 bags of 80 lb mix; mixing that by hand or in a small mixer is a full, hard day.
- 1 yard or more — ready-mix delivery usually wins on both effort and price, typically $125–$165 per yard delivered. Note that most plants charge a short-load fee for orders under 3–4 yards.
- In between — some suppliers offer towable mixing trailers (about 1 yard) that split the difference.
Tips Before You Pour
- Order for the measured slab, not the plan. Measure formed dimensions after forms are set — forms move.
- Check thickness in several spots. Subgrade high spots silently eat volume; that's what the waste factor is for.
- Concrete sets on its schedule, not yours. Have help, tools, and finishing gear staged before the truck arrives.
- Cold joints weaken slabs. If in doubt between two quantities, order the larger.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4″ thick, a 10 × 10 slab is 33.3 cu ft (1.23 cubic yards) — about 56 bags of 80 lb mix, 74 of 60 lb, or 112 of 40 lb, before adding 5–10% for waste.
How many 80 lb bags make a yard of concrete?
One 80 lb bag yields about 0.60 cu ft, so roughly 45 bags make one cubic yard.
Should I order ready-mix or use bags?
At about 1 cubic yard or more, ready-mix is usually less work and competitive on price (watch for short-load fees). Under a yard, bags are the practical choice.
How thick should a concrete slab be?
4″ for patios and walkways, 4–6″ for passenger-car driveways, 6″+ where heavy vehicles park — always over a compacted base, and local code governs.
How much concrete per fence post?
A 4×4 post in a 10″ hole, 24″ deep, needs about 0.85 cu ft after subtracting the post — about 1½ bags of 80 lb mix per post.
Why add extra for waste?
Uneven subgrade, flexing forms, and mix left in the equipment all consume concrete. 5–10% extra is standard practice; running short mid-pour creates a weak cold joint.
References
- Quikrete — Concrete Mix product data sheet (bag yields): quikrete.com
- Portland Cement Association — concrete basics and placing guidance: cement.org
- American Concrete Institute — residential concrete guidance (ACI 332): concrete.org