Concrete Calculator
Calculate exactly how much concrete you need for a slab, footing, post hole, column or wall — in cubic yards, cubic feet and bags, with cost estimates.
A 10×10 ft slab at 4 in thick needs about 1.36 cubic yards of concrete (1.23 yd³ without waste — 56 bags of 80 lb mix), roughly 62 bags of 80 lb with waste.
How to Use This Concrete Calculator
- Choose the shape that matches your project: slab, footing, wall, post hole or column.
- Enter the dimensions — length and width in feet, thickness (or depth) in inches.
- Set the waste percentage (10% is a good default for most projects).
- Add your local price per cubic yard if you want a cost estimate.
- Read the results: cubic yards to order from a ready-mix plant, or bags of 40/50/60/80 lb to mix yourself.
The Formula
Concrete volume is simple geometry. For a slab, footing or wall, multiply length × width × thickness. For a post hole, subtract the post volume from the hole volume. Convert to cubic yards by dividing by 27 (one cubic yard = 27 cubic feet), add your waste percentage, then divide by the yield of the bag size you plan to use.
V = L × W × T Volume of a slab, footing or wall, in cubic feet (T in feet) V = π × r² × H Volume of a column or circular pad V = (π·D²/4 − π·d²/4) × depth × N Post hole: hole minus post, times the number of posts yd³ = ft³ ÷ 27 One cubic yard equals 27 cubic feet Bags = V ÷ yield 40 lb = 0.30 ft³ · 50 lb = 0.375 ft³ · 60 lb = 0.45 ft³ · 80 lb = 0.60 ft³ per bag Worked Example: 10×10 Slab
A 10 ft × 10 ft slab at 4 in thick: V = 10 × 10 × (4 ÷ 12) = 33.3 ft³. That is 33.3 ÷ 27 = 1.23 cubic yards without waste. With the default 10% waste you get 1.36 yd³ — about 62 bags of 80 lb mix, or 1.5 yards to order from the plant (rounded up to the nearest quarter yard).
Concrete Needed for Common Slab Sizes
| Slab size | Thickness | Cubic yards (10% waste) | 80 lb bags |
|---|---|---|---|
| 8 × 10 ft | 4 in | 1.09 yd³ | 49 bags |
| 10 × 10 ft | 4 in | 1.36 yd³ | 62 bags |
| 10 × 12 ft | 4 in | 1.63 yd³ | 74 bags |
| 12 × 12 ft | 4 in | 1.96 yd³ | 88 bags |
| 20 × 20 ft | 4 in | 5.43 yd³ | 245 bags |
| 10 × 10 ft | 6 in | 2.04 yd³ | 92 bags |
| 12 × 12 ft | 6 in | 2.93 yd³ | 132 bags |
Yards and bags include a 10% waste factor. Bag counts use 80 lb mix (0.60 ft³ per bag).
Ready-Mix vs Bags
For small jobs — walkways, small patios, or setting posts — bags are the practical choice: a 10×10 slab needs about 62 bags of 80 lb mix. For anything larger, ready-mix from a local plant is cheaper and faster: delivered concrete averaged about $180 per cubic yard nationally in 2025 (range $160–195 depending on region), and ordering by the yard avoids mixing dozens of bags by hand. Ready-mix orders are typically rounded up to the nearest quarter yard.
How Thick Should a Concrete Slab Be?
4 inches is the standard thickness for residential slabs — patios, walkways and driveways. Garage floors and heavy-duty driveways typically use 5–6 inches, and commercial applications 6–8 inches or more. Footings are sized by soil and load; a common minimum for light residential walls is 12 in wide by 6 in thick.
Quick Tips
- Overestimate your order — Quikrete and Concrete Network both recommend ordering extra so you never come up short on the jobsite (5–10% is the usual guidance).
- For setting posts, use fast-setting mix in 50 lb bags and make the hole about 3× the post diameter.
- One 80 lb bag yields about 0.60 cubic feet — 45 bags make a full cubic yard.
- For a 4 in slab, divide the square footage by 81 for a quick yardage estimate.
Frequently Asked Questions
How many 80 lb bags of concrete are in a yard?
45. One cubic yard equals 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet (27 ÷ 0.60 = 45).
How many 60 lb bags of concrete are in a yard?
60. A 60 lb bag yields about 0.45 cubic feet (27 ÷ 0.45 = 60).
How much concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 in thick needs 1.23 cubic yards without waste — about 56 bags of 80 lb mix. With the usual 10% waste, order 1.36 cubic yards or 62 bags.
How many bags of concrete do I need for a 4×4 fence post?
For a 6 ft fence post with a hole about 8 in wide and 24–30 in deep, plan on 2 bags of 50 lb fast-setting concrete per post. The hole should be about 3× the post diameter.
What does the 1-2-4 mix ratio mean?
It is the classic nominal mix by volume: 1 part cement, 2 parts sand, 4 parts coarse aggregate. Modern bagged concrete mixes already contain the correct proportions — just add water.