Concrete Calculator
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Enter your slab dimensions or post hole size to find out how many bags of concrete to buy. Choose your bag size — 40, 60, or 80 lb.
1. Project type
2. Dimensions
Standard slab: 4" · Driveway: 6" · Walkway: 3–4"
3. Bag size, waste & price
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How to measure your project
- 1. Slabs and pads: Measure length × width in feet. Standard thickness is 4" for patios and walkways, 6" for driveways.
- 2. Post holes: Measure or select the hole diameter and depth. A standard fence post hole is 6–8" diameter, 36" deep.
- 3. Footings: Measure the rectangular cross-section length × width × depth.
- 4. Bag size: 80 lb bags are the most efficient per bag but heaviest to carry. 60 lb bags balance size and yield. 40 lb bags are easiest to handle.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 slab needs 1.23 cubic yards — about 56 bags of 80 lb concrete, 74 bags of 60 lb, or 111 bags of 40 lb. Always add 10% extra.
How many bags of 80 lb concrete in a cubic yard?
45 bags of 80 lb concrete make one cubic yard. Each 80 lb bag yields 0.60 cubic feet (27 cu ft ÷ 0.60 = 45). For 60 lb bags: 60 per yard. For 40 lb: 90 per yard.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks, and garage floors. Driveways need 5–6 inches. Post holes for fence posts typically need 6–8 inches of concrete.
Should I use bags or ready-mix concrete?
For projects under 1 cubic yard (about 80 sq ft at 4 inches thick), bagged concrete is practical. For larger slabs, ready-mix truck delivery is significantly cheaper and faster.
Formula: Volume (cu ft) = Area × Thickness ÷ 12. Bags = Volume × (1 + waste) ÷ bag yield, rounded up. Bag yields per Quikrete specifications. See our methodology page.