Concrete calculator: how many bags for a slab or post holes
Enter the size of a small slab, a pad or a set of post holes. You get cubic yards and the number of 80, 60 and 40 lb bags, based on the yield printed on the bag.
- Slab or post holes
- 80, 60 and 40 lb bags
- 10% extra included
Bag yields are from the Quikrete Concrete Mix data sheet: 0.60 cu ft for 80 lb, 0.45 for 60 lb and 0.30 for 40 lb. Post holes are treated as round and the post itself isn't subtracted, so you'll have a little left over. Source: Quikrete Concrete Mix data sheet.
Do the job
How it works
Slab = length × width × thickness (inches ÷ 12), in cubic feet.
Post holes = π × (width ÷ 2)² × depth for each hole, times the number of holes.
Cubic yards = cubic feet ÷ 27. 10% extra covers spills, an uneven base and forms that bow.
Bags = cubic feet with extra ÷ the bag's yield, rounded up.
About this tool
Who it's for
For a shed pad, a small walkway, a step, or setting fence and deck posts.
Example
You enterA 10 x 10 ft pad, 4 in thick
You get1.23 cubic yards: 62 bags of 80 lb, or 82 of 60 lb.
What's different
- Slabs and post holes in one tool.
- All three common bag sizes side by side.
- Bag yields from the maker's own data sheet.
Gotchas and tips
- Four inches is typical for walkways and shed pads; driveways need more and usually a pro.
- An 80 lb bag is heavy. Have the bags delivered or use 60 lb.
- Above about a cubic yard, compare the price of ready-mix delivery.
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Built and maintained by the GrabTheHammer team. Updated November 2026. Spot a problem? Tell us.
Bags for common jobs
Worked out with the same math as the calculator, including 10% extra.
| Job | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| Fence post, 10 x 24 in | 0.04 | 2 | 3 |
| 4 x 4 ft landing, 4 in | 0.20 | 10 | 14 |
| Shed pad 8 x 10 ft, 4 in | 0.99 | 49 | 66 |
| 10 x 10 ft pad, 4 in | 1.23 | 62 | 82 |
| Walkway 3 x 20 ft, 4 in | 0.74 | 37 | 49 |
| Patio 12 x 12 ft, 4 in | 1.78 | 88 | 118 |
Before you pour
- Set forms and a 4 inch bed of compacted gravel under slabs.
- Mix to a thick oatmeal texture. Too much water makes weak concrete.
- Keep a new slab damp and covered for about a week so it cures, not just dries.
- Don't pour if freezing temperatures are expected within a day or two.
Concrete questions
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet.
Bags or ready-mix?
Bags make sense for small jobs like posts, steps and small pads. Past a yard or so you're mixing dozens of heavy bags, and a ready-mix truck is often easier and not much more expensive.
How wide should a post hole be?
A common rule of thumb is about three times the post's width, so roughly 10 inches for a 4x4. Check the post or fence maker's instructions too.
How long before I can walk on it?
Usually about a day for foot traffic in mild weather, and several days before heavy loads. Follow the times on the bag, since temperature changes them.
Do I need rebar or wire mesh?
For a small walkway or pad, many people skip it or use wire mesh to help hold cracks together. Driveways and anything structural should follow local code, and are often a job for a pro.
Can I use fast-setting concrete for posts?
Yes. Fast-setting mixes are made for posts and are poured dry into the hole, then watered. Check that bag's own yield, since it can differ from regular mix.