How many bags of concrete do I need? Quick answers
Bags needed for common 4-inch slabs, including 10% extra for waste, rounded up to whole bags. For other sizes and thicknesses, use the calculator above.
| Slab (4 in thick) | Cubic yards | 40 lb bags | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 4 × 4 ft | 0.22 | 20 | 16 | 14 | 10 |
| 4 × 8 ft | 0.43 | 40 | 32 | 27 | 20 |
| 6 × 6 ft | 0.49 | 44 | 36 | 30 | 22 |
| 8 × 8 ft | 0.87 | 79 | 63 | 53 | 40 |
| 10 × 10 ft | 1.36 | 123 | 98 | 82 | 62 |
| 10 × 12 ft | 1.63 | 147 | 118 | 98 | 74 |
| 12 × 12 ft | 1.96 | 176 | 141 | 118 | 88 |
| 10 × 20 ft | 2.72 | 245 | 196 | 163 | 123 |
| 12 × 20 ft | 3.26 | 294 | 235 | 196 | 147 |
| 20 × 20 ft | 5.43 | 489 | 392 | 326 | 245 |
How the concrete calculator works
First the calculator finds the volume of your pour in cubic feet, then adds your waste allowance, then divides by how much concrete one bag makes. Bags are always rounded up, because a store won't sell you 0.4 of a bag.
Slab or footing: length × width × thickness (ft) = cubic feet
Column: π × (diameter ÷ 2)² × height × count − post volume
Steps: width × run × rise × (1 + 2 + … + n) + landing
Bags = cubic feet × (1 + waste %) ÷ yield per bag, rounded up
Cubic yards = cubic feet ÷ 27
Example
A 10 ft × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.33 cubic feet. Adding 10% gives 36.67 cubic feet, and 36.67 ÷ 0.60 cubic feet per 80 lb bag = 61.1, so you buy 62 bags.
Yield per bag and bags per cubic yard
| Bag | Yield (cu ft) | Bags per cu yd | Bags per 10 sq ft at 4 in | Avg. price | Price per cu ft |
|---|---|---|---|---|---|
| 40 lb | 0.3 | 90 | 11.11 | $6.59 | $21.97 |
| 50 lb | 0.375 | 72 | 8.89 | $6.47 | $17.25 |
| 60 lb | 0.45 | 60 | 7.41 | $7.99 | $17.76 |
| 80 lb | 0.6 | 45 | 5.56 | $7.13 | $11.88 |
Yields are the approximate figures on the QUIKRETE Concrete Mix No. 1101 data sheet; Sakrete lists the same 0.30, 0.45 and 0.60 cubic feet for its 40, 60 and 80 lb bags. They assume a flat, even base, which is why a waste allowance matters. How much concrete does a bag make? explains the numbers in detail.
Why 10% extra?
Forms bow out, the ground under a slab is never perfectly level, and some mix always stays in the wheelbarrow. Sakrete recommends adding 10% to every estimate, and cost guides such as HomeGuide suggest 5%–10%. Running short in the middle of a pour leaves a cold joint, so the extra bags are cheap insurance. Unopened bags can usually be kept for your next project if stored dry.
Bags or a ready-mix truck?
Ready-mix is priced per cubic yard. HomeGuide puts it at $119–$147 per yard delivered within 20 miles, Angi at $110–$150 with a $130 national average, and orders smaller than a full truck usually pay a short-load fee of $40–$60 per yard. The comparison table in the calculator prices both options for your pour, so you can see where the truck becomes the better deal. Full price tables are in our concrete cost guide.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick holds 33.3 cubic feet of concrete (1.23 cubic yards). With 10% extra for waste you need 62 bags of 80 lb mix, 82 bags of 60 lb, 98 bags of 50 lb or 123 bags of 40 lb. With no extra at all it is 56, 75, 89 and 112 bags.
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags. The same yard takes about 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb mix, before any extra for waste.
Is it cheaper to buy bags of concrete or ready-mix?
Per cubic yard, ready-mix is usually cheaper. At the average prices on this page, 45 bags of 80 lb mix cost about $321, while a yard of ready-mix averages $130 plus a short-load fee of around $53 per yard on small orders. Bags still make sense for post holes, small pads and places a truck can't reach; ask local suppliers about minimum orders and delivery charges before you decide.
How many bags of concrete do I need per fence post?
For a 4×4 post (3.5 in actual) in a hole three times as wide (10.5 in) and 2 ft deep, the concrete around the post is about 1.03 cubic feet: 2 bags of 80 lb mix or 3 bags of 50 lb mix. Use the Column shape above with a 4×4 post selected to size your own holes. Hole width and depth rules come from QUIKRETE and Sakrete post-setting guides; check your frost depth locally.
Should I buy 60 lb or 80 lb bags of concrete?
Compare the price per cubic foot, not per bag. At our average prices an 80 lb bag works out to $11.88 per cubic foot and a 60 lb bag to $17.76, so 80 lb bags are the better buy on average; at some stores the two cost exactly the same per cubic foot. Smaller bags are easier to lift and mix by hand, which matters on a big pour.
Sources
Prices are national shelf prices and cost-guide ranges read on September 17, 2026. The calculator uses the typical price (the median of the shelf prices we saw) and shows the low–high range; type your own local price to update the total.
All sources for this page (19)
Coverage, yield and building specs
- QUIKRETE Concrete Mix No. 1101 product data sheet accessed 2026-09-17
- Sakrete High-Strength Concrete Mix product page accessed 2026-09-16
- Sakrete — Estimating bagged concrete jobs accessed 2026-09-17
- Inch Calculator — Concrete calculator accessed 2026-09-16
- QUIKRETE — Setting posts (project sheet) accessed 2026-09-16
- Sakrete — Setting posts & poles accessed 2026-09-16
Prices
- Ace Hardware — QUIKRETE Concrete Mix 40 lb accessed 2026-09-16
- The Home Depot — Concrete Mix category listing accessed 2026-09-16
- HomeGuide — Concrete prices accessed 2026-09-17 · page dated 2026-01-06
- Ace Hardware — QUIKRETE Fast-Setting Concrete 50 lb accessed 2026-09-16
- Ace Hardware — Sakrete Fast Setting Concrete 50 lb accessed 2026-09-16
- Ace Hardware — QUIKRETE Concrete Mix 60 lb accessed 2026-09-16
- Toolbox Supply — QUIKRETE 110160 60 lb accessed 2026-09-16
- Schillings — Sakrete Concrete Mix 80 lb accessed 2026-09-16
- Cox Hardware — Sakrete Hi-Strength Concrete Mix 80 lb accessed 2026-09-16
- Ace Hardware — QUIKRETE Concrete Mix 80 lb accessed 2026-09-16
- Denver True Value — QUIKRETE Concrete Mix 80 lb accessed 2026-09-16
- Angi — Concrete delivery cost accessed 2026-09-17 · page dated 2026-09-10
- HomeAdvisor — Concrete slab cost accessed 2026-09-17 · page dated 2026-06-20
Estimates for planning purposes only, not professional construction advice. Check local building codes and your product's instructions before you build. Prices are national averages collected from public retail listings and cost guides and will differ in your area. BuildHowMuch is independent and not affiliated with, endorsed by or paid by any retailer or manufacturer mentioned on this page. Nothing you enter is stored or sent anywhere.