How Many Bags of Concrete Do I Need? (80lb, 60lb, 50lb & 40lb Yields)
Quick answer: one 80-lb bag of concrete makes 0.60 cubic feet, so you need 45 bags per cubic yard (60 of the 60-lb size, 72 of the 50-lb, 90 of the 40-lb). A 10x10 slab 4 inches thick takes 62 bags of 80-lb mix or 82 bags of 60-lb mix, including 10% extra for waste.
Bagged concrete is the easy way to pour a small slab, a landing, a set of fence posts or a few deck footings. The only trick is buying the right number of bags. Buy too few and you end up with a cold joint while someone races back to the store. This guide gives you bag yields for every common size, ready-made tables for typical slabs, and the formula to work out anything else.
Bag Yields by Size
| Bag size | Yield per bag | Bags per cubic foot | Bags per cubic yard | Covers at 4 in thick | Typical price |
|---|---|---|---|---|---|
| 80 lb | 0.600 ft³ | 1.67 | 45 | 1.80 sq ft | $6.50 |
| 60 lb | 0.450 ft³ | 2.22 | 60 | 1.35 sq ft | $5.25 |
| 50 lb | 0.375 ft³ | 2.67 | 72 | 1.13 sq ft | $4.75 |
| 40 lb | 0.300 ft³ | 3.33 | 90 | 0.90 sq ft | $3.95 |
Per cubic foot, the 80-lb bag is the cheapest ($10.83 per ft³ against $13.17 for the 40-lb size). Smaller bags make sense if you cannot safely lift 80 lb many times over, or if you are filling a few post holes where a big bag would be left half used.
The Formula
If your ground is rough or the edges are irregular, go toward 15%. On a perfectly graded base with rigid forms, 5% can be enough. Our guide to the concrete waste factor explains when to use which.
Bags Needed for Common Slab Sizes (4 Inches Thick)
| Slab size | Area | 80-lb bags | 60-lb bags | 50-lb bags | 40-lb bags |
|---|---|---|---|---|---|
| 3 × 3 ft | 9 sq ft | 6 | 8 | 9 | 11 |
| 4 × 4 ft | 16 sq ft | 10 | 14 | 16 | 20 |
| 4 × 8 ft | 32 sq ft | 20 | 27 | 32 | 40 |
| 6 × 6 ft | 36 sq ft | 22 | 30 | 36 | 44 |
| 8 × 8 ft | 64 sq ft | 40 | 53 | 63 | 79 |
| 8 × 12 ft | 96 sq ft | 59 | 79 | 94 | 118 |
| 10 × 10 ft | 100 sq ft | 62 | 82 | 98 | 123 |
| 10 × 12 ft | 120 sq ft | 74 | 98 | 118 | 147 |
| 12 × 12 ft | 144 sq ft | 88 | 118 | 141 | 176 |
Bags Per 100 Square Feet by Thickness
For any other size, find the bags per 100 sq ft for your thickness and scale it. A 150 sq ft slab needs one and a half times the number shown.
| Thickness | Concrete with waste | 80-lb bags | 60-lb bags | 50-lb bags | 40-lb bags |
|---|---|---|---|---|---|
| 3.5 in | 1.19 yd³ | 54 | 72 | 86 | 107 |
| 4 in | 1.36 yd³ | 62 | 82 | 98 | 123 |
| 5 in | 1.70 yd³ | 77 | 102 | 123 | 153 |
| 6 in | 2.04 yd³ | 92 | 123 | 147 | 184 |
Go to 5 inches if a riding mower or small vehicle will sit on the slab, and 6 inches for a hot tub or heavier loads. Past about 1.5 cubic yards (68 bags of 80-lb mix) a ready-mix truck is usually cheaper and far less work. See ready-mix vs. bagged concrete for the break-even.
Worked Example: 8x12 Shed Pad
That is 2.4 tons of dry mix. Check your vehicle's payload before you load it, or pay for store delivery. With a rented mixer and three people working steadily at 20 to 30 bags an hour, the pour takes about 2.0 to 3.0 hours. Our concrete bags calculator does this math for any size, and the shed pad guide covers the rest of the build.
Fence Posts and Footings
For round holes, use the cylinder formula and subtract the post if it sits in the concrete:
| Hole | Use | Concrete | 80-lb bags | 50-lb bags |
|---|---|---|---|---|
| 8 in wide × 24 in deep | 4x4 fence post, shallow frost | 0.53 ft³ | 1 | 2 |
| 10 in wide × 36 in deep | 4x4 fence or mailbox post | 1.38 ft³ | 3 | 4 |
| 12 in wide × 42 in deep | Deck footing in a tube form | 2.75 ft³ | 5 | 8 |
For square pads, piers and stepped footings, the concrete footing calculator handles the shapes.
Mixing Tips That Save Bags (and Your Slab)
- Measure the water. About 3 quarts per 80-lb bag is typical; follow the amount printed on your bag. Extra water makes mixing easier but weakens the concrete and adds shrinkage cracks.
- Mix whole bags. Splitting bags throws off the water ratio from batch to batch.
- Rent a mixer for anything over a few bags. A drum mixer runs about $50 to $100 a day and saves your back. Hand mixing in a tub works for posts and small patches.
- Stage the bags beside the mixer before you start, and keep them off wet ground and covered from rain.
- Keep a steady pace. Place each batch against the last one while it is still workable to avoid weak seams.
- Watch the weather. Below about 40°F concrete gains strength slowly and needs protection (ACI 306R); above about 85°F it sets fast and needs shade and extra hands (ACI 305R).
- Keep it moist while it cures. Light foot traffic is usually fine after 24 to 48 hours.
Most stores take back unopened bags. Running short in the middle of a pour costs far more than returning two spare bags the next day.
Sources and Method
Bag counts use the standard volume formula, the published yields for general-purpose concrete mix and 10% waste, rounded up to whole bags. Prices are typical 2026 big-box prices and vary by store and region (see our methodology). Packaged concrete mixes are covered by ASTM C387. The minimum slab thickness comes from the International Residential Code, section R506. For a full project cost with forms, base and labor, use the slab cost calculator.
Frequently Asked Questions
45 bags. Each 80-lb bag yields 0.60 cubic feet, and a cubic yard is 27 cubic feet. With the 60-lb size you need 60 bags per yard.
At 4 inches thick with 10% waste: 62 bags of 80-lb mix, 82 bags of 60-lb mix, 98 bags of 50-lb mix or 123 bags of 40-lb mix.
At 4 inches thick with 10% waste, 88 bags of 80-lb mix or 118 bags of 60-lb mix. That is close to 1.96 cubic yards, where a ready-mix truck is often cheaper.
About 1.80 square feet at 4 inches thick, or about 1.20 square feet at 6 inches. Add 10% when you buy.
For a 4x4 post in a 10-inch hole 3 feet deep, about 3 bags of 80-lb mix or 4 bags of 50-lb mix. Hole size varies, so buy a spare bag.
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