How many bags of concrete do I need?
It takes 45 bags of 80 lb mix to make a cubic yard, and a cubic yard pours about 81 square feet at 4 inches. The bag count for your project is just volume ÷ yield — the trick is getting the volume right before you start loading a cart.
Bagged concrete is sold by weight and used by volume, which is why the numbers feel disconnected. A 60 lb bag and an 80 lb bag look similar on the shelf but yield 0.45 and 0.60 cubic feet respectively — a 33% difference. Buy the wrong size and you either make an extra trip or carry 20 extra bags into the garage.
Bag yields and bags per cubic yard
| Bag size | Yield when mixed | Bags per cubic yard | Bags per 100 sq ft at 4 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 111 |
| 50 lb | 0.375 cu ft | 72 | 89 |
| 60 lb | 0.45 cu ft | 60 | 74 |
| 80 lb | 0.60 cu ft | 45 | 56 |
Those figures are for standard sand-and-gravel concrete mix. Fast-setting mixes, high-strength mixes and mortar mix have different yields, so check the bag. The yield printed on the bag is measured in a lab; a slightly dry mix gives you a little more volume, a wet mix a little less.
Bag counts for common projects
| Project | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft slab, 4 in | 1.23 cu yd | 56 | 74 |
| 12 × 12 ft slab, 4 in | 1.78 cu yd | 80 | 107 |
| 12 × 12 ft slab, 6 in | 2.67 cu yd | 120 | 160 |
| 20 × 20 ft garage, 5 in | 6.17 cu yd | 278 | 371 |
| 24 × 24 ft driveway, 5 in | 8.89 cu yd | 400 | 534 |
| Deck post footing, 12 × 36 in | 2.36 cu ft | 4 | 6 |
| Fence post, 9 × 30 in hole | 1.10 cu ft | 2 | 3 |
These are before waste. Add 5–10% to the bag count and round up, because half a bag cannot be saved once it is wet. For the full calculation including waste, use the concrete calculator.
The point where bags stop making sense
A cubic yard of concrete mixed from 80 lb bags costs about $250–$400 in material in 2026, before you count water, mixer hire or your own time. The same yard delivered ready-mix runs $150–$210. Bags win on convenience and small quantities; they lose on anything measured in yards.
| Pour size | 80 lb bags | Best option |
|---|---|---|
| Under 0.25 cu yd | Up to 11 | Bags, no question |
| 0.25–0.5 cu yd | 11–23 | Bags, or a mixer and a helper |
| 0.5–1 cu yd | 23–45 | Close call — price the short load |
| 1–5 cu yd | 45–225 | Ready-mix with a short-load fee |
| Over 5 cu yd | 225+ | Ready-mix, full truck |
Mixing 45 bags by hand is a four to six hour job and every batch has to match the last. Ready-mix arrives at a consistent slump and places in under an hour. On anything above one cubic yard, the "saving" from bags is usually paid back in labour and risk.
Mixing without ruining the concrete
- Water first, then powder. Put most of the water in the mixer or wheelbarrow, add the bag, then adjust. Adding water to a dry mix is how you overshoot.
- Target 3 quarts per 80 lb bag. That is the manufacturer's figure for a workable mix. A little more is fine on a hot day; a hose is not.
- Mix for 2–3 minutes. Uniform colour and no dry pockets. Too short and you get weak spots; too long and the mix stiffens.
- Place in one lift where possible. For thicker pours, place in 6–8 inch lifts and consolidate each one with a tamper or a quick vibrate of the form.
- Finish, then cure. Broom or trowel within an hour of placing, then cover with plastic or spray curing compound for 7 days.
Bagged concrete is not all the same
- Concrete mix — gravel aggregate, 4,000 psi typical. The default for slabs, footings and posts.
- Sand mix / topping mix — no gravel, used for thin repairs and bedding. Do not pour a structural slab with it.
- Fast-setting mix — sets in 20–40 minutes, useful for post holes, expensive for slabs.
- Mortar mix — for brick and block bedding, not for concrete.
Buying the cheap "concrete mix" for a slab is fine. Buying sand mix because it was on sale is how a patio ends up crumbling at the edges after one winter.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
Forty-five bags. An 80 lb bag yields about 0.60 cubic feet when mixed, and a cubic yard is 27 cubic feet. With 60 lb bags it takes 60 bags per yard, with 50 lb bags 72, and with 40 lb bags 90.
How many bags of concrete do I need for a 10x10 slab?
A 10x10 ft slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards. That is 56 bags of 80 lb mix before waste, or 62 bags with a 10% allowance. It is also about the point where a ready-mix short load starts to be the better deal.
How much water goes in an 80 lb bag of concrete?
About 3 quarts, or roughly 2.5–3 litres. Add most of it first, then the rest slowly while mixing. The mix should hold its shape on a shovel, not flow like soup. Extra water makes placing easier and the finished concrete weaker.
How many bags of concrete do I need for a fence post?
Two to three 50 lb bags per post for a hole 8–10 inches across and 30 inches deep, or about two 80 lb bags. That fills the hole to roughly 2–3 inches below grade so soil and grass can cover the top.
Is it cheaper to mix my own concrete?
For pours under about half a cubic yard, yes — bagged mix is cheaper than paying a short-load fee. Above one cubic yard, ready-mix is cheaper per cubic yard and far less work. Between the two, compare bag price against the delivered yard price plus the minimum charge.