How Many Bags of Concrete Do I Need?

Here is the whole calculation in one line: work out how many cubic feet of concrete your job needs, then divide by the yield of one bag. An 80 lb bag yields 0.60 cubic feet, a 60 lb bag yields 0.45, and a 40 lb bag yields 0.30. So a job needing 10 cubic feet takes 17 bags at 80 lb (10 ÷ 0.60), 23 at 60 lb, or 34 at 40 lb — always rounded up, and with a spare bag or two on top. If you do not have your volume yet, our concrete calculator works it out from your measurements in seconds.

Bag yields: the only numbers you need

Bagged concrete is sold by weight, but holes and slabs are filled by volume, so manufacturers print a yield for every bag size. The three common sizes behave like this:

• 80 lb bag: 0.60 cu ft. The best value per cubic foot and the size contractors reach for, but the heaviest to lift.
• 60 lb bag: 0.45 cu ft. A reasonable compromise between weight and coverage.
• 40 lb bag: 0.30 cu ft. Easy to carry and fine for tiny repairs, but you need twice as many as the 80 lb size.

The formula never changes: bags = cubic feet ÷ yield, rounded up to the next whole bag. The rest of this guide is that one sum applied to real jobs.

Bag counts for common jobs

Take the patio slab used throughout this series: 10 × 10 ft at 4 in thick, which is 33.3 cubic feet, or 1.23 cubic yards. Dividing 33.3 by each yield gives the counts below, and the last column shows the bags needed for a full cubic yard of concrete — the quickest way to see why big jobs stop making sense in bags.

Bag size	Yield per bag	Bags for a 10 × 10 ft slab, 4 in thick	Bags per cubic yard
80 lb	0.60 cu ft	56	45
60 lb	0.45 cu ft	75	60
40 lb	0.30 cu ft	112	90

Check the maths on the first row: 33.3 ÷ 0.60 = 55.5, which rounds up to 56 bags. In practice you would buy a couple of spares on top of any figure in this table, because a slab that comes up half a bag short cannot be paused while someone drives to the store. For shapes other than a simple slab, how much concrete do i need walks through measuring slabs, footings and post holes step by step.

Worked example: concrete for a fence post

The classic bag job is setting fence posts. A standard hole is 12 in across and 36 in deep. As a cylinder, its volume is π × radius² × depth: 3.1416 × 0.5² × 3 = about 2.36 cubic feet. But the post itself sits in that hole and takes up space: a 4 × 4 post (a true 3.5 in square) displaces roughly 0.25 cubic feet over its buried length, leaving about 2.1 cubic feet of concrete to place.

Divide by the 80 lb yield: 2.1 ÷ 0.60 = 3.5, which rounds up to 4 × 80 lb bags per post. Shallower 30 in holes, common for shorter fences, need nearer 2–3 bags. Multiply by your post count before you shop: a ten-post run needs around 40 bags, which is a pallet, not a car boot — plan delivery accordingly. Our guide to the concrete calculator formula shows where the cylinder calculation comes from if you want to check it for different hole sizes.

When bags beat ready-mix — and when they do not

Bags win on small, awkward or spread-out jobs. Anything under about half a cubic yard — a few fence posts, a small equipment pad, a repair — is cheaper and simpler in bags, especially where a mixer truck cannot reach, or where you want to place one post at a time over a weekend. There is no minimum-order charge, no truck clock ticking, and leftovers keep for the next job.

Ready-mix wins on volume. A single cubic yard mixed by hand means 45 bags of 80 lb — over 3,600 lb of lifting, plus mixing and placing all of it before the first batch starts to stiffen. Two people can do it, but it is a hard, rushed day, and the concrete quality suffers if batches are placed too far apart. As a rule of thumb, once a job passes half a yard and has truck access, ready-mix is kinder to your back and usually to your wallet.

Mixing notes that protect the finished job

• Use the water on the bag. Follow the printed instructions — roughly 3 quarts of water for an 80 lb bag. Too little and the mix will not consolidate; too much and it weakens.
• Never add extra water for workability. A soupy mix is easier to shovel but cures weaker and cracks more readily. If the mix feels stiff, mix it a little longer rather than reaching for the hose.
• One bag at a time in a wheelbarrow. Add most of the water first, then the bag, then mix to an even consistency before placing. It keeps batches uniform from the first bag to the last.
• Rent a mixer beyond about 10 bags. A small electric mixer costs little for a day and turns an exhausting slog into steady work — worth it well before the slab example above.

Still deciding between bags and a truck? Run your dimensions through the concrete calculator and look at the yardage: under half a yard, think bags; over it, think ready-mix. And if the count surprises you, the concrete estimating calculator on the same page also reports bag counts and total weight, so you can see exactly what a job asks of you before committing.

Frequently asked questions