Bags per cubic foot and cubic yard

Every calculation starts from one fact: how much volume a single bag fills once it’s mixed. That’s the bag’s yield, and it’s printed on the label. Here are the standard figures.

Notice how the count works: a cubic foot is small, so it takes fewer than two of the big 80 lb bags but more than three of the little 40 lb ones. The bigger the bag, the fewer you open — which matters a great deal once you’re mixing dozens of them.

Why the exact number matters

Concrete doesn’t forgive guesswork. Buy too few bags and you’re making a second trip to the store mid-pour — while the concrete you’ve already placed starts to set, leaving a cold joint where the old and new mix never properly bond. That’s a permanent weak line through your slab.

Buy too many and you’ve wasted money and earned yourself the chore of hauling heavy unopened bags back for a refund. Getting it right the first time is the whole game. A single miscalculation on a 10 × 10 patio can leave you five to eight bags short — which is exactly enough to ruin a pour.

Cement vs concrete — they’re not the same

This is the single most common confusion, so let’s settle it before going further. Cement is the grey powder — the binder. Concrete is what you get when cement is mixed with sand, gravel (aggregate) and water. A bag of “concrete mix” already contains all of it; you just add water.

When people ask “how many bags of cement per cubic foot,” they almost always mean pre-mixed concrete bags — the kind at Home Depot or Lowe’s — because that’s what the overwhelming majority of DIY projects use. This article is about those bags. If you’re mixing from raw Portland cement, sand and gravel separately, the maths is different, and we cover it near the end.

The simple formula

Only one formula to remember:

Say you need to fill 5 cubic feet with 80 lb bags (yield 0.60 cu ft each):

Always round up, never down. There’s no such thing as pouring 0.33 of a bag and calling the slab finished.

How to find your volume in cubic feet

Before the bag count, you need the volume. The shape decides the formula.

• Flat slab or patiolength × width × depth, all in feet.
• Round column or tube3.14 × radius² × height, all in feet.
• Post hole (round)3.14 × radius² × depth, then × the number of holes — and subtract the volume of the post itself, since it takes up space the concrete won’t fill.

The one conversion that trips everyone up: depth is almost always in inches, while length and width are in feet. Divide inches by 12 first. A 4-inch slab is 4 ÷ 12 = 0.333 feet, not 4.

Worked examples

Three common projects, every step shown, so you can follow the same path for your own.

Example 1 — small patio slab, 8 × 10 ft × 4 in, 80 lb bags

Example 2 — 6 fence post holes, 12 in dia × 30 in deep, 60 lb bags

This ignores the volume the posts themselves take up, so it’s a slight over-estimate — which is fine for buying, since you want a little surplus anyway. Our fence post calculator subtracts the post for a tighter number.

Example 3 — walking path, 3 × 20 ft × 3 in, 40 lb bags

Fifty-five 40 lb bags for a garden path is a lot of lifting and opening — a good illustration of why bigger bags, or ready-mix, win once a job grows.

Never skip the waste factor

Every pour loses some material — to spillage while mixing, to uneven ground that swallows extra concrete, to mix left clinging in the wheelbarrow, and to over-digging the form area. Order the exact calculated volume and you’ll almost always come up short.

For most standard projects, add 10%. Two spare bags in the garage beats being one bag short on pour day, with the truck idling and the mix going off.

Mixing from raw cement instead of pre-mix

If you’re buying pure Portland cement (94 lb bags) and combining it with your own sand and gravel, the calculation changes completely, because now you’re counting cement bags, not concrete bags.

Per cubic foot, a standard 1:2:3 mix needs roughly 0.2 bags of 94 lb cement, plus the matching sand and gravel. Unless you have a mixer and bulk aggregate on hand, though, pre-mixed bags are almost always the more practical choice for a small project — you trade a little cost for a lot less hassle.

7 mistakes that ruin a bag count

• Forgetting to convert inches to feetA 6-inch slab is 0.5 feet, not 6. This one slip can multiply your order many times over. Divide inches by 12 first, always.
• Ignoring the waste factorCalculating exactly what you need and buying exactly that almost always leaves you short. Add 10%.
• Not accounting for rebar or embedded objectsRebar, mesh, post brackets and the posts themselves displace concrete. On small pours it’s enough to matter.
• Rounding downNeed 8.2 bags? Buy 9. Concrete doesn’t stretch to cover the gap.
• Measuring the form, not the excavationIf you dug two inches deeper than the form in places, that extra volume still needs filling.
• Adding too much waterIt weakens the mix badly, and it slightly changes the volume you get, throwing off your yield.
• Assuming every bag yields the sameHigh-strength, fibre-reinforced and crack-resistant mixes can yield less than a standard bag. Read the label.

When to stop buying bags and order ready-mix

There’s a clear tipping point. Below about one cubic yard — 27 cubic feet, or roughly 45 × 80 lb bags — bags are cheaper and perfectly practical. Above it, mixing by hand becomes brutal, the pour is no longer continuous, and ready-mix delivered by the yard wins on both cost and quality.

The comfortable DIY range is anything up to about 25–30 bags — manageable for one person with a mixer or a wheelbarrow and a helper. Beyond that, phone a ready-mix supplier. Fifty bags is most of a day’s mixing, and a slab poured batch by batch never sets as one solid piece.

Not sure which side of the line you’re on? The bags calculator flags the crossover for you, and the slab calculator shows the ready-mix volume side by side with the bag count.

Quick reference: common projects

Save this. The most common DIY concrete projects, using 80 lb bags with 10% waste already included. Volume shown is before waste.

What can change a bag’s yield

The “0.60 cu ft per 80 lb bag” figure is a reliable standard, but real-world yield shifts a little with:

• Water amountToo much water raises the volume slightly but weakens the mix; too little leaves it unworkable. Mix to a stiff, workable consistency and no wetter.
• TemperatureHot weather speeds the set, shortening your working time and risking wasted mix that goes off before you place it.
• Brand and mix typeHigh-early-strength, crack-resistant and fibre-reinforced products can yield differently. The label is the authority.
• Mixing methodHand-mixing in a wheelbarrow often leaves dry material stuck in the corners, quietly reducing your effective yield.

Calculate your bag count in five steps

1. Measure the project. Length, width and depth. Convert every inch measurement to feet by dividing by 12.
2. Find the volume in cubic feet. L × W × D for slabs; 3.14 × r² × H for round holes and columns.
3. Add your waste factor. Multiply by 1.10 for the usual 10%; use 1.15–1.20 for irregular or sloped jobs.
4. Divide by the bag’s yield. 0.60 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb.
5. Round up and buy. Always to the next whole bag. Never down.

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