Concrete Bags Calculator: How Many Bags of Mix Do You Need?

Updated 2026-08-06 · Yardly

A standard 80 lb bag of concrete mix yields about 0.60 cubic feet of finished concrete — so one cubic yard takes exactly 45 bags. Everything else about bag math is just that one number worked backward from your project’s volume. Here’s the yield table, the formula, and the point at which you should stop loading bags and call the batch plant instead.

The formula

Bagged concrete is sold by weight (40, 50, 60, 80, or 90 lb) but it finishes by volume. You measure your job in feet and inches, find the volume in cubic feet or cubic yards, then divide by what one bag produces:

(Length × Width × Depth ÷ 12) = cubic feet of concrete

Cubic feet ÷ bag yield = number of bags  •  +10% for waste

The concrete slab calculator does the volume half for flatwork; the fence post concrete calculator does it for holes. This article is about the second half — turning that volume into a bag count you can actually load into a cart.

The bag-yield table (this is the whole trick)

Bag size is not arbitrary, but bag yield is consistent across the standard mixes because the dry density is set by the formula. Quikrete’s own product data sheet for Concrete Mix (Product No. 1101) lists these yields per bag, and the same numbers hold for the equivalent Sakrete standard mix — see Concrete Mix technical data (QUIKRETE 1101):

Bag size	Yield per bag	Bags per cubic yard (27 ft³)
40 lb	0.30 ft³	90
50 lb	0.375 ft³	72
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45
90 lb	0.675 ft³	40

The “bags per yard” column is just 27 ÷ the yield, rounded. Memorize one of them — 45 for the 80 lb size is the common one — and you can sanity-check any store estimate: if a quote says a 10×10 slab needs 100 bags of 80 lb mix, you know immediately that’s wrong, because one yard is only 45 bags and that slab is barely over a yard.

One important caveat: these are the standard mixes. A few products break the pattern on purpose. Sakrete Maximizer, for example, uses lightweight aggregate to yield about 1.0 cubic foot per 80 lb bag — only 27 bags per yard. The trade-off is slightly lower density. Unless your label says otherwise, assume the table above, and always read the yield printed on the bag you’re buying rather than trusting the size.

Worked example 1: a 10×10 patio at 4 inches

This is the most-asked slab on the internet, so it’s worth doing in full:

• Volume: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet = 1.23 cubic yards.
• 80 lb bags: 33.3 ÷ 0.60 = 55.5 → round up to 56.
• Add 10% waste (spillage, an uneven subgrade, a form that isn’t perfectly square): 56 × 1.10 = 62 bags.

With 60 lb bags the same slab is 33.3 ÷ 0.45 = 74 → 75 before waste, about 83 with it. The volume is identical; you’re just carrying more, lighter bags. A 4-inch slab is the residential default for walkways and patios — if you’re pouring something a vehicle will use, the depth (and therefore the bag count) climbs, which is covered separately in the slab thickness guide.

Worked example 2: one fence post hole

Post holes look tiny and then surprise you. An 8-inch-diameter hole, 30 inches deep:

• Radius = 4 inches = 0.333 ft; depth = 30 inches = 2.5 ft.
• Volume = π × 0.333² × 2.5 ≈ 0.87 cubic feet.
• 80 lb bags: 0.87 ÷ 0.60 = 1.45 → round up to 2 bags per hole.

Most fence posts land at 1–2 bags each; a 12-inch hole 42 inches deep pushes to 3. Multiply by the number of posts, not by the length of fence — the fence post concrete calculator totals that across the whole run and tells you how many bags (or how many pallets) to buy. Fast-setting mix is the right call here: it’s formulated to cure without a separate aggregate step and lets you brace and move on.

Worked example 3: a small shed pad

An 8×10 ft pad at 4 inches:

• Volume: 8 × 10 × 0.333 = 26.7 cubic feet = 0.99 cubic yards.
• 80 lb bags: 26.7 ÷ 0.60 = 44.4 → 45, or 50 with a 10% buffer.

Notice this is essentially one cubic yard and it already takes 45 bags. That’s the moment the next section becomes relevant.

When bags stop making sense: the ready-mix crossover

Bagged concrete has two hidden costs the per-bag price hides:

1. Hauling. 45 bags of 80 lb mix is 3,600 lb of dry material — close to two tons. A half-ton pickup carries maybe 20–25 bags per trip, so a single yard is two or three store runs.
2. Mixing. Mixing 45 bags one at a time in a wheelbarrow is a solid half-day for one person, even with a rented mixer. Ready-mix shows up already mixed.

On price alone, the crossover is around half a cubic yard:

• Bagged: ~45 bags × $5–$8 = $225–$360 per yard in materials.
• Ready-mix delivered: typically $125–$175 per yard, sometimes with a short-load fee under 3 yards.

So the rule of thumb: under about 22 bags (half a yard) of work spread across several small pours — post holes, a mailbox footing, a step repair — bags are the right tool. Over half a yard in a single pour, at least get a quote from a batch plant; you’ll usually save money and a backache. Between half a yard and a yard, it’s a judgment call based on truck access and how much you value your Saturday.

A few practical buying notes

• Round up to whole bags, never down. You can’t buy 0.5 of a bag, and running short means a second trip plus a visible color seam if the second batch cures a week later.
• Buy by the pallet for big jobs. Retailers band 80 lb bags about 42 per pallet, so a yard (45 bags) is just over one pallet — ask for a broken pallet rather than hauling loose bags.
• Water changes nothing about the count. The yield figures assume the bag’s specified water dose. Over-watering lowers real yield (more water, same cement), so follow the label and add water gradually to a stiff, workable consistency instead of pouring it in.
• Match bag size to your body, not just the math. 80 lb bags mean fewer trips and fewer opens, but if lifting is a concern, 60 lb or 40 lb bags cost a little more per yard and save your back. The concrete is identical; only the handling differs.

Keep reading

• Need the volume first? The concrete slab calculator totals cubic yards for any flat pour from your dimensions.
• Setting a row of posts? The fence post concrete calculator sizes every hole and sums the bags across the run.
• Want the full walkthrough from area to yards? How much concrete do you need? breaks down depth, shape, and waste in one place.

Frequently asked questions