How Many Bags of Concrete Do I Need?

The formula is boring and it never changes: length x width x thickness, converted to cubic feet, divided by the yield of whatever bag is on the pallet. The reason people get it wrong isn't the math. It's the two places where reality diverges from the formula, so let's do the math first and then deal with reality.

The math, once

A 10x10 slab at 4 inches thick: 10 x 10 x (4/12) = 33.3 cubic feet. An 80-lb bag of Quikrete or Sakrete yields about 0.60 cubic feet of concrete once mixed. 33.3 divided by 0.60 is 55.6 bags. Round up, that's your bare minimum of 56 bags.

Now reality, divergence number one: you will never pour exactly 33.3 cubic feet. The excavation isn't perfectly flat, some of the form boards bow, the screed drags low spots full, and somewhere around bag 40 you'll splash a good half-bag onto the grass. That's why every decent estimate adds 10% waste. 56 bags x 1.1 = 61.6, which rounds to 62 bags of 80-lb for a 10x10x4 slab. If you'd rather wrestle 60-lb bags, the same slab takes 82 of them, since they only yield 0.45 cubic feet each.

Bag counts for common slabs

Everything below includes the 10% waste allowance and assumes 80-lb bags:

Spot-check any row (or build your own size) with the concrete volume & bags calculator. It also gives you 60-lb, 50-lb, and 40-lb counts.

Reality, divergence number two: weight

Here's the part the bag math doesn't show you. That 10x10 slab is about 5,000 pounds of concrete, wet. You are buying that in 80-pound increments, which means roughly 62 trips from pallet to slab if you're batching in a wheelbarrow. A full pallet of 80-lb bags runs around 3,300 pounds, so a half-ton pickup is already at its payload limit with one pallet. Most people doing a 10x10 take two trips to the store or pay for delivery, and that delivery fee quietly changes the "bags are cheaper" math.

Bags or truck?

At roughly $7 to $8 an 80-lb bag in 2026, that 10x10 slab costs you $430 to $500 in bags, plus sand or gravel for the base and whatever your back is worth. The same slab is about 1.24 cubic yards net. Ready-mix in most markets runs $175 to $210 a yard, so a full truck comes to $220 to $260 before the short-load fee that suppliers tack on for anything under 3 or 4 yards. With fees, the truck lands around $350 to $450 — comparable to bags, delivered in ten minutes instead of three hours of batching.

The crossover most builders actually use: under about a yard, bags, no question. Over 1.5 to 2 yards, get truck quotes. Right around one yard (a 10x10 at 4 inches is close), it's a coin flip — call two suppliers and ask what they charge for a short load. And if you're pouring anything where you can't take a break mid-pour, like a slab with reinforcing mesh, the truck wins on quality even when the price is a wash. Cold joints between batches are the death of a slab.

Three things worth knowing before you load the cart

• Unopened bags can go back. Big box stores take returns on intact bags for months. Buy the 62, return the 4 you didn't need. Don't however buy 58 "to be safe" — running short two bags from the end of a pour is the single worst way to end a Saturday.
• Check the bag dates. Bags absorb moisture through the paper and start forming rock nuggets in the corner. A bag with hard lumps when you squeeze it has already begun to hydrate. If the clearance pallet is cheap for a reason, you now know the reason.
• Thickness is where budgets die. Going from 4 inches to 5 inches on that 10x10 adds roughly 15 bags. People eyeball "about four inches" and end up at six where the grade dips. Set your forms with a string line and check the depth in several spots before you order anything.

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