How much extra concrete should I order?

Add about 5–10%. The subgrade is never perfectly flat, the forms flex, and you can't pour a slab in two batches — running short mid-pour ruins it. Ready-mix is ordered to the quarter-yard, so round up.

Bags make sense only for very small pours — a few post footings or a tiny pad. Even a 10 × 10 slab is ~56 eighty-pound bags to mix by hand. Once you're past roughly half a cubic yard, a ready-mix truck (sold by the cubic yard) is cheaper, faster, and more consistent.

How thick should the slab be?

4 inches is standard for patios and walkways; 5–6 inches for a driveway or anything bearing vehicles, usually over a compacted gravel base and often with rebar or wire mesh. Thickness, reinforcement, and the base for a structural slab are a code/engineering call — this tool sizes the concrete once you've set them.

Concrete Slab Calculator

Enter your slab's length, width, and thickness for the concrete it needs — by the cubic yard (how ready-mix is ordered) and by the bag (for small pours).

Concrete

1.23cu yd

Volume: 33.3 cu ft
Bags of mix: 56 bags

Concrete-volume estimate only. Slab thickness, reinforcement, and the compacted base for a structural or load-bearing slab follow local code and engineering — this doesn't design the slab.

Concrete for common slab sizes

Worked out with this calculator. Yards are the exact volume (order 5–10% extra); bag counts use Quikrete's published yields and are rounded up.

Slab size and thickness	Cubic yards	80 lb bags	60 lb bags
10 × 10 ft at 4 in	1.23	56	75
10 × 10 ft at 6 in	1.85	84	112
12 × 12 ft at 4 in	1.78	80	107
16 × 16 ft at 4 in	3.16	143	190
20 × 20 ft at 4 in	4.94	223	297
24 × 24 ft at 4 in	7.11	320	427
24 × 24 ft at 6 in	10.67	480	640

How these numbers are worked out

The formula

Volume = length × width × (thickness ÷ 12) cubic feet, ÷ 27 for cubic yards. Bags = volume ÷ the yield of one bag, rounded up — an 80-lb bag of concrete mix yields about 0.60 cu ft (Quikrete; 60 lb ≈ 0.45, 50 lb ≈ 0.375). The bag count climbs fast, which is why all but the smallest slabs use ready-mix.

Constants and sources

Volume = L × W × (thickness ÷ 12) ÷ 27 (geometry). Bag yields: Quikrete Concrete Mix published values (80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45, 50 lb ≈ 0.375) — confirm against your bag.

How it was checked

Last reviewed June 16, 2026. Before this calculator was published, its formula, constants and worked example were checked in a separate, AI-assisted review that redid the arithmetic and tested the factual claims; the site owner then approved it for publishing. No engineer or tradesperson reviews these pages. Where a figure depends on the product (a density, a coverage rate, a bag size), it is either an input you can change above or a fixed planning value stated on this page, so check it against your product's label or your supplier.

Spot a wrong number or an outdated figure? Send a correction.

Worked example

A 10 × 10 ft slab, 4 in thick: 10 × 10 × (4 ÷ 12) = 33.3 cu ft = 1.23 cu yd. In 80-lb bags that's ⌈33.3 ÷ 0.60⌉ = 56 bags — so a slab this size is really a ready-mix order.

FAQs

How much extra concrete should I order?: Add about 5–10%. The subgrade is never perfectly flat, the forms flex, and you can't pour a slab in two batches — running short mid-pour ruins it. Ready-mix is ordered to the quarter-yard, so round up.
Bags or ready-mix?: Bags make sense only for very small pours — a few post footings or a tiny pad. Even a 10 × 10 slab is ~56 eighty-pound bags to mix by hand. Once you're past roughly half a cubic yard, a ready-mix truck (sold by the cubic yard) is cheaper, faster, and more consistent.
How thick should the slab be?: 4 inches is standard for patios and walkways; 5–6 inches for a driveway or anything bearing vehicles, usually over a compacted gravel base and often with rebar or wire mesh. Thickness, reinforcement, and the base for a structural slab are a code/engineering call — this tool sizes the concrete once you've set them.