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Concrete Slab Calculator (Yards & Bags)

Feet and inches straight to cubic yards and bag counts.

Also known as concrete yards · slab concrete calculator · yardage calculator

Quick answer

With the default inputs: Length 10 ft · Width 10 ft · Thickness 4 in etc.Concrete required 1.36 yd³

Use your own dimensions ↓

Inputs

ft
ft
in
%

Result

Concrete required
1.36yd³
Net volume33.3ft³
Volume with waste36.7ft³
Cubic yards1.36yd³
Truckloads at 9 yd³0.15loads
80 lb bags62bags
Total weight5317lb
Short tons2.66ton
How this is worked out

Volume is length x width x thickness, with thickness divided by 12 to become feet. One cubic yard is 27 cubic feet, and concrete weighs about 145 lb per cubic foot. Bag counts use the manufacturer yield (0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag) and are rounded up to whole bags after waste. Above roughly 3 cubic yards (about 130 bags of 80 lb) mixing by hand stops making sense - it will not be consistent and will not keep up with the set, so order a truck instead.

Next up: Concrete Volume Calculator · Concrete Mix Design Calculator

Formula

Volume (ft³) = L × W × T(in)/12convert inches to feet first
Cubic yards = ft³ ÷ 271 yd³ = 27 ft³
Bags = volume with waste ÷ bag yield80 lb = 0.60 ft³, 60 lb = 0.45 ft³
Weight = volume × 145 lb/ft³typical concrete unit weight
A 10 ft x 10 ft driveway slab, 4 in thick, 10% waste: 10 x 10 x (4/12) = 33.3 ft³, 36.7 ft³ with waste, which is 1.36 cubic yards, about 3.7 tons, or 62 bags of 80 lb. That is a half day of mixing for one person and still too slow for two - order 1.5 yards.

How to work it out

  1. Convert thickness from inches to feet first: 4 in is 0.333 ft, which is the divide by 12 step.
  2. Volume = length x width x thickness with everything in feet, giving cubic feet.
  3. Divide by 27 for cubic yards, the unit concrete is ordered and priced in.
  4. Add waste: 5% flat work, 10% where forms gap or you need fall, 15-20% for footings and trenches.
  5. Divide by bag yield for bag counts: an 80 lb bag is about 0.60 ft³ and a 60 lb bag 0.45 ft³, rounded up to whole bags.

Where it goes wrong

Material and cost

Ready-mix in North America runs roughly 130-180 per cubic yard for 3500-4000 psi, including short haul, and varies widely by region with winter and peak season higher. Orders under a full truck (typically 9 yd³) attract a short-load fee of about 70-150, and waiting time starts after 90 minutes on site. Bagged 80 lb 3500 psi mix is about USD 5-8 per bag, which is 220-400 per cubic yard - so above 3 yards order a truck, and keep bags for patching.

On site

Basis

FAQ

How much concrete do I need per square foot at 4 inches?
At 4 in thick a square foot holds 0.333 ft³, so one cubic yard covers about 81 ft². Handy numbers: 3 in covers about 108 ft² per yard, 4 in about 81, 5 in about 65, 6 in about 54. Six inches is the practical minimum for light vehicles - 4 in is fine for walkways and patios, but driveways want 5 to 6 in.
Should I mix my own or order a truck?
Under about 1 cubic yard (roughly 45 bags of 80 lb) mixing yourself makes sense. From 1 to 3 yards it depends on crew - two people with a mixer can do it. Above 3 yards order a truck. The reason is time, not money: concrete has to be placed and finished before initial set, hand mixing cannot keep up, and inconsistent water per batch leaves patchy strength and a dusty surface. Small orders usually carry a short-load fee, so ask before you book.
How much waste should I allow?
5% on a flat slab with tight forms, 10% where forms gap, the subgrade is uneven or you are sloping to a drain, and 15-20% for footings, trenches and irregular shapes. Better slightly over than short - a top-up load costs a full delivery and a day. Pour the surplus into a spare form as precast blocks; it beats returning it.

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Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Concrete Slab Calculator (Yards & Bags). 2026-10-25. https://jianzhujisuan.com/en/concrete-slab-yards

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.