Do I need rebar or wire mesh?

For a 4 in patio, welded wire mesh or #3 bar on a 16 in grid controls cracking fine. Step up to #4 bar for driveways and anything carrying vehicle loads. Keep steel in the middle third of the slab, never on the ground.

Bags or a ready-mix truck?

Under about 1 cu yd, bags win — no short-load fee, no waiting on a truck. Between 1 and 1.5 cu yd it is a coin toss. Past 1.5 cu yd, order ready-mix: it is cheaper per yard and pours as one monolithic slab instead of dozens of hand-mixed batches.

How many 80-lb bags are in a cubic yard?

A cubic yard is 27 cu ft, and an 80 lb bag yields about 0.6 cu ft, so it takes roughly 45 bags to make a cubic yard — before any waste allowance.

Concrete Slab Calculator

Enter your pad size and get cubic yards to order, bag count, and a bags-vs-truck call — waste allowance built in.

Inputs

Pad size

ft ×

Pad size, second side

Thickness — 4″ typical for patios

Bag size

Waste

Results update as you type — no submit

Concrete to order

1.6 cu yd

≈ 1.25 m³

Or in 80-lb bags

74 bags

≈ 5,920 lb of material

Or in 80-lb bags74 bags

Slab volume (before waste)40 cu ft = 1.48 cu yd

Slab area120 sq ft

Call a truck. Past 1.5 cu yd, ready-mix beats bags on cost and cure quality — even with a short-load fee.

Formula

volume = 12 × 10 × (4 ÷ 12) = 40 cu ft = 1.48 cu yd
order = 1.48 × 1.1 waste = 1.6 cu yd · bags = ceil(44 ÷ 0.6) = 74

Diagram — slab plan & sectionNTS

MeasureShedmeasureshed.com/calculator/concrete-slab-calculator

Concrete slab — material listUNITS: FT · SCALE: NTS

Inputs

Pad size10 × 12 ft

Thickness4″

Bag size80 lb

Waste+10%

Results

Concrete to order1.6 cu yd

80-lb bags74

Slab volume (before waste)40 cu ft

Slab area120 sq ft

Slab plan & sectionNTS

Shopping list

☐ 74 × 80 lb bags concrete

Formula

volume = 12 × 10 × (4 ÷ 12) = 40 cu ft

order = 1.48 × 1.1 = 1.6 cu yd

bags = ceil(44 ÷ 0.6) = 74

Estimates for planning only — verify before purchaseSHEET 1 OF 1

How it works

A slab is a simple box: length × width × thickness. The only catch is that thickness is in inches while the pad is in feet, so the calculator divides the thickness by 12 before multiplying. That gives cubic feet; dividing by 27 converts to cubic yards, which is how ready-mix is sold.

Concrete is never a perfect pour — forms bow, subgrade dips, and some spills. A waste allowance (5–10% is typical) is added to the volume you order so you are not short mid-pour. Bag counts round up from that same padded volume, because you cannot buy a fraction of a bag.

Past about 1.5 cubic yards, a ready-mix truck usually beats bags on both cost and cure quality — mixing that many bags by hand is slow, and the cold joints between batches weaken the slab.

Worked example

Pad 10 ft × 12 ft · thickness 4″ · 80 lb bags · +10% waste
volume = 10 × 12 × (4 ÷ 12) = 40 cu ft = 1.48 cu yd
order = 1.48 × 1.10 = 1.6 cu yd → bags = ceil(44 ÷ 0.6) = 74

Questions from the jobsite

4 in for patios and walkways; 5–6 in where vehicles park. Thicken the edges to 8 in where a slab meets a driveway.

SOURCES — ACI 332 residential concrete · bag yield per manufacturer spec

Estimates for planning only — verify before purchase