Size a concrete slab for a patio, driveway or shed floor, including thickness advice and reinforcement.
Quick answer: A slab is length × width × thickness, with thickness converted to feet first. A 12 ft × 20 ft driveway at 5 in thick is 240 sq ft × 0.42 ft = 100 cubic feet, or about 3.7 cubic yards — roughly 167 bags of 80 lb mix. One cubic yard covers about 81 sq ft at 4 in thick.
What are you pouring?
ft
ft
in
4 in suits patios and shed floors; 5–6 in for driveways carrying vehicles.
Concrete needed
2.47 cu yd
66.7 cu ft · 1.89 m³
Order this
2.75 cu yd
Rounded up with 5% waste — ready-mix is sold in 1/4 yd steps
Bag equivalent
80 lb bags (0.60 cu ft each)112 bags
60 lb bags (0.45 cu ft each)149 bags
Weight of the pour4.9 tons (about 4,000 lb per cu yd)
Ready-mix trucks usually carry 8–10 cu yd. A short load under the minimum often carries a fee, and some suppliers charge for the return of unused concrete — say yes to 5% extra rather than a second trip.
Concrete is ordered in cubic yards and almost always rounded up. Add 5–10% for spillage, uneven subgrade and washout. Bag counts assume 80 lb bags at 0.60 cu ft each.
Core facts
Patio thickness
4 in, typical
Driveway thickness
5–6 in, typical
Coverage per cubic yard
about 81 sq ft at 4 in
Reinforcement
mesh or rebar in most vehicle slabs
What is Concrete Slab Calculator?
A slab is the simplest concrete shape and the one homeowners pour most often: a flat rectangle, usually 4 to 6 inches thick, sitting on compacted gravel. Because the shape is regular the estimate is straightforward. Length times width gives the area, and area times thickness gives the volume. What catches people out is thickness. An extra inch across a 400 sq ft driveway is another 1.2 cubic yards, which is real money, and a slab that is too thin cracks under a vehicle. This calculator accepts inches and returns cubic yards and bag counts, so you can compare a bagged weekend pour against a delivered load.
Common uses for Concrete Slab Calculator
Work out the concrete for a backyard patio slab
Estimate a driveway slab at the thickness your soil needs
Size a shed or garage floor slab before you frame it
See how much a 1 inch thickness change adds to the price
Plan bag counts for a slab you will mix yourself
Thickness drives the whole estimate
Thickness is the variable that matters most and the one most often guessed. Moving a 400 sq ft slab from 4 to 5 inches adds about 1.2 cubic yards, which is roughly $170 to $240 of delivered concrete in many US markets. Decide thickness from what the slab will carry and what your soil does, not from what happens to be left in the truck.
Base preparation is not optional
A slab cracks when the ground under it moves. That means excavating down to stable soil, compacting in lifts, and laying 4 inches of crushed stone that drains. In cold climates, frost-susceptible soils need a deeper base or a thickened edge. The concrete calculator covers none of this, so count the gravel separately and budget a full weekend for preparation.
Joints, curing and finishing
Concrete gains most of its strength in the first week, and it needs moisture to do that. Keep it damp or covered for at least seven days. Cut control joints within the first day or two, at intervals of roughly two to three times the slab thickness in feet, and the cracks that do appear will sit neatly inside them instead of across the middle of your new patio.
Frequently Asked Questions
How thick should a patio slab be?
Four inches is the usual minimum for a patio on firm, well-compacted ground, with a 4 inch gravel base underneath. If the slab will hold a hot tub, a heavy planter or a vehicle, go to 5 or 6 inches. Thicker slabs also need control joints so cracking happens where you want it.
Do I need rebar or wire mesh?
For a patio, welded wire mesh or fiber reinforcement is usually enough and is sometimes omitted on small slabs. Driveways, garage floors and anything carrying vehicles should have rebar or mesh placed in the middle of the slab thickness. Reinforcement holds cracks tight; it does not stop them forming.
How do I convert inches of thickness to feet?
Divide by 12. Four inches is 0.33 ft, five inches is 0.42 ft and six inches is 0.50 ft. Multiply length times width times that decimal to get cubic feet, then divide by 27 for cubic yards. The calculator does the conversion for you.
How many square feet does a cubic yard cover?
About 81 sq ft at 4 inches thick, 65 sq ft at 5 inches and 54 sq ft at 6 inches, because a cubic yard is 27 cubic feet. That is handy for quick mental checks: a 20 by 20 ft patio at 4 inches is just under 5 cubic yards.
Why did I need more concrete than my area suggests?
Because thickness, subgrade variation and spillage all consume volume. Slabs rarely sit on a perfectly flat base, and thickened edges, footings under a stem wall and any over-excavation add concrete. Order 5 to 10 percent extra and spread the leftovers into a small pad.