Concrete Calculator — Concrete Slab Calculator
The slab view answers the most common concrete question — how much for a patio, walkway, shed base, or driveway — from three tape-measure numbers: length, width, and thickness. It reports cubic feet, cubic yards, and a waste-adjusted order figure side by side, so a 10 × 10 ft patio at 4 inches reads as 1.23 cubic yards formed and 1.36 to actually order, while a 20 × 20 ft driveway at 5 inches scales to 6.79 cubic yards with waste — a number that settles the truck-versus-bags question by itself.
Concrete volume
Concrete to order
1.36 yd³
33.33 ft³ · 1.23 yd³ before waste · 62 × 80 lb bags
Breakdown
Volume = shape geometry × a disclosed waste factor; 1 cubic yard = 27 cubic feet. Bag counts use standard pre-mixed yields (80 lb ≈ 0.60 ft³). A planning estimate for ordering, not an engineering specification.
Slab thickness conventions
Thickness is the input people guess at, and it moves the answer linearly. The common conventions: 4 inches for patios, walkways, and shed bases carrying foot traffic; 5 to 6 inches for driveways and anything that parks a vehicle. The same 20 × 20 ft footprint is 6.17 cubic yards at 5 inches before waste — going to 6 inches adds a fifth more concrete, which is why thickness, not footprint, is usually the budget lever.
These are conventions, not a design. Soil conditions, climate, reinforcement, and load can all push a slab thicker, and local code may set minimums — confirm the section with your building department or contractor before ordering for anything structural.
Ordering ready-mix for a slab
Order the waste-adjusted figure, not the formed volume: 6.79 cubic yards for the 20 × 20 ft driveway example, not 6.17. Suppliers sell in fractional-yard increments and charge short-load fees on small deliveries, so get the quote on the rounded-up quantity and ask how the fee schedule works below a full truck.
A slab is a continuous pour — the whole placement should be screeded and finished before any of it sets. That is the practical argument for ordering slightly long: leftover concrete costs a few dollars; a mid-slab cold joint from running short costs the slab.
Questions
- How thick should a concrete slab be?
- 4 inches is the convention for patios, walkways, and shed bases; 5–6 inches for driveways and vehicle loads. Structural slabs are a code and engineering question, not a calculator default.
- How much concrete does a 10 × 10 slab need?
- At 4 inches thick, 33.33 cubic feet = 1.23 cubic yards, or 1.36 cubic yards with the default 10% waste factor — the figure to actually order.
- Should I order ready-mix or use bags for a slab?
- Around one cubic yard is the crossover. A 10 × 10 ft slab already needs about 62 80-lb bags, so most slab-sized pours are ready-mix orders; bags suit pads and repairs well under a yard.