Concrete Calculator
How much concrete for your slab? Enter length, width, and thickness to get cubic yards to order plus 80 lb bag equivalents.
This free concrete calculator handles patios, sidewalks, shed pads, and driveways. Enter your slab dimensions and thickness, optionally add a 10% waste allowance, and get the cubic yards for a ready-mix order plus how many 80 lb bags of premix you would need instead.
How to use this calculator
- Step 1: Measure your slab length and width in feet.
- Step 2: Enter the thickness in inches (4 for patios, 5-6 for driveways).
- Step 3: Leave the 10% waste allowance on unless your forms and subgrade are laser-perfect.
- Step 4: Click Calculate — use the cubic yards number when calling a ready-mix plant, or the bag count for premix.
The formula
Cubic yards = length x width x (thickness/12) / 27. Bags = round up(cubic feet / 0.6).
Worked example
A 10x10 ft patio at 4 inches thick needs 1.23 cubic yards of concrete — that is 56 eighty-pound bags of premix. With the 10% waste allowance, order 1.36 yards or buy 62 bags.
Frequently Asked Questions
How many 80 lb bags of concrete do I need?
One 80 lb bag of premix yields 0.6 cubic feet of concrete. Divide your total cubic feet by 0.6 and round up. A 10x10 ft slab at 4 inches thick is 33.3 cu ft, which needs 56 bags. For anything over about 45 bags (1 cubic yard), ordering ready-mix is usually cheaper and gives a more consistent mix.
When should I order ready-mix instead of mixing bags?
Order ready-mix for pours over roughly 1 cubic yard — that is about 45 eighty-pound bags. Mixing that many bags by hand takes all day and the first batches start setting before you finish. Ready-mix also costs less per yard at that volume and arrives with a consistent, properly proportioned mix.
How thick should a concrete slab be?
Four inches is standard for patios, sidewalks, and shed pads that only carry foot traffic or light loads. Use 5-6 inches for driveways, especially if trucks or RVs will park on them. Thicker slabs also need rebar or wire mesh — 4 inch residential slabs typically use wire mesh, 6 inch slabs use rebar.
Should I order extra concrete?
Yes — add 5-10% over the calculated volume. Forms are rarely perfectly level, subgrades settle unevenly, and spillage happens. A short load costs you a second delivery fee and a cold joint in the slab, which is far more expensive than one extra bag or a tenth of a yard. This calculator has a one-click 10% waste option.