Concrete Calculator
Calculate everything a slab needs. Enter the dimensions for concrete volume and bag counts, then add a ready-mix price, rebar spacing, and gravel base depth for a complete materials estimate.
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Check it outA full slab materials list
Beyond concrete volume, most slabs need a compacted gravel base and a rebar or wire grid. This estimates rebar as a grid at your chosen spacing (total linear feet to buy) and the gravel base as its own volume. Past about a cubic yard, ready-mix delivery usually beats bagging it by hand — enter a price per yard to compare. The waste allowance covers uneven subgrade and spillage.
How it’s calculated
Volume = length × width × thickness ÷ 27 cubic yards, plus waste. Bags = cubic feet ÷ yield (0.6 for 80 lb, 0.45 for 60 lb). Rebar is a grid at your spacing; the gravel base is its own volume.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Worked example
A 10x10 slab at 4 inches with 10% waste is ~1.36 cubic yards (~62 80-lb bags), plus rebar and gravel base.
Common mistakes
- Mixing up inches and feet for thickness.
- Ordering bags for a job where ready-mix is cheaper.
Where it is used
- Ordering concrete for a slab, footing, or driveway.
- Deciding between bags and ready-mix delivery.
Frequently asked questions
How is rebar length estimated?
As a grid at your spacing: bars run both directions across the slab, and the calculator totals their linear feet. Add overlap for splices on big pours.
Bags or ready-mix?
An 80 lb bag yields ~0.6 cu ft. Above roughly one cubic yard, ready-mix is usually cheaper and far less work.
Why a gravel base?
A compacted gravel base improves drainage and load support, reducing cracking. Four inches is common under slabs.
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