How this calculator works
The calculator multiplies your slab’s length, width and thickness to get the volume, converts it to cubic yards (the unit concrete is sold in), then works out how many bags that equals.
10 × 10 × (4 ÷ 12) = 33.3 cubic feet → 33.3 ÷ 27 = 1.23 cubic yards. Add 10% and you’d order about 1.35 cubic yards.
Bags vs. ready-mix
For small jobs under about 1 cubic yard, bagged concrete is easiest. Above that, ready-mix delivered by truck is normally cheaper and far less work than mixing dozens of bags. For a slab that slopes for drainage, calculate with the average thickness—not the thin edge or thick edge alone. Measure any thickened edges, steps, or separate footings as their own volumes and add them before the 10% allowance. Order ready-mix by the calculator's “With 10% waste allowance” result, then ask the supplier about its minimum-load charge and delivery timing before scheduling the pour. For a reinforced slab, estimate the grid separately with the rebar calculator.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
At 4 inches thick, about 1.23 cubic yards (≈74 of the 60lb bags). Order ~1.35 cubic yards to cover waste.
How many 80 lb bags are in a cubic yard?
About 45 bags — each 80lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.