Blog · Floors & Walls
Concrete Slab Material Planning
From the size of the slab to bags, ready-mix and the base underneath: a step-by-step plan for a small concrete slab, with a worked 10 × 12 ft example.
What you need to work out
A slab estimate is really four quantities: the concrete itself, any gravel base underneath, the reinforcement, and the form boards around the edge. Each one has its own calculator, and this guide walks through them in order using one example: a 10 × 12 ft shed base, 4 in thick.
Concrete thickness, base depth and reinforcement are design decisions. Use the figures your plans, local code or engineer give you; the numbers here only show the arithmetic.
Step 1: Concrete volume
Length × width × thickness, with thickness in feet: 10 × 12 × (4 ÷ 12) = 40 ft³, which is 1.48 yd³. The concrete calculator gives the same answer in cubic feet, cubic yards and cubic metres.
Step 2: Add an allowance once
An uneven base or slightly bowed forms soon eat into the volume. With a 10% allowance the example becomes 44 ft³ (1.63 yd³). Apply the allowance once, in the calculator’s extra-material field.
Step 3: Bags or ready-mix
| Option | For 44 ft³ (1.63 yd³) | Based on |
|---|---|---|
| 80 lb bags | 74 bags | 0.60 ft³ per bag |
| 60 lb bags | 98 bags | 0.45 ft³ per bag |
| Ready-mix | 1.63 yd³, rounded as your supplier orders | Supplier’s own minimums and increments |
Bag yields above are the approximate figures on QUIKRETE’s Concrete Mix data sheet; always use the yield printed on the bag you buy. At this size, 74 bags of 80 lb is nearly 6,000 lb of mix to carry and mix, so it is worth pricing a mixer rental or a small ready-mix load alongside the bags.
Step 4: Gravel base
If your plan calls for a compacted gravel base, estimate it as its own layer. A 4 in base under the same 10 × 12 ft area is another 40 ft³ (1.48 yd³), or about 2.1 tons at a planning density of 1.4 tons per cubic yard. Use the crushed stone calculator with your supplier’s density.
Step 5: Reinforcement and forms
If the design uses rebar on a grid, the rebar calculator counts bars and total length for a rectangular layout. The forms run around the perimeter: 2 × (10 + 12) = 44 ft of edge board, which you can price with the lumber calculator.
Shopping list for the example
- Concrete: 1.63 yd³ including 10% allowance (74 × 80 lb bags, or ready-mix).
- Gravel base, if specified: about 1.48 yd³ for 4 in.
- Reinforcement: from your design, counted with the rebar calculator.
- Form boards: 44 ft of perimeter, plus stakes.