Estimate a shed slab: concrete bags vs. ready-mix

Start with the specified slab dimensions

Choose Concrete slab in the shed planner. The estimate uses the outside shed footprint and one uniform slab thickness. Enter the thickness from the foundation detail and a separate concrete allowance. Thickened edges, footings, aprons and changes in level need additional quantities.

The floor choice changes the material estimate; it does not design a foundation or establish the finished floor elevation above grade.

Net volume (ft³) = footprint (ft²) × thickness (in) ÷ 12
Allowed volume = net volume × (1 + concrete allowance ÷ 100)

Use finished yield per bag

The quantity comes from the volume of concrete a mixed bag produces. The SAKRETE Canada 30 kg presets use approximately 14 litres per bag. Select a custom yield when the product or bag size differs. Use the supplier’s current data sheet.

A higher strength or faster setting time does not by itself change the geometric volume. Select the specified mix, then replace the example bag price with your quote.

Bags = round up (allowed litres ÷ finished litres per bag)

Separate the ready-mix order from the bill

Ready-mix quantity rounds up to the entered order increment, in cubic metres or cubic yards. Minimum billed volume affects the price; it does not increase the displayed physical order. Confirm any minimum delivery quantity directly with the supplier.

Delivery and short-load fees are totals for the order. Check pumping, waiting time and access separately. The starting CAD prices are examples, not live local quotes.

Supply cost = max(rounded order, minimum billed volume) × unit price + fees
Supply cost per ft² = supply cost ÷ shed footprint

Worked example: a 10 × 12 ft slab

At an example thickness of 4 inches, 120 ft² needs 40 ft³, or about 1.133 m³, before allowance. Adding 10% gives about 1.246 m³. At 14 litres per bag, round about 88.996 up to 89 bags. If the ready-mix order increment is 0.1 m³, the rounded order is 1.3 m³.

Those thickness and allowance inputs illustrate the calculation. They are not a slab specification. Compare the complete supplier prices against the same 120 ft² footprint.

Keep the two budgets separate

Concrete has its own supply comparison. The shed Materials budget covers its listed framing, surfaces, trim and selected insulation, so concrete must be added separately once when you assemble a project total.

Concrete supply excludes excavation, base preparation, forms, reinforcement, pumping, tools, placing and finishing labour, and tax. Record those items separately to avoid presenting a supply-only figure as an installed cost.

Build the rest of the shed budget