Shed planning · Guide 3 of 7 · 3 min readUpdated
Estimate a shed slab: concrete bags vs. ready-mix
Decide when bagged concrete is manageable and when ready-mix makes more sense by comparing access, mixing effort, placement time and the complete supply cost.
Choose a supply method your crew can place
Bags can suit a small pour, restricted vehicle access or work where a mixer can be set close to the forms. You control the batching, but every bag must be transported, opened, mixed and placed. Count that work as well as the purchase price.
Ready-mix can make a larger continuous placement more practical when the forms, access and finishing crew are ready. The trade-off is a scheduled delivery, possible short-load or waiting charges, and a plan to get the concrete from the truck to the slab. A cheap cubic-metre rate is not the whole delivered job. Neither method makes an undersized crew or unprepared base disappear.
Example: the small slab that needs a lot of bags
The 10 × 12 ft example below comes to 89 bags at the stated thickness, allowance and yield. At 30 kg each, that is 2,670 kg of dry material to handle before adding water. That does not automatically make ready-mix cheaper, but it is a strong reason to price delivery and compare a realistic mixing-and-placing plan.
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.
Technical references
Confirm the current product literature and requirements for the material and assembly being installed.