How many bags of concrete do I need for a shed base? A table by shed size
An 8 × 6 ft shed base (2.44 × 1.83 m) is 4.46 m², and 100 mm thick with 10% extra it needs 0.491 m³ of concrete: 50 bags of 20 kg, each making 0.01 m³. 100 mm of Type 1 under it is 1 bulk bag of about 0.45 m³. Mixed on site instead, it takes 7 bags of cement and 1 bulk bag of ballast: 25 kg bags of cement and all-in ballast, at a cement maker's published quantities that the planner names. The thickness, the allowance and the bag’s yield are generic example figures, not a product’s and not a design: the thickness is decided for the job, and the yield is printed on the bag. The sub-base’s bulk bag and the mix on site are typical figures, each named on its line in the planner.
| Shed base | In metres | Base area | Concrete with 10% extra | Bags of 20 kg | Or mixed on site | Type 1, 100 mm deep | Your shed base |
|---|---|---|---|---|---|---|---|
| 6 × 4 ft | 1.83 × 1.22 m | 2.23 m² | 0.245 m³ | 25 | 4 bags of cement and 1 bulk bag of ballast | 1 bulk bag | Open 6 × 4 ft in the planner |
| 7 × 5 ft | 2.13 × 1.52 m | 3.25 m² | 0.358 m³ | 36 | 6 bags of cement and 1 bulk bag of ballast | 1 bulk bag | Open 7 × 5 ft in the planner |
| 8 × 6 ft | 2.44 × 1.83 m | 4.46 m² | 0.491 m³ | 50 | 7 bags of cement and 1 bulk bag of ballast | 1 bulk bag | Open 8 × 6 ft in the planner |
| 10 × 6 ft | 3.05 × 1.83 m | 5.57 m² | 0.613 m³ | 62 | 9 bags of cement and 2 bulk bags of ballast | 2 bulk bags | Open 10 × 6 ft in the planner |
| 8 × 8 ft | 2.44 × 2.44 m | 5.95 m² | 0.654 m³ | 66 | 10 bags of cement and 2 bulk bags of ballast | 2 bulk bags | Open 8 × 8 ft in the planner |
| 10 × 8 ft | 3.05 × 2.44 m | 7.43 m² | 0.818 m³ | 82 | 12 bags of cement and 2 bulk bags of ballast | 2 bulk bags | Open 10 × 8 ft in the planner |
| 12 × 8 ft | 3.66 × 2.44 m | 8.92 m² | 0.981 m³ | 99 | 14 bags of cement and 2 bulk bags of ballast | 2 bulk bags | Open 12 × 8 ft in the planner |
| 12 × 10 ft | 3.66 × 3.05 m | 11.15 m² | 1.226 m³ | 123 | 18 bags of cement and 3 bulk bags of ballast | 3 bulk bags | Open 12 × 10 ft in the planner |
| 14 × 10 ft | 4.27 × 3.05 m | 13.01 m² | 1.431 m³ | 144 | 21 bags of cement and 3 bulk bags of ballast | 3 bulk bags | Open 14 × 10 ft in the planner |
| 16 × 10 ft | 4.88 × 3.05 m | 14.86 m² | 1.635 m³ | 164 | 23 bags of cement and 4 bulk bags of ballast | 4 bulk bags | Open 16 × 10 ft in the planner |
Each row is the planner’s answer for that shed base, worked out when this page was built. The link at the end of a row opens the planner with that shed base filled in, to change the measurements and the product figures.
For the 10 × 6 ft, 8 × 8 ft, 10 × 8 ft, 12 × 8 ft, 12 × 10 ft, 14 × 10 ft and 16 × 10 ft shed bases, the planner warns: The bags come to more than a tonne to carry and mix by hand. The other ways are cement with sand and aggregate (or ballast) mixed on site, or ready-mixed concrete delivered: choose on the slab step.
This is a material quantity, not a design: the thickness, what goes under the slab, a membrane and any reinforcement are for the product's instructions, a designer or building control.
The 100 mm is an example thickness, not advice: what a base needs depends on what stands on it and on the ground, and is for the shed's instructions or a builder. Concrete is counted by volume, so a base 150 mm thick takes half as much again.
A bag's yield is printed on it. The planner's example, 0.01 m³ from a 20 kg bag, is one maker's figure for a multi-purpose mix: enter your bag's on the concrete calculator, and the bags follow.
Mixed on site is at a cement maker's published quantities for a general purpose mix: cement in 25 kg bags and all-in ballast in bulk bags of about 0.5 m³, typical figures whose sources the planner names on its lines. Which mix a base needs is not BuildBatch's to say.
The Type 1 (MOT) is 100 mm deep, a depth chosen for this table, not advice: the depth is decided for the job. A bulk bag fills about 0.45 m³ once compacted, from a merchant's own words named on the planner's line, so nothing is added for settlement. How much concrete do I need? works through a base.
What the table assumes
Each shed base is a rectangle.
- Doors and windows are not used for a slab.
- The volume is rounded once, at the end, up to whole bags. Working uses unrounded figures.
- The figures for bagged concrete mix are generic examples, not the figures of any product. Enter the thickness you have decided on and the yield printed on the bag for a closer answer.
How this table was made
It was worked out when this page was built, by the same room planner and formulas as the editable list: each row is one shed base sent through the planner, in feet. The sizes are common shed sizes. The thickness, the allowance and the bag’s yield are the planner’s labelled generic examples, not figures for a product; the mix on site and the bulk bags are typical figures whose sources it names.
AI tools drafted the page and automated tests compare every cell with the planner. It has not been checked by a tradesperson or anyone outside the build. No price is shown or used.
Before you use it
Measure the base you will lay, inside the formwork: often a little bigger than the shed itself. Check the figures against the slab, and follow each product's own instructions: they decide the mix, what a bag makes and how it is laid and cured.
Built and checked with AI tools, not yet by a tradesperson or anyone outside the build. Estimates are arithmetic on your measurements and the figures shown with them: follow each product's own instructions.