Gravel Under Concrete Slab Calculator
Use this gravel under concrete slab calculator to estimate how much sub-base material you need below a concrete slab. Enter the slab size and base depth to calculate volume, weight, and bulk bag quantities for materials such as MOT Type 1, crushed stone, or gravel. It is useful for shed bases, patios, paths, and slab foundations where you need a quick sub-base estimate before ordering materials.
- Paths and patios (light use): 50–100 mm
- Driveways (light vehicles): 100–150 mm
- Heavier use: 150–200 mm
These are general planning ranges only. The correct depth depends on the ground, load, drainage, and project design.
How this calculator works
This calculator works out the sub-base volume by multiplying the slab length, width, and chosen sub-base depth using consistent units. It then applies the compaction or wastage allowance if selected, and converts the result into weight using the selected material density. If bulk bag estimates are shown, the calculator also divides the adjusted volume by the bag volume (0.6 m³ or 0.8 m³) and rounds up to a whole number of bags.
Formulas used
volume = length × width × sub-base depth
adjusted volume = volume × 1.10
weight = adjusted volume × material density (t/m³)
bulk bags = ⌈adjusted volume ÷ bag volume⌉
Sub-base materials compact when laid and vibrated, so many estimates include extra material to allow for compaction and site waste. Final ordering should be checked against supplier bag sizes, density figures, and the required finished depth. Material densities used: MOT Type 1 = 2.0 t/m³, 20mm gravel = 1.6 t/m³, 10mm gravel = 1.5 t/m³, hardcore = 1.8 t/m³.
Worked example
Imagine a slab area measuring 5 m by 3 m with a sub-base depth of 100 mm. First convert the depth to metres (0.1 m) and calculate the base volume: 5 × 3 × 0.1 = 1.5 m³. If the 10% compaction allowance is checked, the adjusted volume becomes 1.5 × 1.10 = 1.65 m³. To convert to tonnes, multiply by the material density — for MOT Type 1 at 2.0 t/m³ that is 1.65 × 2.0 = 3.3 tonnes. To estimate bulk bags, the calculator divides the adjusted volume by the bag size and rounds up.
Why sub-base matters under a concrete slab
The sub-base helps create a stable, level platform beneath the slab and can improve load distribution and drainage. Getting the quantity roughly right helps avoid delays, under-ordering, and unnecessary extra deliveries. This calculator is for quantity planning only and does not replace project-specific groundwork or structural advice.
Frequently asked questions
Related tools
- UK Gravel Calculator — volume, tonnes and bulk bags for driveways, paths and borders
- Concrete Slab Bag Calculator — how many bags of concrete for a slab
- Weight of Concrete Slab Calculator — estimate slab weight in kg, tonnes and lb
- Dry Pour Concrete Calculator — volume, bags and cost for dry-pour slabs
- Monolithic Concrete Slab Calculator — concrete volume for slabs with thickened edges
Guides worth reading
- How Much Sub-Base Under a Concrete Slab? UK Guide — sub-base depths by use, and how to order MOT Type 1
Disclaimer: This calculator is for planning and estimating only. Actual aggregate quantities vary with compaction, density, waste, and site conditions. Always confirm supplier bag sizes, densities, and final sub-base requirements before ordering.