Slab Base Gravel Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The gravel volume for a slab sub-base is calculated with surface area × thickness. For 50 m² of slab with a 15 cm sub-base, about 7.5 m³ of gravel is needed, roughly 12 tonnes.
Explanation
The sub-base (called "hérisson" in French building practice) is a drainage layer of aggregate (usually crushed 20/40 mm gravel) laid directly on the ground, under a concrete slab poured on grade: its role is twofold — ensuring effective drainage that prevents rising damp from reaching the slab by capillary action, and creating a stable, compacted base that spreads loads over sometimes uneven soil. The French DTU 13.3 standard, the reference document for slab-on-grade construction, treats it as a non-negotiable requirement for any slab poured on grade: its standard thickness is 15 cm in the common case, increased to 20 or 25 cm on difficult ground (clay soil or flood-prone areas), with this calculator leaving that parameter adjustable depending on the ground conditions. This sub-base is clearly distinct from the volume of soil to excavate before it is laid (two opposite material quantities: one to remove, the other to bring in) and from the concrete poured on top of it once the sub-base is compacted and possibly covered with a polyethylene sheet. Since the compacted gravel's density varies between 1.5 and 1.7 tonnes per m³ depending on the exact aggregate and its compaction level, this calculator uses 1.6 t/m³, the median value of this range commonly cited by aggregate suppliers — allow an ordering margin of about 10% to absorb settling during compaction with a plate compactor.
Example: a 50 m² slab, 15 cm sub-base
Inputs
Slab surface area: 50 m². Sub-base thickness: 15 cm.
Calculation
Volume = 50 × 0.15 = 7.5 m³. Mass = 7.5 × 1.6 = 12 tonnes.
Result
About 7.5 m³ of gravel, roughly 12 tonnes, is needed for this sub-base.
Frequently asked questions
Can a sub-base be skipped under a concrete slab?
No, the DTU 13.3 standard treats the sub-base as a non-negotiable requirement for any slab poured on grade: without this drainage layer, soil moisture rises into the slab by capillary action, and insufficient compaction of the natural ground exposes the slab to differential settling and cracking.
What gravel size should be chosen for a sub-base?
Two grain sizes are commonly used depending on the target thickness: 20/40 mm for a standard sub-base, and 40/63 mm for thicker layers or particularly well-draining soils. A geotextile membrane is usually laid between the natural ground and the sub-base to prevent fine soil particles from rising and clogging the drainage.
Why allow a 10% margin on the calculated quantity?
Because gravel delivered in bulk or in bulk bags settles by about 10 to 20% of its apparent volume once compacted with a plate compactor on site: the delivered quantity should therefore slightly exceed the finished sub-base's theoretical volume to compensate for this settling.