Foundation Drain Gravel Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/10/2026
The gravel volume for a perimeter drain is calculated with trench length × width × depth. For a trench 20 m long, 0.4 m wide, and 0.6 m deep, you need 4.8 m³ of gravel, about 7.68 tonnes.
Explanation
A perimeter drain is a gravel-filled trench (usually paired with a perforated drainage pipe) dug all around a building's foundation, to collect and carry away runoff and groundwater before it presses against the buried wall or seeps into the foundation. Here, the gravel acts as a filtering and draining layer: its gaps let water flow freely to the collector pipe, while keeping fine particles from the surrounding soil from clogging it over time (a geotextile often wraps the whole assembly to reinforce this filtering). The gravel volume needed is simply calculated from the trench's three dimensions: its length (the perimeter of the building concerned), its width (generally 30 to 50 cm, enough to house the drainage pipe and surround it with gravel), and its depth (which must reach at least the base of the foundation wall, often 50 to 80 cm). This calculator uses the same compacted gravel density (1.6 t/m³) as our slab hardcore gravel calculator, the same material being used in both cases for a different geometric purpose. Before laying this drain, you of course need to have dug the trench itself: see our excavation volume calculator for that preliminary earthwork.
Example: a 20 m trench around a foundation
Inputs
Length: 20 m. Width: 0.4 m. Depth: 0.6 m.
Calculation
Volume = 20 × 0.4 × 0.6 = 4.8 m³. Mass = 4.8 × 1.6 = 7.68 t.
Result
You need about 4.8 m³ (7.68 tonnes) of gravel to fill this drainage trench.
Frequently asked questions
Why pair a drainage pipe with the gravel instead of just filling with gravel?
Gravel alone already eases water pressure against the wall by giving it a preferred flow path, but a perforated drainage pipe laid at the bottom of the trench effectively concentrates and carries the collected water to a discharge point (a sump, a ditch, or the storm drain system), rather than simply letting it seep more slowly into the surrounding gravel.
What type of gravel should I use for a perimeter drain?
Clean, washed gravel with a uniform grain size (often 20 to 40 mm) is recommended, since gravel that's too fine or mixed with soil particles clogs faster and loses its draining effectiveness over time. A geotextile wrapping the gravel and pipe assembly further reinforces this protection against clogging by fines from the surrounding soil.
Is this drain enough to fully protect a buried wall from moisture?
No, the perimeter drain handles water that runs off or seeps toward the foundation from outside, but it's generally combined with waterproofing of the wall itself (bituminous coating, membrane) for complete moisture protection: the drain reduces water pressure against the wall, while the waterproofing keeps any residual water from passing through it.