Attenuation Crate Quantity & Excavation Calculator
Enter the attenuation volume your drainage design requires. This returns the StormGrid module count, the excavation footprint, and how that excavation compares with a granular (gravel) alternative at the same storage.
Modules required—
Delivered storage—
Tank plan area—
Tank depth (crate block)—
Excavation depth incl. cover—
Excavation volume (crates)—
Excavation volume (gravel, 20% voids)—
Excavation avoided vs gravel—
Suggested load grade—
Cover check: enter your figures to see the governing load case.
What this does not do
This is a sizing aid, not a structural design. It converts a storage volume into module counts and excavation geometry. It does not calculate whether a grade survives your traffic load at your cover depth — that is a structural calculation to CIRIA C737, using the crate’s long-term (EN 17151) strength, with partial safety factors and the wheel load derived at the installed depth.
Two cover depths must be checked, not one: the shallowest cover governs dynamic wheel loads, and the deepest cover governs permanent (static) loads.
Module reference
| Module | Dimensions | Storage | Gross volume | Void ratio |
|---|
| H500 | 1000 × 500 × 500 mm | 237.5 L | 250 L | 95.0% |
| H400 | 1000 × 500 × 400 mm | 190 L | 200 L | 95.0% |
Excavation comparison assumes a granular alternative at 20% voids, the same required storage, and the same cover depth — a volume comparison only, not a cost comparison. Grade suggestion is an indication from the surface type; the specified grade must come from the structural calculation. Storm Manage holds short-term compression test data (Intertek 190702149GZU-001) and does not hold an EN 17151 long-term creep report.
