Trench Calculator
Works out trench excavation, bedding, backfill and surplus spoil for a drain, service or footing run.
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The formula
excavation = length × width × depth spoil = excavation × (1 + bulking) bedding = width × (bedding + pipe + 150 mm) × length − pipe volume backfill = excavation − bedding − pipe
A trench is a long box. The pipe and its granular surround displace part of it, so the backfill is less than the hole — and because the spoil bulked when it came out, there is always more of it than fits back, even before the pipe is in.
What to watch out for
Things that change the order and the day:
- Width is the pipe plus room to work. 300 mm wider than the pipe is a minimum; a mini-digger bucket sets it at 450 or 600 mm in practice.
- Bedding is a separate material. Pea gravel or sand, under and around the pipe to 150 mm above it, so nothing sharp bears on it and it does not settle.
- Depth is often a rule. Water services below the frost line — 750 mm in much of the UK, far more in cold climates. Drains under a drive want cover or protection.
- Over 1.2 m deep, shore it. This is not optional and the risk does not announce itself.
Frequently Asked Questions
How much spoil does a trench produce?
Length × width × depth, times the bulking factor. A 25 m trench 450 mm wide and 750 mm deep is 8.4 m³ in the ground, about 10.5 m³ of loose spoil at 25% bulking.
How much pea gravel for pipe bedding?
Trench width × (100 mm under + pipe + 150 mm over) × length, less the pipe. For a 110 mm pipe in a 450 mm trench that is about 0.15 m³ per metre of trench — roughly 0.27 tonnes per metre.
How deep does a trench need shoring?
From about 1.2 m, and sooner in loose or wet ground. Trench collapses are fast and the soil is far heavier than it looks — a cubic metre is over a tonne.