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Groundworks · m³ · lengths
How much pipe and pea shingle for a drainage run?
A foul or surface water run: pipe lengths, the pea shingle round it, and what comes out of the trench. Bedding and surround depths follow Part H unless the drawing says different.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Shingle = trench width × (bed + pipe + surround) × run, less the volume the pipe itself takes up
Trench dig = width × invert depth × run
Backfill reused = dig less the shingle zone; anything left over is carted away, bulked
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
Rate
Default
Why
Shingle density
1.6 t/m³
10–20 mm pea shingle, loose.
Spoil density
1.9 t/m³
Bulking
30 %
Swell on the spoil once it is out.
Bulk bag weight
0.85 t
A worked example
With the defaults the page opens on:
How long is the run? 24 metres
How wide is the trench? 600 millimetres
How deep is the invert? 0.9 metres
Which pipe? 110 mm — foul branch
How much bedding under the pipe? 100 millimetres
How much surround over it? 150 millimetres
What length does the pipe come in? 6 metres
Pipework
Pipe lengths: 4 lengths— 24 m ÷ 6 m
Couplings: 4 no. — one per length
Bends & junctions: 0 no. — count these off the drawing — they are not in the run length
Bedding & surround
Surround zone depth: 360 mm — bed + pipe + surround
Pea shingle volume: 4.96 m³ — trench × zone × run, less the pipe
Pea shingle weight: 7.93 t— × 1.6 t/m³
Bulk bags: 10 bags — at 0.85 t
Earthworks
Trench dig: 12.96 m³ — width × invert × run
Backfill reused: 7.78 m³ — as-dug above the surround, if it is suitable
Surplus to cart away: 6.74 m³ — bulked 30%
Surplus weight: 9.85 t
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How much pea shingle do I need per metre of drain?
The trench width × (bed + pipe + surround), less the volume the pipe itself takes up, at 1.6 t/m³. For a 600 mm trench and 110 mm pipe with 100 mm bed and 150 mm surround, that is about 0.34 t per metre.
How much bedding goes under a drain?
The defaults are 100 mm of granular bed under the pipe and 150 mm of surround over the crown, which is the usual Part H arrangement. Change them if the drawing says different - the drawing wins.
How wide should a drainage trench be?
Pipe diameter plus about 300 mm is the usual minimum for someone to work in it. The calculator defaults to 600 mm.
Does it count bends and junctions?
No - it shows a row for them with a zero so you remember to count them off the drawing. They are not in the run length and no formula can guess them.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
