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Groundworks · mm · 1 in N · %
How much fall does a pipe need over its run?
The drop over a run of pipe at a gradient - and, if you measured a drop, the gradient it gives. The gradient a drain needs comes from its design or Building Control; this does the arithmetic either way.
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.
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.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Drop =
run ÷ Nat a gradient of 1 in N - Drop per metre =
1 ÷ N; as a percentage =100 ÷ N - From a measured drop:
N = run ÷ drop
A worked example
With the defaults the page opens on:
- How long is the run? 10 metres
- At what gradient - 1 in …? 40
- The drop you measured (optional) 0 millimetres
At 1 in 40
- Run: 10 m
- Drop over the run: 250 mm — run ÷ 40
- Drop per metre: 25 mm — 1 ÷ 40
- As a percentage: 2.5 % — 100 ÷ 40
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 fall over a pipe run?
Run ÷ N for a gradient of 1 in N. A 10 m run at 1 in 40 drops 250 mm - 25 mm a metre, a 2.5% slope.
What gradient does a drain need?
That comes from the drainage design or Building Control, not from this calculator. Type the gradient you have been given and it works out the drop.
I measured the drop - what gradient is it?
Give the drop as well and the calculator divides the run by it: 250 mm over 10 m is 1 in 40.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
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.