How to Work Out Drainage Fall

What you need

A tape measure, a spirit level (or a level and a string line), pegs, and the drainage slope calculator. Measure the run horizontally — the fall is measured over the horizontal length, not along the slope of the pipe.

Step 1 — Measure the run

Measure the horizontal length of the pipe or channel, from the high end to the low end. For a drain run, that is the distance from the start of the pipe to where it joins the next pipe or the sewer. Write it down in metres — the calculator uses the same unit for length and fall, so keep them consistent.

Step 2 — Work out the slope from the fall

Divide the fall by the length and multiply by 100: that is the slope percentage. The drainage slope calculator does it and gives you the gradient too: a pipe 10 m long with a 0.25 m fall has a slope of 0.25 ÷ 10 × 100 = 2.5%, which is a gradient of 1 in 40. The gradient is how drainage is usually described — 1:40 means one unit of fall for every forty units of length.

Step 3 — Work out the fall from the slope

The calculator works backwards too: give it the length and the slope you want, and it gives you the fall. A 20 m run at 1% needs a fall of 20 × 0.01 = 0.2 m, which is a gradient of 1:100. That is useful when you know the gradient the code asks for and need to know how deep to dig at each end.

Step 4 — Know the guides

Common guides are 1:40 (2.5%) for 110 mm drains and 1:100 (1%) as a minimum — but those are guides, not rules. Your local code decides the final design: the pipe size, the gradient and the inspection points all depend on it. Never design a drain from a guide alone; check the local requirements first.

Step 5 — Check the levels on site

Set a string line between pegs at the high and low ends and check the fall with the level. The fall must be continuous — a dip in the middle holds water and solids, and a high spot blocks the flow. Check it before you lay the pipe, because a buried pipe with the wrong fall is a pipe you dig up again.

Worked example

A pipe 10 m long with a 0.25 m fall has a slope of 0.25 ÷ 10 × 100 = 2.5%, and a gradient of 1 in (10 ÷ 0.25) = 1:40. The other way round: a 20 m run at 1% needs a fall of 20 × 0.01 = 0.2 m. Those are exactly the numbers the drainage slope calculator gives you.

Common mistakes

• Measuring along the slope instead of horizontally. The run is the horizontal distance. Measuring along the pipe gives a longer run and a smaller slope than the drain really has.
• Too little fall. Water stands, solids settle, and the drain blocks. If the fall is below the local minimum, the design is wrong — not the water.
• Too much fall. Water runs faster than the solids, which are left behind to block the pipe. Steeper is not better.

Safety

Check your local requirements before you design or dig any drainage: connections to public sewers need approval, and the gradient, pipe size and inspection points are set by code, not by preference. Never direct water towards a building or a neighbour's property — the fall must take water away from structures, not towards them. Check for underground cables and pipes before you dig, and get a qualified professional for any work that connects to a public sewer.

Try these calculators

• Drainage Slope Calculator — slope, fall and gradient for any run.
• Pipe Size Calculator — the pipe diameter a flow rate needs.
• Pipe Length Calculator — pipe length from the run and the fittings.