How to calculate drainage pipe slope and fall
Pipe gradient is the vertical fall divided by horizontal distance. At 2%, a pipe drops 2 cm per horizontal metre: over 8 m it drops 16 cm. Calculate the fall, the existing gradient or the theoretical horizontal run for a known fall and gradient. The calculator cannot measure the actual space at your site: you need to measure it yourself. It also shows the geometrical length of a straight sloping pipe.
Example: 8 m horizontal run × 2 cm/m = 16 cm fall (160 mm), equivalent to 2% or 1 in 50.
Results are estimates and depend on the supplied values.
Drainage gradients: formulas and worked examples
How much does a pipe drop at 2%?
At 2% the inside bottom of the pipe drops 2 cm per horizontal metre. Multiply the run by the drop per metre. An 8-metre run gives 16 centimetres.
Fall (cm) = horizontal run (m) × gradient (%) · 8 × 2 = 16 cm
Find the slope from a measured fall
A 12 cm drop across 8 horizontal metres gives a 1.5% gradient. That also means 1.5 cm of drop per metre.
Gradient (%) = fall (cm) ÷ horizontal run (m) · 12 ÷ 8 = 1.5%
Find the horizontal run from known fall
Use this when the run is not fixed: with 12 cm of available fall and a 2% target gradient, you use that fall over 6 horizontal metres. It cannot measure the available space at your site. If the actual space is 5 m, use that measurement to calculate the gradient.
Horizontal run (m) = fall (cm) ÷ gradient (%) · 12 ÷ 2 = 6 m
Horizontal run versus actual sloping pipe length
The slope calculation uses the horizontal run, not the length along the sloping pipe. For a straight pipe segment, its geometrical length is the hypotenuse of the triangle. With an 8 m horizontal run and 16 cm drop, the pipe length is approximately 8.0016 m, excluding fittings, cuts and waste.
Sloping pipe length = √(horizontal run² + fall in metres²)
Understanding 1 in 50 and invert levels
The ratio 1 in 50 means one vertical unit of drop per 50 horizontal units. With a starting pipe invert level of 100.000 m, a 16 cm drop gives a final invert level of 99.840 m.
Ratio 1:N = 1:(100 ÷ gradient %) · End level = start level − fall (cm) ÷ 100
Why does a steeper gradient give a shorter run for the same fall?
Imagine the pipe may drop by only 12 cm from its starting level. The more it drops per metre, the sooner you use all 12 cm. Therefore the theoretical horizontal run decreases as the gradient increases.
| Gradient | Fall per metre | Run with 12 cm fall |
| 1% | 1 cm/m | 12 m |
| 2% | 2 cm/m | 6 m |
| 3% | 3 cm/m | 4 m |
| 4% | 4 cm/m | 3 m |
These are mathematical runs, not permitted maximum pipe lengths or the physical space available at your property. If your run is already known, enter it into one of the other calculations.
Important: slope geometry alone does not establish whether a drain is suitable. Drain design depends on flow rate, diameter, material, velocity, network type, traps, access points, applicable regulations and construction details. This calculator uses horizontal distance; an invert elevation is not necessarily the depth below ground.
Planning a drainage trench too? Estimate gravel, pipe and geotextile with our trench calculator. Open drainage trench calculator
Frequently asked questions
How many centimetres per metre are 1% and 2%?
1% is 1 cm/m and 2% is 2 cm/m. In millimetres these are 10 and 20 mm of drop per metre.
How much fall is needed over 10 metres at 2%?
The total is 20 cm: 10 m × 2 cm/m = 20 cm.
What does a 1 in 100 pipe fall mean?
One vertical unit of fall per 100 horizontal units, equivalent to 1% or 1 cm per metre.
Should I measure the pipe length or horizontal distance?
Slope uses horizontal distance. The sloping pipe is slightly longer: we show it separately using Pythagoras for a straight section. This is not a substitute for measuring the pipe you need to buy.
How does the calculator know how long my available space is?
It does not. It uses only your inputs. If you have already measured the physical run, enter it in «Vertical fall in cm» or «Existing gradient». The third mode calculates a theoretical horizontal run from gradient and fall.
Why does a higher slope give fewer metres?
Because the available drop is fixed. With 12 cm, at 2% you drop 2 cm per metre and cover 6 m; at 4% you drop 4 cm per metre and cover only 3 m. Increasing available fall while keeping the same gradient would give a longer run.
What if the available space is exactly 5 metres?
Measure the horizontal run. With 12 cm fall over 5 m, the geometric gradient is 12 ÷ 5 = 2.4%. To find the required drop for a chosen gradient, use the first mode.
Does this determine the legal minimum gradient?
No. Requirements vary by drain type, pipe diameter, location and applicable design codes. It is a geometry calculator, not a hydraulic design approval.