What this calculator does
Drainage pipes need a consistent fall so that solids are carried along rather than left behind. Too little fall and the pipe blocks; too much and the water can run away ahead of the solids, which causes the same problem for a different reason. The required grade comes from the plumbing code and the pipe diameter, and is usually written as a ratio such as 1 in 60.
This calculator converts between that ratio and the actual drop over a given run, in both directions, and also expresses the grade as a percentage and an angle.
The formula
Fall is the run length divided by the grade ratio, so a 12 metre run at 1 in 60 drops 0.2 metres. Working backwards, the grade ratio is the run length divided by the fall. The percentage is the fall divided by the run, and the angle is the arctangent of the same figure.
| Term | Meaning |
|---|---|
| Grade | The slope of the pipe, usually written as a ratio like 1 in 60, meaning it drops one unit for every sixty travelled. |
| Fall | The total vertical drop over the run, which is what gets measured on site. |
| Run | The horizontal length of the pipe between the two points being set out. |
The inputs explained
| Field | What to enter |
|---|---|
| Calculate | Choose whether to find the fall from a known grade, or the grade from a measured fall. |
| Pipe run length (m) | The horizontal length of the pipe run in metres. |
| Grade as 1 in … | The grade written as 1 in this number. A larger number is a shallower grade. |
| Fall (drop over the run) (mm) | The vertical drop over the run in millimetres, when working back to a grade. |
When to use it
Setting out a drain
Knowing the total fall over the run gives the invert level at the far end, which is what the trench depth is set from.
Checking an existing pipe
Measuring the drop between two points and converting back to a grade shows whether an installed pipe meets the required minimum.
Working between units
Codes state grades as ratios, site measurements come out in millimetres, and some drawings use percentages. All three describe the same slope.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much fall does a 12 metre run need at different grades?
The same run length at a range of grades.
| Grade | Fall over the run | Grade as a percentage |
|---|---|---|
| 1 in 40 | 300 mm | 2.500% |
| 1 in 60 | 200 mm | 1.667% |
| 1 in 80 | 150 mm | 1.250% |
| 1 in 100 | 120 mm | 1.000% |
Questions
What grade should a drain have?
It depends on the pipe diameter, the application and the plumbing code in force, so it must come from the applicable standard rather than from a general figure. This calculator converts whatever grade you are required to use, it does not tell you which one applies.
Can a pipe have too much fall?
Yes. If water runs away faster than the solids it is carrying, the solids are left behind and the pipe blocks. This is why codes usually specify a maximum grade as well as a minimum.
Is 1 in 60 the same as 1.67 per cent?
Yes. Dividing 1 by 60 gives 0.0167, which is 1.67 per cent. The ratio and the percentage are two ways of writing the same slope.
Does the fall depend on pipe diameter?
The arithmetic here does not, since it converts a grade into a drop over a length. The required grade itself very much does depend on diameter, with larger pipes generally permitted shallower grades.
For slope expressed as a percentage or angle in general, see the slope percentage calculator. For flow in an open channel at a given grade, see the open channel flow calculator.