What this calculator does
Drainage and plumbing pipework needs a consistent fall (slope) along its run so waste flows by gravity rather than pooling. That fall is usually specified in millimetres of drop per metre of pipe (mm/m), while the underlying angle from horizontal is what a spirit level or laser is actually reading, so converting between the two comes up constantly on site.
This calculator works out 1 degree fall over 1 metre in mm, and the same conversion for any other angle, using mm/m = 1000 × tan(angle in degrees), a standard trigonometric relationship. Enter the fall angle and the length of the run to get the fall per metre, the total drop over that run, and the equivalent grade percentage and ratio.
The formula
The fall per metre is 1000 times the tangent of the angle in degrees, since tangent is rise (or fall) over run, and multiplying by 1000 converts a run of 1 metre into millimetres. Multiplying that fall-per-metre figure by the actual run length gives the total drop over the whole pipe.
| Term | Meaning |
|---|---|
| Fall | The vertical drop of a pipe or drain over its horizontal run, here expressed in millimetres per metre. |
| Grade (slope percentage) | The fall expressed as a percentage: rise or fall divided by run, times 100. |
| Ratio | The same slope expressed as 1 unit of fall for every so many units of run, such as 1:57. |
The inputs explained
| Field | What to enter |
|---|---|
| Fall angle (°) | The fall angle from horizontal, in degrees. Common drainage falls, such as 1, 2, 3 or 5 degrees, all work. |
| Pipe or drain run length (m) | The total length of the pipe or drain run, used to work out the total drop over that distance. |
When to use it
Setting out a drainage pipe fall
Many drainage designs specify a fall angle rather than a mm/m figure directly; converting it first gives the number that can actually be checked with a spirit level or laser on site.
Checking a minimum fall requirement
Plumbing codes commonly specify a minimum fall in mm/m for different pipe diameters; converting a design angle into mm/m checks it against that minimum before installation.
Working out total drop over a long run
A shallow fall adds up over a long run: entering the full pipe length alongside the angle gives the total vertical drop from one end to the other, not just the per-metre figure.
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.
What fall in mm/m does each common drainage angle give?
The angles most often quoted for drainage and plumbing falls, over a 1 m run.
How total drop builds up over a longer run at a 1 degree fall
A fixed 1 degree fall angle, over a range of pipe run lengths.
| Run length | Total fall over the run |
|---|---|
| 5 m | 87 mm (0.087 m) |
| 10 m | 175 mm (0.175 m) |
| 15 m | 262 mm (0.262 m) |
| 20 m | 349 mm (0.349 m) |
Questions
What is 1 degree fall over 1 metre in mm?
About 17.5 mm, from 1000 × tan(1°). That figure is the fall per metre; over a longer run, multiply it by the total run length in metres.
What is 3 degree fall over 1 metre in mm?
About 52.4 mm, from 1000 × tan(3°), or roughly a 5.24% grade.
Is pipe fall the same calculation as roof pitch?
The underlying trigonometry is the same, angle to rise-over-run, but roofing conventionally expresses pitch as a ratio against a run of 12 (an "X in 12" figure), while drainage and plumbing fall is conventionally expressed in millimetres of drop per metre of run. See the pitch calculator for the roofing version.
Why does a small angle still matter for drainage?
Because pipe runs can be long even when the angle looks tiny: a fall that seems negligible per metre still adds up to a meaningful total drop over a run of several metres, which is why installers work in mm/m rather than trying to judge a shallow angle by eye.
For the equivalent slope calculation on a roof, see the pitch calculator. For a ramp gradient using the same rise-over-run idea, see the ramp slope and length calculator.