What this calculator does
Drainage pipes, driveways and floor slabs are often specified with a fall written as a ratio, such as "1 in 100" or "1 in 60". That notation means the surface drops by 1 unit of height for every X units it travels horizontally, so a run written as 1 in 100 drops 1 millimetre for every 100 millimetres of run, or 1 metre for every 100 metres, the ratio holding at any scale.
The figure that actually matters on site is the total fall over the real length of pipe or slab being laid, not the abstract ratio. This calculator takes the X from a "1 in X" ratio and a run length and returns the fall in millimetres, centimetres and metres, along with the equivalent percentage grade and angle for cross-checking against other formats a spec might use.
The formula
Divide the run length by X, since a slope of 1 in X drops one unit of height for every X units of run. The result is the fall in the same units as the run length; converting that into millimetres, centimetres and a percentage grade covers the ways a drainage fall is typically quoted.
| Term | Meaning |
|---|---|
| 1 in X | A slope ratio meaning a fall of 1 unit for every X units of horizontal run; a smaller X means a steeper fall. |
| Run length | The horizontal distance the pipe, drain or slab actually covers. |
| Grade | The same slope expressed as a percentage: 100 divided by X. |
The inputs explained
| Field | What to enter |
|---|---|
| Slope ratio (the X in "1 in X") | The X in the "1 in X" ratio quoted on the drawing or spec, such as 100 for a 1 in 100 fall. |
| Run length (m) | The actual length of the run the fall applies to. |
When to use it
Setting out a drainage pipe fall
A drainage spec calling for a 1 in 60 fall over a 15 metre run needs converting into an actual drop in millimetres before it can be marked out or checked with a level.
Checking a driveway or path against council minimums
Councils and building codes often set a minimum or maximum fall as a 1 in X ratio; converting that into a real fall over the paved length confirms a design meets the requirement.
Cross-checking against a percentage or angle spec
Some documents quote slope as a percentage grade or an angle instead of a ratio. Converting a 1 in X figure into both lets you compare specs written in different formats without a separate lookup.
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 does a 1 in 100 slope give over different run lengths?
A fixed 1 in 100 ratio applied to a range of run lengths.
How does the fall change with a steeper or flatter ratio, over a fixed 10 m run?
A fixed 10 metre run at a range of common drainage fall ratios.
| Ratio (1 in X) | Total fall over the run |
|---|---|
| 1 in 40 | 250.0 mm |
| 1 in 60 | 166.7 mm |
| 1 in 80 | 125.0 mm |
| 1 in 100 | 100.0 mm |
| 1 in 150 | 66.7 mm |
| 1 in 200 | 50.0 mm |
Questions
What does "1 in 100" actually mean?
It means the surface drops 1 unit of height for every 100 units it runs horizontally. Over a 1 metre run that is a 10 mm fall; over a 10 metre run it is 100 mm, since the ratio scales with distance.
Is a bigger or smaller X a steeper fall?
Smaller. A 1 in 40 fall is steeper than a 1 in 100 fall, because the height drops faster relative to the horizontal run.
How does this relate to percentage grade?
A 1 in X ratio equals a grade of 100 divided by X percent. A 1 in 100 fall is a 1% grade, and a 1 in 50 fall is a 2% grade.
What is a typical minimum fall for a drainage pipe?
This varies by pipe diameter, material and local plumbing code, so check the applicable standard for the job rather than assuming a figure. This calculator converts whatever ratio your spec calls for into an actual fall; it does not set the minimum itself.
If your fall is specified as an angle in degrees rather than a 1 in X ratio, use the pipe fall calculator instead. For roof slope and rafter length from a pitch ratio, see the roof pitch calculator.