Calculate drainage fall from run and a 1-in-N ratio, measure an existing slope, or convert percent, angle, per-mille, mm per meter, and inches per foot.
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Fall Ratio Formula
The calculator uses three related formulas depending on which tab you select.
Drop needed (from a target ratio):
Measured slope (from rise and run):
Convert between formats:
- Drop – the vertical fall over the run, in the same units as the run.
- Run – horizontal distance the surface covers.
- Rise – measured vertical change between the two ends.
- N – the denominator in a “1 in N” ratio. Larger N means a shallower slope.
- Slope% – rise divided by run, expressed as a percentage.
- Angle – slope expressed in degrees from horizontal.
- Permille (‰) – parts per thousand, common in civil drainage specs.
The Drop needed tab takes a run and a target ratio and returns the vertical drop you need to set out. The Measure slope tab takes the rise and run you already have and returns the actual slope as a ratio, percentage, angle, and permille. The Convert tab takes any one slope format and returns the others.
Common Fall Ratios and Equivalents
Use this table to sanity check your input or pick a target before you measure.
| Ratio | Slope % | mm per m | Angle |
|---|---|---|---|
| 1 in 200 | 0.50% | 5.0 | 0.29° |
| 1 in 100 | 1.00% | 10.0 | 0.57° |
| 1 in 80 | 1.25% | 12.5 | 0.72° |
| 1 in 60 | 1.67% | 16.7 | 0.95° |
| 1 in 40 | 2.50% | 25.0 | 1.43° |
| 1 in 20 | 5.00% | 50.0 | 2.86° |
| 1 in 12 | 8.33% | 83.3 | 4.76° |
Typical fall ratios by use
| Application | Typical fall |
|---|---|
| Patios and paving | 1 in 60 to 1 in 80 |
| Driveways | 1 in 40 to 1 in 60 |
| Foul drain pipes | 1 in 40 to 1 in 80 |
| Surface water drains | 1 in 100 to 1 in 150 |
| Flat roof drainage | 1 in 40 to 1 in 80 |
| Wheelchair ramps (max) | 1 in 12 |
Worked Examples
Example 1 – Patio drop. You are laying a 5 m patio that needs a 1 in 60 fall away from the house. Drop = 5000 mm / 60 = 83.3 mm. Set the far edge 83 mm lower than the edge against the house.
Example 2 – Checking an existing slope. A driveway drops 120 mm over a 6 m run. Slope = 120 / 6000 × 100 = 2.0%. That works out to 1 in 50, or about 1.15°. That sits in the normal driveway range.
FAQ
Does “1 in 60” mean steeper or shallower than “1 in 40”? Shallower. A larger denominator means less drop per unit of run.
Can I mix units? Yes. The calculator converts everything internally. You can enter run in metres and rise in millimetres.
What slope do I need for water to drain? A minimum of about 1 in 100 (1%) is standard for surface water on smooth materials. Use 1 in 60 to 1 in 80 for paving to allow for settlement and surface texture.
Why does the angle look so small? Drainage falls are shallow by design. A 1 in 60 fall is under 1°, which still moves water effectively without being noticeable underfoot.
