A 1 in 60 fall is 16.7 mm per metre, about 0.95 degrees. Free fall and gradient calculator: convert 1 in N, degrees, percent and mm per metre.
A gradient gets written down four different ways depending on who wrote it. This converts between all of them and turns any one into the total drop over a measured run: the millimetres you actually set a string line to. It does the geometry; it does not tell you what gradient anything requires.
| Fall | mm per metre | Percent | Degrees | Ratio |
|---|---|---|---|---|
| 1 in 40 | 25.0 | 2.50 | 1.43 | 1 in 40 |
| 1 in 50 | 20.0 | 2.00 | 1.15 | 1 in 50 |
| 1 in 60 | 16.7 | 1.67 | 0.95 | 1 in 60 |
| 1 in 80 | 12.5 | 1.25 | 0.72 | 1 in 80 |
| 1 in 100 | 10.0 | 1.00 | 0.57 | 1 in 100 |
| 1° | 17.5 | 1.75 | 1 | about 1 in 57 |
| 2° | 34.9 | 3.49 | 2 | about 1 in 29 |
| 3° | 52.4 | 5.24 | 3 | about 1 in 19 |
| 4° | 69.9 | 6.99 | 4 | about 1 in 14 |
| 5° | 87.5 | 8.75 | 5 | about 1 in 11 |
Millimetres per metre of horizontal run. Multiply by the run in metres for the total drop: 1 in 60 over 12 metres is 200mm. Degrees and ratios are rounded.
A 1 in 60 fall drops 16.7 mm per metre, about 0.95 degrees.
The same conversion for the other common ratios: 1 in 40 is 25.0 mm per metre, 1 in 50 is 20.0 mm per metre, 1 in 80 is 12.5 mm per metre, 1 in 100 is 10.0 mm per metre. The table above has the degrees for each.
Multiply the millimetres per metre by the run in metres to get the total drop over the run.
A 2 degree fall drops 34.9 mm over 1 metre, about 1 in 29.
A 1 degree fall drops 17.5 mm over 1 metre, about 1 in 57. A 3 degree fall drops 52.4 mm over 1 metre, about 1 in 19.
The table above runs from 1 to 5 degrees, each with the nearest whole ratio. The ratio is rounded to a whole number, so the millimetres per metre is the figure to set out from.
A plumber writes 1:60. A spec sheet says 1.67%. A laser level reads 0.95 degrees. The person holding the tape wants 16.7 millimetres per metre. Those are one quantity written four ways, and the conversion between them is where mistakes creep in.
Type the one you have been given and the page shows the other three, plus the drop over the run you entered. Over twelve metres, 1:60 is a 200mm fall.
It works in reverse too: measure the height difference between two ends, give it the run, and it tells you what gradient you have actually got.
Ratio, percent and millimetres per metre are all measured against the horizontal distance, which is the convention on a plan and what a drainage gradient means. Degrees is the angle from horizontal. Measure the run flat, not along the slope.
The page also gives the sloped length: the surface itself, which is longer than the run. On a shallow drainage fall the difference is negligible. On a ramp it is not, and the sloped length is the one you order material to.
The ratio is how a gradient is written down. The total fall in millimetres is what you build to. Working from the ratio at each point along a run is how a fall ends up uneven; setting the two ends and pulling a line between them is how it ends up right.
Bring the required gradient from your plans, the relevant Acceptable Solution or your BCA. This turns it into millimetres, it does not set it. Confirm the run and the finished levels on site before you set out.