How do I convert a fall between 1 in X, %, degrees and mm per metre?
Published · Updated · By Sai Nath
Divide 1 by X to get the slope: 1 in 60 is 0.0167. Multiply the slope by 100 for a percentage (1.67%) or by 1000 for millimetres per metre (16.7 mm), and take its arctangent for degrees (0.95°). The drop over a run is the run divided by X: 1 in 60 over 3 m falls 50 mm.
Fall calculator
Lengths in millimetres. The same maths as the Pitch calculator in the MeisterCalc app.
This calculator needs JavaScript, which is turned off or didn't load. You can still do it by hand: the formulas, the conversion chart and the worked examples below give the same answers.
How does the fall calculator work?
Every way of writing a fall describes one number, the slope: how far it drops for each unit it runs, measured level. The calculator turns your figure into that slope, then writes it the other ways.
- 1 in X (also written 1:X): slope = 1 ÷ X. 1 in 60 is 0.0167.
- Percentage: slope = % ÷ 100. 2% is 0.02, which is 1 in 50.
- Millimetres per metre: slope = mm per m ÷ 1000. 10 mm per metre is 0.01, which is 1 in 100.
- Degrees: slope = tan(degrees), and degrees = arctan(slope). 1° is 1.75%, not 1%.
- Drop = length × slope, which is the same as length ÷ X. Length for a drop = drop ÷ slope, or drop × X. Fall from a drop: X = length ÷ drop.
These are the formulas MeisterCalc's Pitch calculator uses for rise, run, angle and % grade; this page adds the 1 in X and millimetres-per-metre forms that falls are usually written in.
Fall conversion chart
The same fall written four ways. Lower X is steeper: 1 in 50 falls faster than 1 in 100.
| 1 in X | Percentage | Degrees | mm per metre |
|---|---|---|---|
| 1 in 8 | 12.50% | 7.13° | 125.0 |
| 1 in 10 | 10.00% | 5.71° | 100.0 |
| 1 in 12 | 8.33% | 4.76° | 83.3 |
| 1 in 14 | 7.14% | 4.09° | 71.4 |
| 1 in 20 | 5.00% | 2.86° | 50.0 |
| 1 in 40 | 2.50% | 1.43° | 25.0 |
| 1 in 50 | 2.00% | 1.15° | 20.0 |
| 1 in 60 | 1.67% | 0.95° | 16.7 |
| 1 in 80 | 1.25% | 0.72° | 12.5 |
| 1 in 100 | 1.00% | 0.57° | 10.0 |
| 1 in 120 | 0.83% | 0.48° | 8.3 |
| 1 in 200 | 0.50% | 0.29° | 5.0 |
| 1 in 500 | 0.20% | 0.11° | 2.0 |
Going the other way from degrees: tan 1° = 0.01746, so 1° is 1.75% and 1 ÷ 0.01746 = 1 in 57.3, 2° is 1 in 28.6 (3.49%) and 5° is 1 in 11.4 (8.75%). There's more on percentages and degrees in how to convert a slope percentage to degrees, and on roofs in roof pitch in degrees.
Common falls, and where the figures come from
Each figure below is quoted from the official document, with its clause. Checked on 28 October 2026. Building rules are set by the NCC, the NZ Building Code or the Approved Documents and by your council or building control, not by this page or the app. A plumber, certifier or the drawings settle what a particular job needs.
Australia (NCC Housing Provisions)
The figures below are the same in NCC 2022 and NCC 2025, so they hold whichever edition your state has adopted. The one change of wording, on eaves gutters, is noted in the table.
| What | Fall | Source |
|---|---|---|
| Eaves gutter | Not less than 1:500 (2 mm per metre). NCC 2025 adds “unless designed in accordance with AS/NZS 3500.3”; NCC 2022 gives 1:500 alone. | Housing Provisions 7.4.4(1)(a) |
| Ground or paving next to a slab-on-ground house | At least 50 mm over the first 1 m from the building (1 in 20) | Housing Provisions 3.3.3(a)(ii) |
| Concrete or clay paving next to the house, in a low rainfall intensity area | At least 25 mm over the first 1 m (1 in 40) | Housing Provisions 3.3.3(a)(i) |
| Subsoil drain | Uniform fall of not less than 1:300 | Housing Provisions 3.3.4(a) |
| Wet-area floor, including a shower, to a floor waste | Continuous fall between 1:80 and 1:50 | Housing Provisions 10.2.12 and 10.2.14 |
| Ramp serving an external door, or inside a house | Not steeper than 1:8 | Housing Provisions 11.2.3(b) |
| Livable Housing access path to the dwelling entrance | Not steeper than 1:14. Ramping in total no more than 9 m at 1:14, or 15 m at 1:20. State variations apply: it doesn't apply to houses in NSW. | Livable Housing Design Standard 1.1(2)(a)(ii) and 1.1(4)(a) |
New Zealand (Building Code Acceptable Solutions)
| What | Fall | Source |
|---|---|---|
| Roof gutters | No figure: “Roof gutters shall fall to an outlet.” | E1/AS1 5.3.1 (PDF) |
| Surface water drains | 85 mm: 1 in 90. 100 mm: 1 in 120. 150 mm: 1 in 200. 225 mm: 1 in 350. | E1/AS1 3.4.1, Table 2 (PDF) |
| Foul drains | Set by pipe size and discharge units. Flattest allowed: 100 mm at 1:120 (up to 104 discharge units), 150 mm at 1:200 (up to 515). | G13/AS2 3.5.3, Table 2 (PDF) |
| Shower with no upstand, or no wall, screen, door or curtain | No less than 1:50 towards the floor waste, within 1500 mm of it | E3/AS1 3.3.2.3 (PDF) |
| Membrane-covered deck | At least 1.5° (1:40) | E2/AS1 4th edition, 8.5.1.2 (PDF) |
| Ramps | Accessible ramp 1:12. Common ramp 1:10 if it gets wet, 1:8 if normally dry. | D1/AS1 Table 3 (PDF) |
United Kingdom (Approved Documents, England)
The Approved Documents apply in England. Wales and Scotland publish their own guidance, which can differ.
| What | Fall | Source |
|---|---|---|
| Gutters | No figure: “laid with any fall towards the nearest outlet” | Approved Document H, H3 1.6 (PDF) |
| Rainwater drains | 75 and 100 mm: not less than 1:100. 150 mm: 1:150. 225 mm: 1:225. | Approved Document H, H3 3.15 (PDF) |
| Foul drains | 75 or 100 mm at 1:40 for a peak flow under 1 litre a second. 100 mm at 1:80 with at least one WC. 150 mm at 1:150 with at least five WCs. | Approved Document H, H1 2.34, Table 6 (PDF) |
| Paving and other hard surfaces | At least 1 in 60 recommended. Across a path, no more than 1 in 40. | Approved Document H, H3 2.3 (PDF) |
| Approach to a dwelling | Level: not more than 1:60. Gently sloping: 1:60 to 1:20. Ramped: 1:20 to 1:12. | Approved Document M Vol 1, Appendix A (PDF) |
| Ramp flights, Category 1 (visitable) dwelling | Up to 1:15: flights no more than 10 m long. Up to 1:12: no more than 5 m. | Approved Document M Vol 1, 1.7 (PDF) |
| Ramps, buildings other than dwellings | Going of 10 m: 1:20. 5 m: 1:15. 2 m: 1:12. | Approved Document M Vol 2, Table 1 (PDF) |
Left out on purpose: Australian drain grades and box gutter falls are in AS/NZS 3500, and accessible ramp limits for other buildings in AS 1428.1. Both are paid standards that couldn't be checked for this page, so their figures aren't quoted here.
Worked examples
A 9 m eaves gutter at 1:500. 9000 ÷ 500 = 18 mm. The outlet end sits at least 18 mm lower than the far end.
A shower floor 900 mm from the far wall to the waste, in Australia. Between 1:80 and 1:50 means 900 ÷ 80 = 11.3 mm and 900 ÷ 50 = 18 mm. The floor at the wall sits between 11.3 and 18 mm above the waste.
A ramp rising 600 mm at 1 in 14. Choose Length for a drop: 600 × 14 = 8400 mm measured level, 8421.4 mm along the slope. That's inside the Livable Housing limit of 9 m of ramping at 1:14. More in how long a 1 in 14 ramp needs to be.
A path that drops 25 mm over 3 m. Choose Fall from a drop: 3000 ÷ 25 = 1 in 120, which is 0.83%, 0.48° or 8.3 mm per metre.
Common mistakes
- Reading a percentage as degrees. 1% is 0.57°, and 1° is 1.75%. On a 10 m run that's 100 mm against about 175 mm.
- Getting minimum and maximum backwards. A minimum fall of 1:80 means 1:80 or steeper, so X of 80 or less. 1:100 is flatter and doesn't meet it.
- Measuring the length along the slope. Falls use the level (plan) length. At 1 in 60 the difference is under a millimetre over 3 m, but on a steep ramp it isn't.
- Rounding too early. 1 in 60 is 16.7 mm per metre, not 16 or 17. Over 10 m that's 167 mm, not 160 or 170.
- Mixing millimetres and metres. 1 in 100 over 6 m is 60 mm, not 600 mm and not 6 mm.
Questions people ask
Is 1 in 100 the same as 1:100 and 1%?
Yes. All three are 10 mm of fall for every metre of length.
Which is steeper, 1 in 50 or 1 in 80?
1 in 50: 20 mm per metre against 12.5 mm. The smaller the X, the steeper the fall.
How do I measure an existing fall?
Set a level string line or a straightedge and level from the high end, measure down to the surface at the low end, and measure the level length between the two points. Enter both in Fall from a drop. Step-by-step: how to calculate fall.
What this calculator doesn't do
- It doesn't tell you what fall a job needs, or that a fall complies. The table above quotes the documents; the code, the drawings and your council decide.
- One straight, even fall at a time. No cross-falls, changes of grade or landings.
- It doesn't size pipes or gutters, or work out flow.
- No roof pitch in X/12 inches, here. MeisterCalc's Pitch calculator has that.
- Millimetres only for lengths.
Falls and pitch in MeisterCalc
On Android, MeisterCalc's Pitch calculator works from any two of rise, run, pitch (X/12), angle and % grade, with a live diagram, and Dumpy Level works out reduced levels from staff readings. Both are among the 12 calculators that unlock with a weekly subscription that starts with a 3-day free trial, or with a one-time lifetime unlock; Google Play shows the prices in your currency. Concrete, Stairs and Check Square are free. Everything works offline on site, on Android 7.0 or later. There's no iPhone version yet.