Plumber is connecting a 9-metre wastewater drain run to the sewer junction. Before digging the trench, you need the total fall in mm and a confirmation it meets the minimum 1:80 gradient for a 100mm drain under AS 3500.
Pipe Fall Calculator
Plumbing
AS/NZS 3500: min 1:60 for DN100
Fall = Length ÷ Ratio
Minimum falls: DN40–DN65 = 1:40 · DN80 = 1:60 · DN100 = 1:60 · DN150 = 1:150
Reference: AS/NZS 3500 — Plumbing and Drainage
Results are estimates for planning purposes. Verify with current standards and a qualified professional.
1 What this calculator does
Calculates the total fall (in mm) over a pipe run from the pipe length and gradient ratio. Checks the result against minimum gradient requirements for different pipe types and sizes under AS 3500 (AU) or IPC (US). Flags if the gradient is too flat.
2 Formula & professional reasoning
Fall (mm) = Pipe length (m) x 1000 / Gradient ratio
Gradient ratio 1:N means 1mm of fall per N mm of horizontal run
Minimum gradients (AS 3500):
100mm drain: 1:80 (12.5mm per m)
150mm drain: 1:150 (6.7mm per m)
50mm overflow relief gully: 1:40
Drainage pipe fall ensures that wastewater flows by gravity without ponding. Too flat and solids settle and block the pipe. Too steep and the water runs ahead of the solids, also causing blockage (scarping). The AS 3500 minimum gradients represent the minimum needed to keep solids in suspension in a partially filled pipe. For stormwater drains the gradients may differ from sanitary drainage -- always check which system applies.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
9-metre drain at minimum 1:80 gradient
Given: Length: 9m | Gradient: 1:80 | Pipe: 100mm sanitary drain
Working:
Fall: 9 x 1000 / 80 = 9000/80Answer: Fall: 112.5mm over 9m | 12.5mm per metre -- meets 1:80 minimum
💡 Total depth change at the downstream end is 112.5mm. If the connection point is at a set invert level, dig the upstream end 112.5mm higher than the connection invert.
Standard
Long stormwater run -- is 1:150 sufficient?
Given: Length: 25m | Gradient: 1:150 | Pipe: 150mm stormwater
Working:
Fall: 25 x 1000 / 150 = 25000/150 = 166.7mmAnswer: Fall: 167mm over 25m -- meets 1:150 minimum for 150mm pipe
💡 At 1:150, a 25m stormwater run drops 167mm. Check headroom between the connection invert and the depth available at the downstream end.
Advanced
Limited depth -- what is the maximum gradient ratio?
Given: Available fall: 200mm | Run length: 15m | Solve for maximum gradient
Working:
Maximum gradient ratio N: N = Length in mm / Fall = 15000 / 200 = 75 | So 1:75Answer: Maximum gradient: 1:75 (steeper than minimum 1:80 for 100mm pipe) -- compliant
💡 1:75 is steeper than the 1:80 minimum (a ratio of 75 means more fall per metre than 80). Steeper is generally better for drainage. Confirm the connection invert depth allows this gradient.
4 Sanity check
AS 3500 minimum gradients
50mm pipe: 1:40 | 65mm-80mm: 1:60 | 100mm: 1:80 | 150mm: 1:150 | 225mm: 1:225
These are minimums -- steeper is better where headroom allows.
Gradient ratio direction
1:80 means 1 unit of fall for every 80 units of horizontal run | Lower ratio number (e.g. 1:40) is STEEPER than higher (1:80)
A common source of confusion: 1:40 is steeper (more fall) than 1:80.
Maximum gradient for sanitary drainage
Generally avoid gradients steeper than 1:20 for sanitary drains -- water runs faster than solids causing scarping
Stormwater vs sanitary gradient requirements
Stormwater can be steeper than sanitary | Check which AS 3500 part applies
AS 3500.3 covers sanitary drains, AS 3500.4 covers stormwater
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Confusing gradient ratio direction -- thinking 1:80 is steeper than 1:40 | Treating higher numbers as steeper gradients | Pipe installed at insufficient gradient -- drainage fails, blocked pipe | A lower ratio number means STEEPER gradient. 1:40 is steeper (twice as much fall) as 1:80. Think of it as: in 1:N, N is the horizontal run for 1mm of fall -- shorter N means more fall per unit run. |
| Measuring pipe fall from the top of the pipe instead of the invert | Not specifying invert levels | Apparent compliance but actual invert gradient below minimum | Drainage gradient is measured between pipe inverts (the inside bottom of the pipe), not the top or centreline. The invert level is what matters for flow. Specify and check invert levels on the drawing. |
| Not accounting for depth restrictions at the connection point | Calculating gradient without checking available invert depth | Designed gradient impossible due to insufficient depth at the connection | Start from the fixed point (the connection invert depth). Work backwards upstream: upstream invert = downstream invert + (run length / gradient ratio in mm). Check the resulting upstream depth fits within the site constraints. |
| Using the same gradient for all drain sizes | Applying 1:80 to a 150mm drain that only needs 1:150 | Unnecessary excavation depth for the 150mm run | Larger diameter pipes are self-cleaning at flatter gradients because the hydraulic radius is larger. Use the specific minimum gradient for each pipe size from AS 3500. |
6 Reference & regulatory links
🇦🇺 Australia
7 Professional workflow
Common tools used alongside this one: