Enter the pipe size, run length and planned slope, and the calculator gives the total fall in millimetres and inches, the slope as a ratio, percent, millimetres per metre, inches per foot and degrees, and whether it meets the National Plumbing Code minimum. Add the fixtures the pipe serves to check its load, and the depths at each end to see whether the fall you have is enough.
- The minimums come from the National Plumbing Code of Canada 2020: 1 in 50 for 3-inch pipe, 1 in 100 for 4-inch, 1 in 133 for 5 and 6-inch.
- Fall is run length times slope: 15 m at 1 in 100 is 150 mm.
- The load check uses the code's fixture-unit values and its table of what each size carries at each slope.
- A building sewer to a public sewer must be at least 4 inch; the tool flags a 3-inch sewer.
Sewer Slope & Fall Calculator
01 /What does the calculator work out?
It answers three questions installers and homeowners ask about a gravity drain. How much fall does this run need? Is the slope I plan legal for this pipe size? And will this pipe carry the fixtures that drain into it? It also answers a fourth, optional one: given how deep the pipe is at each end, do I actually have the fall? Every rule it applies is stated on this page, and every number it uses comes from the National Plumbing Code of Canada 2020, which Alberta has enforced since April 1, 2023 without provincial changes.
| Input | Options | Used for |
|---|---|---|
| What is this pipe? | Building sewer to the city; branch or building drain upstream of the main cleanout; line from a suite or addition to a tie-in | Size warnings and a tie-in note |
| Pipe size | 3, 4, 5 or 6 inch (NPS) | The minimum slope and the capacity column |
| Run length | Metres, in half-metre steps | Total fall |
| Planned slope | 1 in 400, 200, 133, 100, 50 or 25 | Everything below |
| Custom slope | A percentage, optional | Overrides the planned slope when filled in |
| Fixtures | Counts of nine fixture types | The hydraulic load in fixture units |
| Fall you have | Depth to the pipe bottom at each end, and how much lower the ground is at the far end | Checks the available slope |
02 /How does it convert a slope into fall?
Slope is the drop per unit of length. A slope of 1 in 100 means 1 unit of drop for every 100 units along the pipe, which is 1 percent, 10 mm per metre, or 0.12 inches per foot (close to the 1/8 inch per foot that tradespeople quote). The calculator shows all five forms together, plus the angle in degrees, because people search in all of them and a level or laser may be set in any of them.
| Ratio | Percent | mm per metre | Inches per foot | Degrees |
|---|---|---|---|---|
| 1 in 400 | 0.25% | 2.5 | 0.03 | 0.14° |
| 1 in 200 | 0.5% | 5 | 0.06 | 0.29° |
| 1 in 133 | 0.75% | 7.5 | 0.09 | 0.43° |
| 1 in 100 | 1% | 10 | 0.12 | 0.57° |
| 1 in 50 | 2% | 20 | 0.24 | 1.15° |
| 1 in 25 | 4% | 40 | 0.48 | 2.29° |
Total fall is simply the run length multiplied by the slope. The calculator reports it in millimetres, inches and metres. A 15 m run at 1 in 100 falls 150 mm; the same run at 1 in 50 falls 300 mm. If you enter a custom percentage, the ratio shown is rounded to the nearest whole number, so 1.5 percent reads as 1 in 67.
03 /Where do the minimum slopes come from?
From two places in the plumbing code. Article 2.4.8.1 says drainage pipes of NPS 3 or less must slope at least 1 in 50. For building drains and building sewers, Table 2.4.10.6.-C lists the maximum hydraulic load each pipe size may carry at each slope, and a blank ("n/a") cell means that size is not permitted at that slope. The flattest slope with a number in it is therefore the minimum for that size, and those are the minimums the tool uses.
| Pipe size | 1 in 133 | 1 in 100 | 1 in 50 | 1 in 25 | Minimum used |
|---|---|---|---|---|---|
| 3 inch | n/a | n/a | 27 | 36 | 1 in 50 |
| 4 inch | n/a | 180 | 240 | 300 | 1 in 100 |
| 5 inch | 380 | 390 | 480 | 670 | 1 in 133 |
| 6 inch | 600 | 700 | 840 | 1,300 | 1 in 133 |
The table's flatter columns (1 in 200 and 1 in 400) only apply to 8-inch and larger pipe, so for the sizes offered here any slope flatter than the minimum is reported as below the code minimum. The code's explanatory note adds that although slopes below 1 in 100 are permitted for pipes over NPS 4, they should be used only where necessary, because steeper slopes and higher velocities help keep pipes clean. Calgary's infill page states the same minimums for services: 1 in 50 for 3-inch and 1 in 100 for 4-inch.
04 /How is the fixture load counted?
Each fixture is worth a number of fixture units, the code's measure of how much and how suddenly it discharges. The calculator multiplies your counts by the values from NPC 2020 Table 2.4.9.3 and adds them up. It then looks up the capacity for your size and slope in Table 2.4.10.6.-C. If you entered a custom slope that falls between two table columns, it uses the next flatter column, which is the conservative choice.
| Fixture | Fixture units |
|---|---|
| Bathroom group: toilet with a tank, basin, and tub or shower | 6 |
| Extra toilet with a tank | 4 |
| Extra basin (domestic lavatory) | 1 |
| Kitchen or bar sink, domestic, with or without a grinder | 1.5 |
| Domestic clothes washer | 2 |
| Laundry tray | 1.5 |
| Separate shower, total flow under 9.5 L/min | 1.5 |
| Separate bathtub | 1.5 |
| Floor drain with an NPS 3 trap | 3 |
The default counts, two bathroom groups, a kitchen sink, a washer and a floor drain, add up to 18.5 fixture units. A 4-inch building sewer at 1 in 100 carries 180, which is why a 4-inch line serves almost any single house. Load becomes the limiting factor on multi-unit buildings, long shared lines and suites tied into an existing sewer, covered in tie-ins for additions and suites.
05 /What do the verdicts and notes mean?
The headline is one of three verdicts. "Meets the NPC table for this size" means the slope is at or above the minimum and the load fits. "Too flat for a ... pipe" means the slope is below the minimum for that size. "Slope is legal, but the pipe is overloaded" means the load exceeds the table value at that slope. Under the headline, a row for each check shows the numbers and a badge.
- 3-inch building sewer: if you choose "building sewer" with a 3-inch pipe, a note explains that NPC 2.4.9.4 requires building sewers connected to a public sewer, downstream of the main cleanout, to be at least 4 inch.
- Suite or addition line: a note reminds you that some municipalities require a separate service, and that the shared pipe downstream carries both loads.
- Below the minimum: options are to steepen the grade, lower the far end, or start the pipe higher at the house.
- Overloaded: a steeper grade adds capacity; if the grade is fixed, the next size up is the usual answer.
- 1 in 25 or steeper: a note explains that the code sets minimums, not a maximum, and that on steep lots the concern is abrupt grade changes and bends between cleanouts. See sewer lines on steep lots.
06 /How does the "fall you have" check work?
Fill in the depth to the pipe bottom (the invert) where the pipe leaves the house, the depth to the connection invert at the far end, and how much lower the ground is at the far end (negative if it is higher). The available fall is the ground drop plus the far-end depth minus the house-end depth. Divided by the run length, that gives the available slope.
- Enough for your plan
The available fall is at least the fall your planned slope needs.
- Enough for the code, not your plan
You can meet the minimum but not the steeper slope you chose. The tool shows both figures.
- Short of the code minimum
It states the shortfall in millimetres and lists the ways out: start higher at the house, connect deeper, or send low fixtures to a sump and pump. It tells you to confirm the connection invert with the municipality before redesigning.
Worked example: a 15 m run leaving the house with its invert 1.8 m down, to a connection invert 1.9 m down on level ground, has 100 mm of fall. That is 1 in 150, short of the 150 mm a 4-inch sewer needs at 1 in 100. Drop the connection to 2.4 m and the fall becomes 600 mm, or 1 in 25. When the fall works, check the cover with the Sewer Depth & Frost Cover Planner, because a shallow start can leave the pipe too close to the frost.
07 /What are the calculator's limits?
It handles one straight gravity run of one size at one slope. It does not size storm drains, which use a different table, and it does not model pumped lines. It does not know your municipality's rules beyond the Calgary note, and some authorities ask for more than the national minimum. It cannot see a sag in an existing pipe; a camera does that. And it is a planning aid: the permit office and the inspector have the final word on any installation.
The reasoning behind the minimums, and how installers hold them on site, is in sewer pipe slope and how a sewer line is installed. To have someone look at your project, send the details.
FAQQuestions people ask
How do I calculate sewer pipe fall?
Multiply the run length by the slope. At 1 in 100, a 20 m run falls 200 mm; at 1 in 50 it falls 400 mm.
Is 1/8 inch per foot enough for a 4-inch sewer?
Yes. One-eighth inch per foot is about 1 in 96, slightly steeper than the NPC minimum of 1 in 100 for a 4-inch building sewer.
Why does the calculator say a 3-inch sewer is not allowed?
NPC 2020 Article 2.4.9.4 requires a building sewer connected to a public sewer, downstream of the main cleanout, to be at least 4 inch. Three-inch pipe is fine upstream.
What is a fixture unit?
The plumbing code's measure of a fixture's drainage load. A bathroom group with a tank toilet counts 6, a domestic clothes washer 2, a kitchen sink 1.5.
Does the calculator work for storm drains?
No. Storm and combined drains are sized from rainfall and a different code table. This tool covers sanitary drains and sewers only.
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