Building sewer fixture load estimator: will one service carry the whole building?

01 /What does the fixture load estimator do?

It answers the sizing question that comes up the moment a lot is densified: once the house becomes a house with a suite, a duplex or a fourplex, how much drainage load reaches the building sewer, what size of pipe does the plumbing code call for, and can the service already in the ground take it? Most calculators size one pipe from a short list of fixtures. This one is built around dwelling units, so you describe a typical unit and a typical suite, say how many of each there are, add the building-wide items, and read the result for the whole building.

Every number the widget uses is published in the National Plumbing Code of Canada 2020 (second printing, with the February 2025 errata), which this site fetched and read in September 2026. Nothing is estimated from rules of thumb. Where the code is silent, for example on whether a city will accept a sixty-year-old clay service, the tool says so and points you to the city.

02 /Which inputs does it ask for?

The form has four groups. Start from a preset and adjust, or choose Custom to keep your own numbers when you change the building type.

The eight fixture types, with their fixture-unit values from Table 2.4.9.3, are: a bathroom group of a tank toilet, basin and tub or shower (6); an extra tank toilet (4); an extra basin (1); a domestic kitchen sink with or without a food waste grinder (1.5); a domestic clothes washer (2); a separate shower whose heads together discharge under 9.5 L/min (1.5); a separate bathtub (1.5); and a laundry tray (1.5). Floor drains carry 3 fixture units with an NPS 3 trap and 2 with an NPS 2 trap.

03 /How does it add up the load?

The total is the per-unit fixture sum times the number of main units, plus the per-suite sum times the number of suites, plus floor drains, plus pumped flow, plus any provision for future fixtures. That follows Article 2.4.10.1, which defines the load on a pipe as every fixture connected upstream plus every fixture for which provision is made for a future connection. The result table shows each part so you can see where the load comes from.

1. Per unit

   Each fixture count times its fixture-unit value, summed. The fourplex preset uses two bathroom groups, a kitchen sink and a clothes washer: 12 + 1.5 + 2 = 15.5 FU per unit.

2. Per suite

   The same for a typical suite. The default suite is one bathroom group, a kitchen sink and a washer: 9.5 FU.

3. Building items

   Floor drains at 3 or 2 FU each. A pump or ejector that discharges into the sewer counts as a continuous flow at 31.7 FU for each litre per second, from Sentence 2.4.10.3.(1).

4. Future provision

   Any roughed-in fixtures you enter, in fixture units, because the code counts provision for future connection now.

5. Total

   The fourplex preset gives 4 × 15.5 + 2 × 3 = 68 FU. Four units with four suites and four floor drains gives 112 FU.

04 /How does it choose a pipe size?

It walks up Table 2.4.10.6.-C at the slope you chose and returns the smallest size from NPS 4 upward whose tabulated capacity is at least the total. NPS 3 is never recommended for the building sewer, because Article 2.4.9.4 requires building drains and sewers connected to the public system downstream of the main cleanout to be at least NPS 4. If the table lists no capacity for a size at that slope, the size is skipped: a 4-inch pipe has no entry flatter than 1 in 100, and 5 and 6 inch pipes none flatter than 1 in 133.

If you enter a city minimum larger than the code result, the widget shows both and treats the larger as the size to plan around. That matters more than it sounds: Toronto's online process for buildings of up to 5 units is built around a 150 mm sewer connection, and older Ottawa infill specifications show a 125 mm sanitary service. The code number is the floor, not the answer. If the load exceeds what NPS 12 carries at your slope, the tool says the building needs an engineered design.

05 /How does it judge the existing service?

The existing-service check is a paper test of size and slope, followed by honest notes about everything paper cannot tell you.

• 3 inch: flagged as too small for a building sewer to a public sewer under 2.4.9.4, whatever the load.
• Size and slope known: the load is compared with the table capacity at that slope and shown as a percentage. If the slope is flatter than the flattest permitted for that size, the table gives no capacity and the pipe is flagged.
• Slope unknown: the tool uses the flattest slope the table permits for that size, which is the most cautious assumption, and asks you to measure the inverts.
• Smaller than your city minimum: flagged, whatever the load.
• Condition: unknown, defects on camera, or clay, fibre or cast iron not yet filmed each add a note. A sound pipe on recent video is the only case where the verdict says the load fits without a condition warning.

Many cities decide reuse on grounds the plumbing code does not cover. On a demolish-and-rebuild, Calgary's design guidelines call for new water, sanitary and storm services; Toronto and Vancouver accept reuse only for PVC in good condition with a video; Halifax Water requires replacement at the owner's cost when a service 30 years or older serves a redevelopment. Our guide to reusing an existing sewer service sets these out city by city.

06 /What do the separate-building and suite notes mean?

If you say the sewer serves one building plus a suite building on the same lot, the tool keeps the suite's fixtures in the total and reminds you of two code points. Article 2.1.2.4 lets an ancillary building on the same property share the service, and Sentence 2.4.7.2.(4) limits the developed length between a building and the building sewer it joins, other than at a manhole, to 30 m. If you say the sewer serves two or more principal buildings, it tells you to run the estimate per building, because each building connected to public services must be connected separately.

Whenever suites are present, the tool also notes that a normally open backwater valve in a building drain may serve only one dwelling unit (Sentence 2.4.6.4.(5)). That rule does not change the fixture count, but it changes the drain layout below the street level, which is covered in the secondary suite plumbing guide.

07 /What are the limits of this estimate?

• It sizes the sanitary building sewer only. Storm and combined building sewers are sized by rainfall load under a different table (2.4.10.9), and water services by a different method.
• It assumes the fixtures listed. Commercial kitchens, flush-valve toilets, large showers over 9.5 L/min and other fixtures carry different values; add them as future fixture units if you know their load.
• It reads the National Plumbing Code 2020. Alberta has had it in force since April 1, 2023. Ontario uses the Ontario Building Code, and Vancouver has its own Building By-law, so the governing table may differ slightly: confirm with the permit office.
• It cannot see the pipe. Roots, offsets, sags and belly sections reduce real capacity, which is why the verdict defers to a camera run.
• It does not decide anything. The designer, the permit office and the utility do.

When the numbers are in, take them to the Service Connection Planner to see what your city asks for, read how building sewers are sized for the reasoning, or ask for quotes with your fixture total in hand.

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