A septic bed in Ontario isn’t sized by eye or by copying the old one: it comes out of two numbers and one formula in the Building Code.
The first number is how much wastewater your house is assumed to produce in a day. The second is how fast your soil soaks up water. Put them together and you get the minimum length of distribution pipe the bed needs, which in turn sets how many trenches there are and how much of your yard they take up.
That’s why two neighbours with the same three-bedroom house can end up with beds of very different sizes. One sits on sandy loam near the lake. The other sits on tight clay a few kilometres inland. Same house, different ground, different bed.
Below is how the sizing works, a worked example on three soil types, a sizing chart by bedroom count, and what “how deep” really means around Kawartha Lakes. One thing up front: this is for understanding your quote, not replacing it. A licensed sewage system designer sizes the real system, and the City of Kawartha Lakes Building Division approves it.
The Quick Answer
| Question | Short answer |
|---|---|
| What sets the size? | Daily design flow (Q, from bedrooms and house size) and soil percolation time (T) |
| Pipe length formula | Total distribution pipe = Q × T ÷ 200, never less than 40 m |
| 3-bedroom daily design flow | 1,600 litres per day |
| Longest single trench | 30 m, and the runs should be about equal |
| Trench spacing | Centred at least 1.6 m apart |
| How deep is a leach field? | Trenches are 600 to 900 mm deep |
| Soil needed below the trench | At least 900 mm above rock, the high water table, or very tight soil |
| Rough size, 3-bed on good soil | About 80 m of pipe in 3 runs, roughly 100 to 120 m² of trench area |
| Who decides | A licensed designer, reviewed by the City |
Step 1: Daily Design Flow by Bedroom Count
Ontario doesn’t size a bed on how much water you actually use. It uses a design flow from Table 8.2.1.3.A of the Building Code, based mostly on bedrooms. Two careful people in a four-bedroom house still get a four-bedroom bed.
| Bedrooms | Daily design flow (Q) |
|---|---|
| 1 | 750 L/day |
| 2 | 1,100 L/day |
| 3 | 1,600 L/day |
| 4 | 2,000 L/day |
| 5 | 2,500 L/day |
| Each bedroom over 5 | add 500 L/day |
Big houses get bumped up. The table adds flow for finished floor area over 200 m² (basements aren’t counted) and for more than 20 plumbing fixture units, and the designer uses whichever calculation comes out highest. A large three-bedroom home with several bathrooms can end up designed at a higher flow than its bedroom count suggests. The same flow figure also drives the minimum tank size, which you can check with our septic tank size calculator.
Step 2: The Soil’s Percolation Time (T)
T is how many minutes it takes water to drop one centimetre in your soil. Low T means fast-draining soil; high T means slow. A designer gets it from a perc test or from a lab analysis of the soil’s grain size.
One unit note, because it trips people up. The Ontario Code works in minutes per centimetre. Some reports and older references quote minutes per inch, which makes the number look about 2.5 times bigger. A T of 10 min/cm is roughly 25 min/inch. If your report uses inches, divide by 2.54 before comparing it with anything below.
For a conventional in-ground trench bed, the Code wants T between 1 and 50 min/cm. Training material from the Ontario government describes 10 to 20 min/cm as close to ideal: slow enough to treat the effluent, fast enough to keep accepting it.
Step 3: Pipe Length, Runs and Trench Layout
The core formula for a trench bed fed by a septic tank is:
L = Q × T ÷ 200
L is the total length of distribution pipe in metres. The Code sets a floor of 40 m for a conventional trench bed no matter how good the soil is. If the effluent first goes through an approved treatment unit, the divisor becomes 300, so less pipe is needed.
Then the pipe gets split into trenches, and the trenches have their own rules under the Code:
- Runs about the same length, none longer than 30 m
- Trench width 500 to 1,000 mm
- Trench depth 600 to 900 mm
- Trenches centred at least 1,600 mm apart
- Pipe laid in washed septic stone, at least 150 mm of stone under it and 50 mm over it
- At least 900 mm of unsaturated soil between the trench bottom and rock, the high groundwater table, or soil slower than 50 min/cm
Equal runs matter because the effluent is split between them, usually through a distribution box. When one run gets more than its share, that end of the bed wears out first. Our post on distribution box problems covers what that looks like.
Septic System Sizing Chart: Pipe Length by Bedrooms and Soil
Here’s Q × T ÷ 200 run for each bedroom count at three soil speeds, with the 40 m minimum applied. Treat it as a way to read a design, not as a design.
| Bedrooms | Daily flow (L) | Sandy soil, T = 10 | Loam, T = 20 | Slow clay loam, T = 40 |
|---|---|---|---|---|
| 1 | 750 | 40 m (minimum) | 75 m | 150 m |
| 2 | 1,100 | 55 m | 110 m | 220 m |
| 3 | 1,600 | 80 m | 160 m | 320 m |
| 4 | 2,000 | 100 m | 200 m | 400 m |
| 5 | 2,500 | 125 m | 250 m | 500 m |
| 6 | 3,000 | 150 m | 300 m | 600 m |
Doubling T doubles the pipe. That’s the whole story of why clay lots cost more.
Worked Example: One Three-Bedroom House on Three Soils
Picture a 1,600 L/day, three-bedroom bungalow. We’ll assume 600 mm wide trenches at the minimum 1.6 m spacing. Footprints below are the trench area only, not the setbacks or working room around it.
Sandy loam, T = 10. 1,600 × 10 ÷ 200 = 80 m of pipe. That’s three runs of about 27 m. Three trenches at 1.6 m centres make a bed roughly 4 m wide, so about 27 m × 4 m, or 100 to 120 m².
Loam, T = 20. 1,600 × 20 ÷ 200 = 160 m. Six runs of about 27 m, roughly 8.6 m wide. Call it 230 m².
Slow clay loam, T = 40. 1,600 × 40 ÷ 200 = 320 m. Eleven runs of about 29 m, close to 17 m wide. Now you’re near 480 m² of trenches before any setbacks. On a lot like that, a designer will usually look hard at a filter bed or raised bed instead.
How does that square with our drain field guide, which puts a typical three-bedroom bed at 50 to 150 m²? That range describes the beds most homeowners actually have: conventional trenches in reasonably fast soil, or more compact designs like filter beds. It’s an area figure, and pipe length is a different number. Once T climbs past the teens, a pure trench layout grows well beyond 150 m², which is exactly when the design usually changes type.
How Deep Is a Septic Leach Field?
For a conventional in-ground bed, the trenches are 600 to 900 mm deep. The pipe sits in the stone near the bottom, so there’s usually somewhere around 30 to 60 cm of backfill and topsoil over it. That’s why you can’t see a bed, and why a heavy truck can crush one.
The depth that matters more is what’s underneath. The trench bottom needs 900 mm of unsaturated soil above rock, the seasonal high water table, or soil slower than 50 min/cm. Add that to the trench and a fully in-ground bed needs roughly 1.5 to 1.8 m of workable soil from the surface down.
Around Kawartha Lakes that plays out three ways:
- Canadian Shield near Norland and Coboconk. Test pits often hit granite well short of 1.5 m. The fix is imported fill to build the bed up, so the “depth” is partly above grade.
- Clay around Lindsay. Depth to rock is often fine, but T is high and spring water can perch on the clay. The bed gets long, or it gets raised on sand.
- Limestone plain near Kirkfield. Thin soil over flat, fractured limestone. Effluent that reaches cracked rock before it’s treated can travel a long way, so designers treat that 900 mm seriously.
When the Soil Won’t Take a Standard Trench Bed
T under 1 min/cm. Coarse gravel and some lakeshore sand drain too fast to treat anything. Conventional trenches aren’t allowed. The designer brings in leaching bed fill with a slower T and builds in that, and on waterfront lots may push the bed further back from the water than the minimum.
T over 50 min/cm. Too tight for in-ground trenches. The usual answers are a raised bed on imported fill, a filter bed, or a treatment unit feeding a shallow buried trench (which the Code allows in soil up to 125 min/cm) or a Type A dispersal bed. Our post on Type A and raised sand beds explains the difference.
Less than 900 mm to rock or water. Same toolbox. A raised bed makes up the missing soil with fill, but it comes with a catch: the Code increases the required clearances from wells, buildings, lot lines and water by twice the height the bed is raised. On a tight lot that can rule it out and point toward tertiary treatment, which our small-lot guide walks through.
Setbacks shape the bed as much as the formula does. Distribution pipe has to stay back from wells, lakes, buildings and lot lines, and a bed that works on paper can still fail to fit the lot.
What Leach Field Design Looks Like in Practice
A real design follows the same steps: site visit, test pits, perc time, flow from Table 8.2.1.3.A, a bed type, then a drawing with pipe lengths, run layout and setbacks. The drawing goes to the City of Kawartha Lakes Building Division for a permit, and an inspector checks the trenches before backfill. The Code itself is published through Ontario’s Building Code page; your designer will be working from the current edition.
Some beds use plastic leaching chambers instead of pipe and stone, which changes the length rules a little. We cover that in septic leaching chambers in Ontario. For a budget number on your own lot, try the septic replacement cost estimator.
Frequently Asked Questions
How deep is a septic leach field in Ontario?
Conventional trenches are 600 to 900 mm deep, with the pipe near the bottom in a layer of stone. Below that, the Code requires at least 900 mm of unsaturated soil above rock or the high water table. Raised beds sit partly or fully above the original ground.
How big is a leach field for a 3-bedroom house?
At 1,600 L/day, a three-bedroom house needs Q × T ÷ 200 metres of pipe. On sandy soil (T of 10) that’s about 80 m, roughly 100 to 120 m² of trenches. On slower loam it can be double that or more. The designer’s drawing is the real answer.
What are typical leach field dimensions?
Individual trenches are 500 to 1,000 mm wide, 600 to 900 mm deep, no more than 30 m long, and centred at least 1.6 m apart. The number of trenches depends on the total pipe length the soil calls for.
Is there a septic system sizing chart for Ontario?
Table 8.2.1.3.A in the Building Code gives the daily flow by bedroom count, and the pipe length formula turns that into a bed size once your soil is tested. The chart above runs those numbers for common soils, but it isn’t a substitute for a perc test.
Can I make my septic bed smaller to save money?
Not below what the Code calculates. An undersized bed is one of the most common reasons beds fail early, and it won’t pass inspection. Treatment units and some chamber products allow shorter lengths, and your designer can tell you whether they make sense on your lot.
Talk to Someone Who Digs These Every Week
If you’ve got a design in hand and want someone to walk through the numbers, or you’re not sure why your quote calls for a raised bed, call (705) 702-5526 or Book A Pump. We work across the county, from clay lots in Lindsay to rock in Norland and Coboconk and the limestone around Kirkfield, with no travel surcharge anywhere in Kawartha Lakes.