In Ontario, septic tank size isn’t a judgment call: the Building Code sets a floor of 3,600 litres, and anything bigger comes from your bedroom count.
That answers how big a tank your house needs. Below we also cover how big that tank is physically, how to measure the one in your yard, the smallest tank Ontario allows, and which gallons someone means when they say “1,000 gallons.”
Want your number in one step? The septic tank size calculator takes your bedrooms, how the property is used and any high-water fixtures, and returns a tank size plus a pumping schedule. It rounds up to the standard sizes installers stock, so for four or more bedrooms its answer can sit a little above the bare Code minimum in the chart below. Both are legal. The chart is the floor.
Quick Answer: Septic Tank Size by Number of Bedrooms
Ontario sizes a septic tank from the house’s daily design flow, which comes mainly from bedrooms. Not bathrooms, and not the number of people living there today. The tank’s working capacity must be at least twice that daily flow, and never less than 3,600 litres.
Here’s the septic tank size chart for Ontario homes, built from those two rules:
| Bedrooms | Daily design flow | Minimum tank (litres) | ≈ US gallons | ≈ Imperial gallons |
|---|---|---|---|---|
| 1 | 750 L | 3,600 L | 950 | 790 |
| 2 | 1,100 L | 3,600 L | 950 | 790 |
| 3 | 1,600 L | 3,600 L | 950 | 790 |
| 4 | 2,000 L | 4,000 L | 1,060 | 880 |
| 5 | 2,500 L | 5,000 L | 1,320 | 1,100 |
| 6 | 3,000 L | 6,000 L | 1,590 | 1,320 |
| Each bedroom over 5 | +500 L | +1,000 L | +265 | +220 |
Two things change the answer in practice. Tanks come in standard sizes, so the one installed is usually the next stock size up: 3,600 L for one to three bedrooms, 4,500 L for four, and around 5,700 L for five are common. And large houses get a higher design flow than their bedroom count suggests, which can push even a three-bedroom home past 3,600 L.
Not sure what size you have now? Give us a call at (705) 702-5526. We can check during a routine pumping.
Litres, US Gallons and Imperial Gallons
The Code works in litres. People search in gallons, and there are two kinds: a US gallon is 3.785 litres, and an imperial gallon is 4.546 litres, about 20% bigger.
Ontario tank makers and most local contractors, us included, mean imperial gallons when they say “gallons.” That’s why a 3,600 L tank gets called an “800-gallon tank” here and a 4,500 L tank a “1,000-gallon tank.” American websites and US-made poly tanks use US gallons, so the same 3,600 L tank shows up there as roughly 950 gallons. If a number on a label or a quote looks off, check which gallon it’s in.
How Ontario Determines Septic Tank Size
Why bedrooms and not people? The Ontario Building Code uses bedrooms because they don’t change with occupants. A four-bedroom house might have two people in it today and eight tomorrow, and the system has to handle the potential load.
Daily Design Flow
Table 8.2.1.3.A of the Code assigns each house a daily design flow: 750 litres for one bedroom, 1,100 for two, 1,600 for three, 2,000 for four, 2,500 for five, and 500 more for each bedroom after that.
It also adds flow for finished floor area over 200 m² (basements aren’t counted) and for more than 20 plumbing fixture units, and the highest calculation wins. Picture a three-bedroom house with 250 m² of finished space above grade. The floor-area adder takes it to roughly 2,100 L a day, so the tank minimum becomes about 4,200 L. Your designer runs this math.
The tank must then hold at least twice the daily design flow (Article 8.2.2.3). That’s roughly two days of wastewater sitting still, long enough for solids to settle and scum to float.
The Code also requires at least two compartments, the first holding at least 2,400 litres. Our post on two-compartment septic tanks explains why both sides need pumping.
The Bedroom Count Isn’t Always Obvious
The designer and the City look at rooms that could be used as bedrooms, not just the ones you use that way now. A home office with a closet likely counts. So does the “den” with a window and a door.
When you apply for a septic permit in Ontario, the inspector reviews the floor plan and makes the call. More bedrooms than you expected means a larger tank.
Soil Conditions Matter Too
Soil mostly drives the size of the leaching bed, not the tank. On slow clay like the ground around Lindsay and Bobcaygeon, though, some designers suggest a bigger tank so the effluent is as clean as possible before it reaches a slow bed.
Septic Tank Dimensions: How Big Is a Septic Tank Physically?
These are typical outside dimensions pulled from Ontario precast and poly tank spec sheets. They vary by manufacturer and model, so use them for planning, not digging.
| Tank | Typical outside size (L × W × H) | Approx. weight |
|---|---|---|
| 3,600 L concrete (≈800 imp / 950 US gal) | 2.3–2.6 m × 1.5–1.8 m × 1.4–1.7 m | 3.5–4.5 tonnes |
| 4,500 L concrete (≈1,000 imp / 1,200 US gal) | 2.3–2.7 m × 1.7–2.0 m × 1.5–1.8 m | 4.5–5.5 tonnes |
| About 6,000 L concrete (5,900–6,800 L models) | 2.6–3.9 m × 1.8–2.4 m × 1.5–1.8 m | 6–7.5 tonnes |
| 9,000 L concrete (≈2,000 imp / 2,400 US gal) | 3.9–4.1 m × 2.1–2.4 m × 1.5–1.7 m | 8–9.5 tonnes |
| 3,600–3,800 L poly | 2.6–3.5 m × 1.25–1.55 m × 1.35–1.6 m | 150–250 kg |
| About 4,700 L poly | 2.9–3.0 m × 1.4–1.5 m × 1.8 m | about 225 kg |
| About 5,700 L poly | 3.4–3.5 m × 1.4–1.5 m × 1.8 m | about 285 kg |
Low-profile models of the same capacity are shorter but longer and wider, which helps where rock or groundwater sits close to the surface.
Liquid depth. The Code requires a tank to hold liquid at least 1,000 mm deep (900 mm where the hole is in rock or to keep groundwater from shifting the tank). Most residential tanks run around 1.2 to 1.5 m of liquid, with air space above.
What it means for the dig. The hole needs working room on every side and a levelled base. For a 3,600 L concrete tank, expect an excavation roughly 3.3 to 3.6 m long, 2.5 to 2.8 m wide and about 2 m deep. A 4-tonne tank also needs a boom truck or excavator that can reach the hole, which matters on tight cottage lots. Poly can be placed with small equipment but has to be anchored where the water table rises; our comparison of concrete vs poly vs fiberglass tanks covers that trade-off.
The tank also has to fit the setbacks: at least 1.5 m from the house, 3 m from a property line, and 15 m from a well or lake. See septic and well water distance for the full table.
How to Measure Your Existing Septic Tank
The quickest answer is on paper. Your permit file often lists the tank size, and our guide to septic records in Kawartha Lakes explains how to request it. No records? A pole and a tape measure get you a decent estimate.
1. Find the lids. Follow the sewer line out from the basement and probe for the tank, as described in how to find your septic tank. Most tanks have two openings, one over each compartment. If they’re buried, a septic tank riser brings them to the surface for good.
2. Measure the liquid depth. With a lid off, lower a straight pole (PVC pipe works) until it touches the tank floor. Mark the liquid surface on the pole, pull it out and measure from the mark to the end. Normally the liquid sits at the outlet pipe; see septic tank water level.
3. Measure inside length and width. Shine a flashlight in. From each opening, hold the pole against the inside end wall and measure back to the edge of the opening, then add the distance between the openings. Do the same side to side for width. If you can’t see the walls, probe the outside outline and subtract the walls; precast walls are often around 8 to 10 cm thick.
4. Do the math.
- In centimetres: length × width × liquid depth ÷ 1,000 = litres
- In inches: length × width × liquid depth ÷ 277.4 = imperial gallons (÷ 231 for US gallons)
Say the inside is 210 cm by 135 cm with 130 cm of liquid: 210 × 135 × 130 ÷ 1,000 is about 3,690 litres. That’s a 3,600 L tank, roughly 810 imperial or 970 US gallons. Round or ribbed poly tanks won’t fit a box formula well, so treat those results as rough.
Safety first. Never enter a septic tank, and never put your head or shoulders into the opening. Tanks hold hydrogen sulfide and methane, and people have died in them, including would-be rescuers. Work from outside with a second person present, keep kids and pets away, and secure the lid before you walk off. More in septic tank lid safety.
Or let us measure it during a septic tank pump-out, when the walls are easy to see.
The Smallest Septic Tank Allowed in Ontario
The smallest septic tank you can install in Ontario today has a working capacity of 3,600 litres. That floor applies to every septic tank under Part 8, from a one-bedroom cabin to a five-bedroom house, and to non-residential buildings too.
You’ll still find smaller tanks in the ground. Older cottages sometimes have 500- to 750-gallon tanks from decades ago. They can stay in service while they work, as our post on grandfathered septic systems explains, but a replacement goes back in at 3,600 L or more.
The Smallest Septic System: Tiny Homes and Bunkies
The smallest conventional septic system is a 3,600 L tank feeding a trench bed with at least 40 metres of distribution pipe. A one-bedroom home on good soil can land right on those minimums. A tiny home on a rural Ontario lot is sized like any other dwelling, so it doesn’t get a tiny tank.
Buildings without running water are different. A no-plumbing bunkie needs nothing from a septic standpoint, and some small setups use a composting toilet or a Class 2 grey water system. Since 1 January 2025, though, a new dwelling unit with a water distribution system needs a flush toilet, which closes that route for most new homes. See bunkie and sleeping cabin septic rules and Ontario’s septic classes explained.
What Size Is a 1,000-Gallon Septic Tank?
It depends on the gallon:
- 1,000 US gallons is about 3,785 litres. That clears the 3,600 L minimum, so it suits a one-, two- or three-bedroom home (design flow up to about 1,890 L a day). It also has to be certified for Canada (CSA B66), which not every US-market tank is, and meet the two-compartment rule.
- 1,000 imperial gallons is about 4,546 litres. That’s essentially the 4,500 L tank sold here as a “1,000-gallon tank.” It covers a four-bedroom home (2,000 L a day needs 4,000 L) but not five bedrooms, which need 5,000 L. A large four-bedroom house with floor-area adders can outgrow it too.
On a Canadian quote, “1,000-gallon septic tank” almost always means the 4,500 L size.
Septic System Size: The Leaching Bed Is Sized Separately
The tank is only half the system. The leaching bed uses the same daily design flow plus your soil’s percolation time (T): total pipe length = daily flow × T ÷ 200, never less than 40 m for a conventional trench bed.
A three-bedroom house at 1,600 L a day needs about 80 m of pipe on fast sandy soil, roughly 100 to 120 m² of trenches, and double that on slower loam. Same house, same tank, very different bed. Our septic bed design guide has the full septic system sizing chart by bedrooms and soil.
Cottage vs. Year-Round Sizing
Seasonal Use Properties
Table 8.2.1.3.A gives a private cottage no seasonal discount. A three-bedroom cottage used May to October is sized the same as a three-bedroom house lived in all year: same design flow, same 3,600 L minimum. Seasonal use changes how hard the system works over a year, not what the Code requires.
Many older cottage systems were built smaller, before current rules, and can keep running while they work. The trouble starts when use changes and nobody looks at the system.
Converting Cottage to Year-Round
Moving in full time usually comes with renovations, and added bedrooms or floor area change the design flow. That typically means a septic permit and a check on whether the tank and bed can take it. Our post on converting a cottage to full-time living walks through it.
Sometimes the tank is already big enough and only the bed needs work. Other times everything goes: a full septic system replacement in Ontario runs $15,000 to $35,000 for most conventional systems, more for raised beds.
Weekend vs. Full-Time Use
Every weekend all year is a different load than three months of summer. It doesn’t change the Code minimum, but it changes pumping frequency and bed life. A pattern we see around Coboconk and Fenelon Falls: a cottage slowly becomes a year-round home, the septic never gets reviewed, and the bed wears out early.
When Your Tank Is Too Small
Signs Your Tank Is Too Small
- Frequent pumping. Needing it far more often than every 3 to 5 years suggests the tank is small for the house.
- Slow drains throughout the house, not just one sink.
- Sewage odours in the yard, especially near the bed.
- Wet spots, or unusually green grass, over the bed. Effluent is reaching the surface.
- Sewage backup into the house. The worst case.
A typical scenario: a 30-year-old system sized for a two-bedroom cottage, now serving a four-bedroom home after an addition.
What Happens to an Overloaded Tank
Wastewater doesn’t stay long enough, so solids flow out with the effluent. They clog the bed’s pipe and stone, the bed stops absorbing, and effluent backs up or surfaces. A clogged bed often can’t be repaired, only replaced. Regular maintenance helps, but it can’t make up for a tank that’s fundamentally too small.
Can You Upgrade Your Tank Size?
Yes, and it’s more straightforward than most people expect.
When Upgrading Makes Sense
- You’ve added bedrooms or significant finished floor area.
- You’ve converted from seasonal to year-round use.
- You’ve added a hot tub, a second laundry or other high-water fixtures.
- You’re on the original system and the house has changed a lot since.
What’s Involved
- Permit. Replacing a tank is permitted work; in the City of Kawartha Lakes the fee for a tank replacement is $461.
- Site evaluation. A designer checks whether the existing bed can handle the flow. Our guide to home additions and septic capacity covers that assessment.
- Excavation. The old tank is pumped, disconnected and removed or decommissioned, and the new one goes in.
- Connections and inspection before backfill.
Cost depends on tank size, material, depth and access, so a tank-only upgrade is quoted after a site visit.
Going Bigger Than Required
More capacity means longer settling, cleaner effluent and a buffer for guests. If you’re installing a new system in Kawartha Lakes, one size up is usually a modest cost for a real long-term benefit.
Frequently Asked Questions
What size septic tank do I need for a 3-bedroom house?
A three-bedroom home has a design flow of 1,600 litres a day, so the Code minimum is 3,600 litres (about 800 imperial or 950 US gallons), the same as a one- or two-bedroom home. A large three-bedroom house can need more, and many owners choose 4,500 litres for margin.
How big is a 4-bedroom septic tank in Ontario?
Four bedrooms means 2,000 litres a day, so the Code minimum is 4,000 litres. Because tanks come in standard sizes, most four-bedroom homes get a 4,500-litre (“1,000-gallon”) tank, which is what our calculator returns.
How do I find out what size septic tank I have?
Check the permit file first, then measure from outside the tank using the steps above. In Kawartha Lakes, we can measure it during your next pump-out and write it down so you never have to guess again.
Can I use a smaller tank if only two people live in a 4-bedroom house?
No. The Code sizes the tank from bedrooms, floor area and fixtures, not current occupants. A four-bedroom home needs at least 4,000 litres no matter who lives there.
Does a septic tank need to be a certain distance from the house?
Yes. A septic tank must be at least 1.5 m from the house or any structure, 3 m from a property line, and 15 m from a well, lake or stream. Setbacks can limit where a larger tank fits, which is why a site evaluation is always part of the process.
Get the Right Size From the Start
Planning a new septic installation in Kawartha Lakes, converting a cottage in Bobcaygeon, or wondering whether the tank at your place in Lindsay or Fenelon Falls can handle an addition? Call Kawartha Septic at (705) 702-5526 or Book A Pump. We’ll tell you exactly what you’re working with and recommend the right path forward.