Septic Tank Sizes: Which One Do You Need?

Septic tanks are sized by the daily wastewater flow your house is expected to produce — and that's usually based on the number of bedrooms, not the number of people. Here's the practical sizing chart most Ontario installs follow.

Standard residential sizing

• 1–2 bedrooms: 750–1000 gallons
• 3 bedrooms: 1000 gallons (most common)
• 4 bedrooms: 1250 gallons
• 5 bedrooms: 1500 gallons
• 6+ bedrooms or large flows: 2000+ gallons

Why bedrooms and not people?

Codes use bedrooms because the people in a house change over time, but bedrooms don't. A 4-bedroom house gets sized for a future family that might fill all 4 bedrooms, even if only two adults live there now.

Holding tanks are different

If you have a holding tank instead of a septic with a leaching bed, size matters even more — because everything you put in stays in until you pump it. Holding tanks are typically 1500–3000 gallons and need pumping monthly or more.

Signs your tank is too small

• You're pumping every 12 months or less and there are only a few people in the house
• The bed is wet or soggy even with normal water use
• You're seeing slow drains shortly after pumping

Can you upsize?

Yes — and if you're adding bedrooms or expanding the house, you'll need to. A bigger tank gives wastewater more time to settle, which protects the leaching bed and extends overall system life.