Septic sizing starts with how much water a household sends to the tank each day. Multiply the number of people by a gallons-per-person-per-day figure and you have the design flow that drives tank and field sizing.
Four people at 60 gallons a day each is a 240-gallon daily flow.
The example
Three homes, each with an occupant count and the gallons-per-person figure used for that home.
| A | B | C | D | |
|---|---|---|---|---|
| 1 | Home | People | Gal/person | Daily flow |
| 2 | Home A | 4 | 60 | 240 |
| 3 | Home B | 6 | 60 | 360 |
| 4 | Home C | 2 | 75 | 150 |
The formula
People times the per-person daily figure is the whole calculation:
How it works
Two inputs:
B2is the number of people the system serves — often taken as two per bedroom for design.C2is the gallons-per-person-per-day figure; 60 is a common design value, higher where water use runs heavy.- The product is the estimated daily flow that tank capacity and drain-field size are based on.
- Keep the per-person figure in its own cell so you can switch between local design codes.
Many codes size by bedrooms, not occupants, to fix a home's rated capacity regardless of who lives there — swap the people count for bedrooms times two.
Try it: interactive demo
Enter the number of occupants and the gallons per person per day.
Variations
Size by bedrooms
Two people per bedroom is a common design assumption.
Peak-day allowance
Add a safety factor for guests and laundry days.
Pitfalls & errors
This is a design estimate, not a metered flow. Actual use varies with fixtures, habits, and leaks — a running toilet can double the real number.
Check the local per-person figure. Design values differ by jurisdiction, and using the wrong one under-sizes or over-sizes the whole system.
Practice workbook
Frequently asked questions
Is 60 gallons per person right?
Why do codes size by bedrooms?
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