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Septic

Daily Wastewater Calculator

Multiply the number of people by the gallons each one uses per day and you have a planning figure for household flow. The per-person rate is fully editable, because the right number depends on the household, the fixtures and any local guidance you are working to.

Your measurements

Results update as you type. Every unit dropdown converts for you.

gal

This is your own planning figure, not a design requirement. Change it to whatever your situation or local guidance calls for.

Estimated daily flow

240gallons per day

Per week
1,680 gal
Per 30 days
7,200 gal
Per year
87,600 gal
Per day in liters
908 L
Rate used
60.00 gal per person per day

Planning estimate: This calculator provides general mathematical estimates and is not a septic-system design, permit determination, soil evaluation, engineering recommendation, or substitute for local requirements. Septic regulations, design flows, loading rates, setbacks, reserve areas, and installation requirements vary by jurisdiction and site conditions.

Show the calculationOpen
  1. Daily flow4 people × 60.00 gal=240 gal/day
  2. Weekly flow240 × 7=1,680 gal
  3. Annual flow240 × 365=87,600 gal

Reading your result

The daily figure is occupants multiplied by your per-person rate. Everything else on the panel is that same number scaled out to a week, a month and a year.

Treat it as a planning figure for your own use — sizing a holding tank, understanding pump-out intervals, comparing against a water bill. It is not a design flow.

If a permit is involved, the flow figure will normally be dictated by your local authority and is often based on bedroom count rather than on how many people actually live there. Use the number they give you.

How this calculator works

The arithmetic is one multiplication: occupants times gallons per person per day. Everything else on the results panel is that daily figure scaled to a week, a month or a year.

The default rate of 60 gallons per person per day is a starting point for planning, not a requirement and not a legal design figure. Households with low-flow fixtures often use less; households with high-use appliances, long showers or a laundry habit can use considerably more.

If you are sizing anything that has to be permitted, the flow figure will usually be dictated by your local authority rather than by measured use, and it is often based on bedrooms rather than people. Use the figure they give you.

For a measured rather than estimated number, read your water meter over a full week and divide by seven. That includes irrigation and outdoor use, so subtract those if they do not reach the septic system.

The formula

Daily flow
occupants × gallons per person per day
Weekly flow
daily flow × 7
30-day flow
daily flow × 30
Annual flow
daily flow × 365
Liters per day
gallons per day × 3.785412

Worked example

A four-person household using an estimate of 60 gallons per person per day.

  1. 01Daily flow4 × 60=240 gal/day
  2. 02Weekly240 × 7=1,680 gal
  3. 0330 days240 × 30=7,200 gal
  4. 04Annual240 × 365=87,600 gal

About 240 gallons a day, or roughly 87,600 gallons a year at that rate.

How to take the measurements

  • Count everyone who lives in the house full time, and decide separately how to treat regular guests or part-time occupants.
  • For a measured figure, take two water meter readings a week apart and divide the difference by seven.
  • Subtract outdoor water use — irrigation, car washing, filling a pool — since that water never reaches the septic system.
  • Consider peaks as well as averages. Laundry day and house guests both move the number well above the daily average.
  • If a permit is involved, ask which flow figure your local authority requires before you rely on your own estimate.

Practical considerations

  • This figure drives several downstream decisions — holding tank size, pump-out interval, absorption area — so an error in it propagates into all of them.
  • A jump in household water use with no change in habits usually means a leaking fixture. A running toilet can add more than an additional occupant's worth of flow.
  • Guests, working from home and a new appliance all move the number. Revisit the estimate when the household changes rather than treating it as fixed.
  • If anything you are sizing needs a permit, ask your local authority which flow basis they require before relying on your own arithmetic.

Assumptions and limitations

  • The default rate is a common planning figure, not a requirement and not a legal design value. It is editable precisely because the right number depends on the household.
  • The annual figure assumes year-round occupancy. For a seasonal property, scale by occupied days instead.
  • Only indoor wastewater is estimated. Groundwater infiltration, foundation drains and stormwater are separate matters and should not be entering a septic system.

Frequently asked questions

Is 60 gallons per person per day the right number?

It is a common planning figure, not a rule. Actual household use varies widely with fixture age, habits and occupancy patterns. The field is editable so you can use whatever figure fits your situation or is specified by your local authority.

Should I use people or bedrooms?

For your own planning, people are the more realistic basis. Permitted designs are often based on bedroom count instead, since occupancy changes over the life of a house. Ask which basis applies to your project.

Does this include rainwater or foundation drains?

No, and neither should your system. This calculator only estimates wastewater from indoor fixtures.

How do I estimate a vacation home?

Estimate the daily flow for the days it is occupied, then scale by the number of occupied days. The annual figure here assumes year-round use.

Should I base this on people or bedrooms?

For your own planning, people are more realistic. Permitted designs are often based on bedrooms because occupancy changes over the life of a house. Ask which basis applies to your project.

Is 60 gallons per person per day the right figure?

It is a common starting point, not a rule. Actual use varies widely with fixtures, habits and occupancy. The field is editable so you can enter whatever fits your situation or matches local guidance.

Does this include water from a washing machine or dishwasher?

It should. A per-person rate is intended to cover all indoor fixture use across a day, appliances included.

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