What this calculator does
A septic tank has to do two things at once: hold the wastewater long enough for solids to settle, and store the sludge that settles until someone pumps it out. A four-person house producing 150 litres each a day needs 1,200 litres of working volume for two days of retention, plus 960 litres of sludge storage for three years between pump-outs, so 2,160 litres in total.
The part that gets left out is the second half. A tank sized only on retention looks fine on the day it is installed and then quietly loses capacity as sludge builds, until the retention time has halved and solids start carrying over into the disposal field. That is how a field fails years early, and it is a sizing question rather than a maintenance one, because the storage either exists or it does not.
The formula
Working volume is people × daily flow × retention days, the part that gives solids time to settle. Sludge allowance is people × sludge per person per year × years between desludging. The tank needs both. The page also works the other way: given a tank size you are considering, the desludging interval is (tank size − working volume) ÷ annual sludge accumulation, which tells you how often that tank has to be pumped out to keep its retention time intact.
| Term | Meaning |
|---|---|
| Working volume | The liquid depth that gives solids time to settle. Sized by retention time. |
| Retention time | How long wastewater stays in the tank. Commonly 24 to 48 hours. |
| Sludge allowance | Storage for settled solids between pump-outs. |
| Desludging | Pumping the tank out. The interval depends on tank size and household size. |
The inputs explained
| Field | What to enter |
|---|---|
| People in the house | Permanent occupants. Size for the bedrooms rather than the current household if the house will be sold. |
| Wastewater per person (L/day) | Wastewater per person per day. Around 150 litres is typical; low-flow fittings reduce it. |
| Retention time (days) | Retention time in days. One to two days is the common range, and longer settles better. |
| Sludge per person (L/year) | Sludge accumulated per person per year. Varies with diet, garbage disposal use and detergent. |
| Years between desludging | How long you want between pump-outs. Longer means a bigger tank. |
| Tank you are considering (L) | A tank size you are considering, for the interval and adequacy checks. |
When to use it
Sizing a tank for a new build
Enter the occupancy the house will support rather than the family moving in. Tanks outlive households.
Checking an existing tank
Put the installed size in the last field. If it is smaller than the volume needed, the answer is a shorter pump-out interval rather than a bigger tank.
Planning a pump-out schedule
The desludging interval for your tank tells you how long it keeps full retention. Going past it does not break anything immediately; it just moves solids toward the field.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What size septic tank does a household need?
Only the number of people changes.
| People in the house | Tank volume needed | Working volume | Sludge allowance |
|---|---|---|---|
| 2 people | 1,080 L | 600 L | 480 L |
| 4 people | 2,160 L | 1,200 L | 960 L |
| 6 people | 3,240 L | 1,800 L | 1,440 L |
| 8 people | 4,320 L | 2,400 L | 1,920 L |
| 10 people | 5,400 L | 3,000 L | 2,400 L |
How does the pump-out interval change the size?
The household is fixed and only the years between desludging change.
| Years between desludging | Tank volume needed | Sludge allowance | The tank you entered |
|---|---|---|---|
| 1 year | 1,520 L | 320 L | large enough, 1,480 L to spare |
| 2 years | 1,840 L | 640 L | large enough, 1,160 L to spare |
| 3 years | 2,160 L | 960 L | large enough, 840 L to spare |
| 5 years | 2,800 L | 1,600 L | large enough, 200 L to spare |
| 10 years | 4,400 L | 3,200 L | too small by 1,400 L |
Questions
Is this the size my local authority will approve?
Not necessarily. Most jurisdictions set a minimum size and many prescribe their own formula, often based on bedrooms rather than occupants. Use this as a sizing check and take the approval question to the authority, since their number governs.
Does this size the drain field too?
No, and the drain field is the part that usually fails. Field sizing depends on soil percolation and needs a site test, so a correctly sized tank discharging into an undersized field is still a failed system.
What happens if the tank is too small?
Retention time falls, solids do not settle fully, and they carry over into the field where they clog the soil. The damage is gradual and mostly invisible until the field stops accepting water.
How often should I actually pump it out?
Every few years for most households, and the interval figure here is a guide rather than a schedule. Have the sludge depth measured at an inspection: the real accumulation rate depends on your household and is the only number worth trusting.
Does a garbage disposal change this?
Yes, substantially. Food waste raises the sludge accumulation rate, so raise the litres per person per year rather than keeping the default, and expect a shorter interval between pump-outs.
For the water going in rather than out, see tank and pool volume and rainwater harvesting yield. Gravel covers the bedding and drainage material around the installation.