Septic Tank Size Calculator
Tank gallons, dimensions, and daily flow from bedroom count.
What it does
Septic sizing starts from bedrooms because each one stands for two people at 75 gallons a day. Tank gallons follow published tables while drainfields follow soils, so the tank is the calculable half of the system.
The tool above takes the bedroom count from 1 to 5. It returns the tank gallons with approximate dimensions and the daily design flow beside them.
The default 4 bedroom home needs 1,200 gallons (about 9 by 4.5 ft) at 600 gpd. A 3 bedroom home needs 900 gallons (about 8 by 4 ft) at 450 gpd.
Tables follow Virginia Tech Extension guidance sourced to state code with EPA onsite manuals behind the method. Both stress the same ending: local health departments permit the real design after soil tests.
How to use
- Enter the bedroom count, 1 to 5.
- Read tank gallons first for the shopping number.
- Use dimensions for placement planning.
- Use daily flow for drainfield talks.
- Confirm everything with the local permit office.
Formula
- x2
- People assumed per bedroom.
- 75 gpd
- Daily gallons per person design flow.
- table
- Tank gallons by bedroom count.
Six plus bedrooms need engineered designs beyond this table. Garbage disposals, hot tubs, and home businesses add flow the bedroom count misses.
Worked example
Example: 4 bedrooms
Bedrooms 4.
- People: 4 x 2 = 8.
- Flow: 8 x 75 = 600 gpd.
- Table: 4 bedrooms maps to 1,200 gal.
- Measures: about 9 x 4.5 ft.
- Tool shows 1,200 gal, 9 x 4.5 ft, 600 gpd.
Reference table
| Bedrooms | Tank | Daily flow |
|---|---|---|
| 1 | 750 gal | 150 gpd |
| 2 | 750 gal | 300 gpd |
| 3 | 900 gal | 450 gpd |
| 4 | 1,200 gal | 600 gpd |
| 5 | 1,500 gal | 750 gpd |
Limits
- Tank half only; drainfields need soil tests.
- 1 to 5 bedrooms; larger needs engineers.
- State tables vary; confirm local code.
- Extra fixtures and businesses add uncounted flow.
Common mistakes
- Sizing by bathrooms instead of bedrooms.
- Skipping permits and soil evaluation.
- Forgetting garbage disposal extra load.
- Planning buildings or parking over the field.
- Never pumping because it still drains.
Try it yourself
Quick checks. Answers sit tucked under each question.
3 bedrooms?
900 gal, about 8 x 4 ft, 450 gpd.
Flow for 2 bedrooms?
4 people x 75 = 300 gpd.
5 bedroom tank?
1,500 gal at 750 gpd.
Questions
What size septic for 4 bedrooms?
1,200 gallons at 600 gpd design flow.
Why bedrooms?
Each stands for two people of flow; baths do not set the count.
How often to pump?
Typically 3 to 5 years by household size and tank; inspect instead of guessing.
What about the drainfield?
Sized by soils and flow with the health department; this page covers the tank.
Do states differ?
Yes. Tables and setbacks vary, so confirm local code before buying.
Garbage disposal effect?
Adds solids and flow; upsize discussions and pump more often.