01
Formula
Flow = bedrooms × 150 gallons per day.
Tank gallons = 1,000 through two bedrooms, then 250 gallons for each extra bedroom. The septic tank calculator rounds the bedroom count to the nearest whole room before it applies that step.
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Septic tank size calculator from bedrooms
This septic tank size calculator uses a planning rule: 150 gallons a day of flow per bedroom, and a tank of 1,000 gallons through two bedrooms, then 250 gallons for each bedroom after that. The septic tank calculator is the same rule. It is not a health-department approval and not a soil test.
Planning tank size
1,500gal
Planning flow600 gal/day
Bedrooms4
Rule150 gal/day per bedroom
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Flow is 150 gallons a day times the bedroom count.
Round the bedroom count to a whole room. One or two bedrooms plan a 1,000 gallon tank. Each bedroom past two adds 250 gallons. Four bedrooms plan 1,000 plus 500, which is 1,500 gallons, and 600 gallons a day of flow. A jurisdiction can require more for a garbage disposal, a large tub, or a poor perk rate. This page does not know your code. Take the result to the permit office as a starting size, not as an approval. Dengaro does not design septic systems.
02
Example
Four bedrooms.
Flow is 600 gallons a day. The tank planning size is 1,500 gallons. Three bedrooms would be 1,250 gallons. Two bedrooms stay at 1,000 gallons under this rule.
03
Tips
Bedrooms are not bathrooms.
The rule counts rooms that can be slept in, which is how many codes estimate flow. A home office that is not a bedroom does not add a step on this page. If your county counts it, add it to the bedroom number yourself.
04
Equipment
The excavator digs the hole. The tank is material.
Dengaro arranges financing for an excavator. The tank size is not a loan amount. Dengaro arranges equipment financing for an excavator.
Important questions
Know what to expect.
Is 1,500 gallons the legal size for four bedrooms?+
Not necessarily. It is a common planning step. Your health department's table controls the permit. Read that table before you order a tank.
Why 150 gallons a day?+
It is a widely used residential planning allowance per bedroom. Some codes use a different number. If yours does, scale the flow note and do not treat this tank step as the code.
Does this size the drain field?+
No. The septic tank calculator stops at the tank. The field depends on the soil test and the code. This page does not estimate perk rate or lateral length.
What about a commercial building?+
Do not use a bedroom rule for a shop or a restaurant. Those flows are fixture counts or seats. This form is the residential bedroom shortcut only.
Financing built into the sale