Septic tank and drainfield for a 6-bedroom house, perc 90 min/in

6-bedroom house, perc 90 min/in

1,750 gal tank

perc slower than 60 min/in — a conventional gravity field is not permitted in most states

Septic tank1,000 gal + 250 per bedroom over 3	1,750gal
Design flow150 gal per bedroom per day — not per person	900gal/day
Soil loading rateoff the standard scale	—
Drainfield area	—
Trench length3 ft wide trenches	—
Field footprint	—

Notes

**The tank is sized by bedrooms, the field by soil.** These are two independent questions and mixing them is the most common mistake. A 6-bedroom house needs a 1,750 gallon tank whether the soil is sand or clay — bedrooms are used instead of occupants because bedrooms do not change when the house is sold.
**At 90 minutes per inch the soil is too slow for a conventional field.** Most states stop permitting gravity systems past 60 min/in. The answer is not a bigger field: it is a different system — a mound, a sand filter, or an aerobic treatment unit that discharges water clean enough for the soil to accept.
**A perc test measures one hole on one day.** Rates vary across a lot by a factor of several, and a test dug after a dry week reads faster than the same hole in March. Health departments require the test for exactly this reason, and they choose where it is dug.
**Reserve area is usually required too.** Most jurisdictions want room set aside for a full replacement field before they will permit the first one — so budget roughly double the field in usable ground, clear of driveways, foundations and big trees.
The local health department has the final say and its rules can be stricter than anything here. Use this to size a lot before you buy it and to sanity-check a quote — not to skip the permit.

A 6-bedroom house, soil at 90 min/in

Tank: 1,750 gallons. Drainfield: — ft², about — ft of trench.

The tank is sized by bedrooms, the field by soil

These are two independent questions, and mixing them is the most common mistake in this whole subject.

A 6-bedroom house needs a 1,750 gallon tank whether the ground is sand or clay. Bedrooms are used instead of occupants because bedrooms do not change when the house is sold.

Good soil does not let you fit a smaller tank, and a bigger tank does not rescue bad soil.

What the perc test actually decides

Only the field. Soil at 90 minutes per inch accepts — gallons per square foot per day, and this house produces 900.

Slower soil does not mean a bigger tank. It means more ground.

One hole, one day

A perc test measures a single hole on a single afternoon. Rates vary across a lot by a factor of several, and a test dug after a dry week reads faster than the same hole in March.

That is exactly why health departments require the test and choose where it is dug.

Reserve area

Most jurisdictions want room set aside for a full replacement field before permitting the first one.

Budget roughly double the field area in usable ground — clear of driveways, foundations and large trees. On a tight lot this is the constraint that decides whether the house can be built at all.

The local health department has the final say, and its rules can be stricter than anything here.