Septic tank and drainfield for a 5-bedroom house, perc 10 min/in

5-bedroom house, perc 10 min/in

1,500 gal tank

plus about 938 ft² of drainfield — roughly 313 ft of trench

Septic tank1,000 gal + 250 per bedroom over 3	1,500gal
Design flow150 gal per bedroom per day — not per person	750gal/day
Soil loading ratefor soil percolating at 10 min/in	0.8gal/ft²/day
Drainfield area	938ft²
Trench length3 ft wide trenches	313ft
Field footprintabout 24 ft across at 6 ft centres	4 × 100 ft

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 5-bedroom house needs a 1,500 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.
**938 ft² comes straight from the perc test.** Soil at 10 min/in accepts 0.8 gallons per square foot per day, and the house produces 750. Slower soil does not mean a bigger tank; it means more ground.
**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 938 ft² 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 5-bedroom house, soil at 10 min/in

Tank: 1,500 gallons. Drainfield: 938 ft², about 313 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 5-bedroom house needs a 1,500 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 10 minutes per inch accepts 0.8 gallons per square foot per day, and this house produces 750.

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.