Septic tank and drainfield size

Septic tank and drainfield size — one page per exact question.

Septic tank and drainfield for a 4-bedroom house, perc 20 min/in

4-bedroom house, perc 20 min/in

1,250 gal tank

plus about 1,000 ft² of drainfield — roughly 333 ft of trench

Septic tank1,000 gal + 250 per bedroom over 3	1,250gal
Design flow150 gal per bedroom per day — not per person	600gal/day
Soil loading ratefor soil percolating at 20 min/in	0.6gal/ft²/day
Drainfield area	1,000ft²
Trench length3 ft wide trenches	333ft
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 4-bedroom house needs a 1,250 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.
**1,000 ft² comes straight from the perc test.** Soil at 20 min/in accepts 0.6 gallons per square foot per day, and the house produces 600. 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 1,000 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.

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