Septic tank size calculator: what size septic tank do I need?
1,000 gallon septic tank
750 sq ft of trenches or pits, 250 per bedroom at 25 min/in · for 3 bedrooms
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Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
1,000 gallon septic tank
750 sq ft of trenches or pits, 250 per bedroom at 25 min/in
Source: International Private Sewage Disposal Code 2021, Tables 802.7.1 (septic tank capacity) and 603.1 (absorption area), and Section 406.3, read October 27, 2026. Read the source
A 3 bedroom house needs a septic tank of at least 1,000 gallons by IPSDC 2021 Table 802.7.1, and at a 25 min/in perc test 750 sq ft of trench bottom, 250 per bedroom by Table 603.1.
A worked example
A 5 bedroom house on a lot whose slowest perc test took 38 minutes per inch, with a seepage bed needs a 1,425 gallon septic tank and 1,875 sq ft of bed. Table 802.7.1 gives 1,425 gallons for 5 bedrooms. 38 min/in falls in the 30 to under 45 band of Table 603.1, where a seepage bed takes 375 sq ft per bedroom: 5 × 375 = 1,875 sq ft. Trenches would take 300 per bedroom, 1,500 sq ft.
What size septic tank for a 3 or 4 bedroom house?
By IPSDC 2021 Table 802.7.1, 1,000 gallons for 3 bedrooms and 1,200 for 4. A 1 or 2 bedroom house still takes the 750 gallon minimum, a 5 bedroom one 1,425 and an 8 bedroom one 2,100.
How big a drain field do I need?
Table 603.1 sets the area per bedroom by the perc test: 165 to 330 sq ft of trench bottom, from under 10 to 60 minutes per inch, or 205 to 415 sq ft for a seepage bed. A 4 bedroom house at 20 min/in needs 1,000 sq ft of trenches.
What if the perc test is slower than 60 minutes per inch?
Then IPSDC 406.3 rules out a trench or bed system, and the slowest of the three tests is the one that counts. The soil under the system also needs 3 ft of depth above ground water or bedrock (406.2).
What else it is called
Also searched as a septic tank size chart, drain field size calculator or leach field size calculator. The code calls the drain field a soil absorption system: seepage trenches, seepage beds or seepage pits.
Tank and drain field by bedrooms
| Bedrooms | Septic tank | Trenches, under 10 min/in | 10 to under 30 | 30 to under 45 | 45 to 60 |
|---|---|---|---|---|---|
| 1 | 750 gal | 165 sq ft | 250 sq ft | 300 sq ft | 330 sq ft |
| 2 | 750 gal | 330 sq ft | 500 sq ft | 600 sq ft | 660 sq ft |
| 3 | 1,000 gal | 495 sq ft | 750 sq ft | 900 sq ft | 990 sq ft |
| 4 | 1,200 gal | 660 sq ft | 1,000 sq ft | 1,200 sq ft | 1,320 sq ft |
| 5 | 1,425 gal | 825 sq ft | 1,250 sq ft | 1,500 sq ft | 1,650 sq ft |
| 6 | 1,650 gal | 990 sq ft | 1,500 sq ft | 1,800 sq ft | 1,980 sq ft |
| 7 | 1,875 gal | 1,155 sq ft | 1,750 sq ft | 2,100 sq ft | 2,310 sq ft |
| 8 | 2,100 gal | 1,320 sq ft | 2,000 sq ft | 2,400 sq ft | 2,640 sq ft |
Tank: IPSDC 2021 Table 802.7.1, minimum liquid capacity. Trench or pit area: Table 603.1 sq ft per bedroom times the bedrooms, bottom area for trenches and sidewall area for pits; seepage beds take 205, 315, 375 and 415 sq ft per bedroom in the same four bands. Percolation in minutes for water to fall 1 inch, the slowest of the three tests.
Where the numbers come from
- Tank: International Private Sewage Disposal Code 2021, Section 802.7 and Table 802.7.1, as adopted by Alabama without amendments. Minimum liquid capacity 750 gallons; for one- and two-family dwellings 750 gallons for 1 or 2 bedrooms, 1,000 for 3, 1,200 for 4, 1,425 for 5, 1,650 for 6, 1,875 for 7 and 2,100 for 8. Read October 27, 2026. IPSDC 2021 chapter 8 on UpCodes
- Drain field: IPSDC 2021 Table 603.1, minimum absorption area per bedroom by percolation rate. Under 10 min/in 165 sq ft for trenches or pits and 205 for beds; 10 to under 30, 250 and 315; 30 to under 45, 300 and 375; 45 to 60, 330 and 415. Trenches and beds count bottom area, pits sidewall area. Read October 27, 2026. IPSDC 2021 chapter 6 on UpCodes
- Site: IPSDC 2021 Section 406.3, no trench or bed system where any of the three percolation tests is slower than 60 minutes per inch, and the slowest rate sizes the area; 406.2, at least 3 ft of soil between the bottom of the system and high ground water or bedrock. Read October 27, 2026. IPSDC 2021 chapter 4 on UpCodes
- The rest is arithmetic: drain field sq ft = sq ft per bedroom × bedrooms.
Table 802.7.1 stops at 8 bedrooms and covers one- and two-family dwellings; other buildings add capacity by use in Table 802.7.2. Pressure distribution systems are sized under chapter 7 of the code, not Table 603.1.
By Javier Miralles · Last updated . How this site is made.
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