Septic Tank Size Calculator
Virginia Table 5.2 lists 900 gallons for a 3-bedroom dwelling. The typical tank beside that row is 8 ft long, 4 ft wide, 4 ft liquid depth, and 1 ft freeboard.
Last updated . Bedroom count starts at 3. The lookup is 12VAC5-610-815 only, and it is not an EPA national schedule.
A septic tank size calculator that reads one state’s dwelling table. Pick the bedrooms. The gallon minimum and the typical dimensions are the published row, not a formula.
Soil moved for a hole is the excavation calculator. Water standing in a supply pipe is the pipe volume calculator. Drain fall is the pipe slope calculator.
Virginia dwelling tank
Virginia only. The menu stops at 5 because Table 5.2 stops at 5. Gallons and the four foot-dimensions are copied from the row. They are not calculated from each other.
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The Virginia bedroom table
12VAC5-610-815.A says the minimum hydraulic detention time is 48 hours based on daily design flow, and that septic tank capacity is in no case less than 750 gallons. Table 5.2 is titled “Septic Tank Capacities for Dwelling Units” and its volume column is “Approximate Tank Volume in Gallons.” The five rows are the whole dwelling table. Source: 12VAC5-610-815, Virginia Administrative Code, statutory authority §§ 32.1-12 and 32.1-164 of the Code of Virginia.
| No. of bedrooms | Approximate tank volume in gallons |
|---|---|
| 1 | 750 |
| 2 | 750 |
| 3 | 900 |
| 4 | 1200 |
| 5 | 1500 |
Bedroom choices end at five, matching the last row of Table 5.2. A six-bedroom house is not given a made-up gallon number here. Section A’s 48-hour sentence needs a daily design flow, and this page does not invent that flow from the bedroom count.
Typical dimensions beside the gallons
12VAC5-610-815.B says tanks are rectangular, liquid depth is at least four feet and at most eight feet, and freeboard is at least one foot. Table 5.3, “Typical Septic Tank Dimensions in Feet,” prints one size set for each approximate gallon figure. The 750-gallon row is shared by the one-bedroom and two-bedroom lines of Table 5.2.
| Approximate gallons | Length | Width | Liquid depth | Freeboard |
|---|---|---|---|---|
| 750 | 7 | 3.5 | 4 | 1 |
| 900 | 8 | 4 | 4 | 1 |
| 1200 | 9 | 4.5 | 4 | 1 |
| 1500 | 9.5 | 5 | 4.7 | 1 |
Multiplying the foot columns does not produce the gallon column. Eight by four by four is 128 cubic feet, and 128 × 7.48052 is 957.51, not 900. The panel therefore shows the gallon cell and the dimension cells as printed. Meters on the panel are those feet times 0.3048. Liters are the gallons times 3.7854. USGS prints the comparison “1 gallon = 4 quarts = 8 pints = 128 fluid ounces = 3.7854 liters” on its water-facts sheet.
Other states are not this table
Rules outside Virginia are different, so do not treat this lookup as a national minimum. Some programs size the tank from the number of people or from gallons per day, and the bedroom steps are not universal. A local permit can also add sludge storage, effluent filters, or a second compartment that this regulation’s bedroom table does not convert into an extra gallon figure. Order the tank only after the health department that issues the permit confirms the volume.
Septic tank questions
What septic tank does Virginia list for 3 bedrooms?
Leave bedrooms at 3. Gallons reads 900. Liters, converted here reads 3,406.86 L. Length reads 8 ft · 2.438 m. Width reads 4 ft · 1.219 m. Liquid depth reads 4 ft · 1.219 m. Freeboard reads 1 ft · 0.305 m.
Why do 1 bedroom and 2 bedrooms show the same tank?
Set bedrooms to 1. Gallons reads 750. Liters, converted here reads 2,839.05 L. Length reads 7 ft · 2.134 m. Width reads 3.5 ft · 1.067 m. Liquid depth reads 4 ft · 1.219 m. Freeboard reads 1 ft · 0.305 m. Set bedrooms to 2. All six lines stay on those same figures. Table 5.2 prints 750 for both of those bedroom counts, and Table 5.3 has one 750-gallon dimension row.
What does the table list for 4 bedrooms and for 5?
Set bedrooms to 4. Gallons reads 1200. Liters, converted here reads 4,542.48 L. Length reads 9 ft · 2.743 m. Width reads 4.5 ft · 1.372 m. Liquid depth reads 4 ft · 1.219 m. Freeboard reads 1 ft · 0.305 m. Set bedrooms to 5. Gallons reads 1500. Liters, converted here reads 5,678.10 L. Length reads 9.5 ft · 2.896 m. Width reads 5 ft · 1.524 m. Liquid depth reads 4.7 ft · 1.433 m. Freeboard reads 1 ft · 0.305 m.
Does 8 ft by 4 ft by 4 ft equal 900 gallons?
No. Those three Table 5.3 figures are 128 cubic feet. 128 × 7.48052 is 957.51, which is not 900. The regulation prints an approximate gallon column and a typical dimension column side by side. This page copies each one. It does not derive the gallons from the feet, and it does not resize the feet so they multiply out to 900.
Where is the row for 6 bedrooms?
There is no sixth choice. Table 5.2 ends at 5 bedrooms and 1500 gallons. Section A also says capacity is never under 750 gallons and that detention is 48 hours based on daily design flow. This page does not turn a flow you type into gallons, because the dwelling minimums it shows are the bedroom rows.
Can I use this tank size in another state?
No. The rows are 12VAC5-610-815, a Virginia Administrative Code section under statutory authority of Code of Virginia §§ 32.1-12 and 32.1-164. A health department somewhere else can require a larger tank, a smaller tank, or a volume based on daily flow. Confirm the rule where the house will be built before you order a tank.
Where do the liter and meter lines come from?
They are not in Table 5.2 or Table 5.3. Liters multiply the printed gallons by 3.7854. USGS prints that comparison as 1 gallon = 3.7854 liters on its facts sheet. Meters multiply the printed feet by 0.3048. For the opening row, 900 × 3.7854 = 3,406.86 and 8 × 0.3048 = 2.438.