Quick answer: A house with three or fewer bedrooms needs at least a 1,000-gallon septic tank in both North Dakota and Minnesota. For four bedrooms, the minimum is 1,200 gallons in North Dakota and 1,500 gallons in Minnesota. Older systems can be smaller than today's minimums. To confirm your tank's size and pumping schedule, call (701) 566-7446.
Minimum tank sizes by bedroom count
Both states size septic tanks by bedrooms, not by how many people live in the house today, because bedrooms predict how many people a house can eventually hold. The Fargo–Moorhead area straddles the Red River, so two sets of rules apply: North Dakota's plumbing code for onsite systems on the Cass County side, and Minnesota's SSTS rules (Minnesota Rules chapter 7080) on the Clay County side.
| Bedrooms (single-family home) | North Dakota minimum (Cass County side) | Minnesota minimum (Clay County side) |
|---|---|---|
| 1–3 | 1,000 gallons | 1,000 gallons |
| 4 | 1,200 gallons | 1,500 gallons |
| 5 | 1,500 gallons | 1,500 gallons |
| 6 | 1,500 gallons | 2,000 gallons |
| 7 | 2,000 gallons | 2,000 gallons |
| 8 | 2,000 gallons | 2,500 gallons |
| Garbage disposal | already allowed for in the table | at least 50% larger, with multiple compartments or tanks |
Sources: North Dakota Administrative Code 62-03.1-03-07 and Minnesota Rules 7080.1930. North Dakota adds 150 gallons for each bedroom beyond eight. Minnesota also requires the 50% increase when sewage is pumped to the tank by an ejector or grinder pump, which is common in homes with basement bathrooms. These are the minimums for systems built under current rules. A system designer can specify a larger tank, and the county permit, not this table, is what governs a new installation. See septic installation for how design and permitting work here.
How to find out what size tank you have
The table tells you what a house should have. Your tank may be different, especially on farmsteads and older acreages where the house has been added onto or the system predates current rules. Work through these in order:
- Old pumping receipts. A pumping receipt often lists the gallons hauled or the tank size. Ask the previous owner, or check the closing paperwork if you bought recently.
- Permit and inspection records. Systems installed under a permit may have records with the county: Fargo Cass Public Health's environmental health office in Cass County, ND, and Clay County Environmental Health in Clay County, MN. If the house changed hands in Clay County, the county's septic compliance inspection report may also describe the system. See the home sale rules guide.
- Measure it at the next pump-out. With the tank open, the operator can work out its capacity from the tank's dimensions and the liquid depth, count the compartments, and note it on your receipt for the future.
- Count bedrooms, cautiously. Bedroom count gives a reasonable guess for a house built under modern rules. It's a poor guess for an old farmhouse, a home with an added bedroom or basement apartment, or a system that also serves a shop.
Can't find the tank at all? Look for a riser or lid in the yard roughly in line with where the main sewer line leaves the house. On tanks built to North Dakota's current code, the maintenance hole has to come up to within about a foot of the finished grade. Older tanks can sit deeper, which is one reason a buried lid adds to the cost of a pump-out.
What tank size means for pumping
A septic tank works by holding wastewater long enough for solids to settle out as sludge and grease to float up as scum. Size buys you storage for those layers. A bigger tank serving the same household fills with sludge more slowly and can go longer between pump-outs. A small tank under a busy house fills faster and gives the drain field less protection. For most households here, that works out to the familiar every 3–5 years. The septic pumping cost guide has a chart by tank size and household size, and the price of a pump-out also rises with the gallons hauled.
Minnesota spells out when a tank is due. Owners must have their tanks assessed at least every three years, including measuring sludge and scum. A tank must be pumped once the top of the sludge is within 12 inches of the bottom of the outlet baffle, or the bottom of the scum layer is within 3 inches of it, and the combined sludge and scum must never take up more than 25 percent of the tank's liquid capacity (Minnesota Rules 7080.2450). Those are sensible yardsticks on the North Dakota side too. The same rule notes two things worth knowing anywhere: pumping must go through the maintenance hole, not a small inspection pipe, and septic additives don't replace pumping.
Ready to schedule, or want the tank measured while it's open? See septic tank pumping, or call (701) 566-7446.
Frequently asked questions
What size septic tank does a 3-bedroom house need?
Under current rules in both North Dakota and Minnesota, a house with three or fewer bedrooms needs a septic tank with at least 1,000 gallons of liquid capacity. Those are minimums for new and replacement systems; a designer can specify more, and a tank installed decades ago under older rules may be smaller.
What size septic tank does a 4-bedroom house need?
It depends which side of the river you're on. North Dakota's code sets a 1,200-gallon minimum for a 4-bedroom single-family home; Minnesota's SSTS rules set 1,500 gallons for 4 or 5 bedrooms. In Minnesota, a garbage disposal or a sewage ejector/grinder pump raises the requirement by another 50 percent.
How do I find out what size my septic tank is?
Start with paperwork: an old pumping receipt often lists the gallons hauled, and permit or inspection records may be on file with Fargo Cass Public Health (Cass County, ND) or Clay County Environmental Health (Clay County, MN). Without records, the pumping operator can measure the tank when it's open. Counting bedrooms gives a rough guess, since tanks are sized by bedroom count, but older and farmstead systems don't always match today's minimums.
Does a bigger septic tank mean I pump less often?
Generally yes. More capacity means more room for sludge and scum before they reach the outlet, so a larger tank serving the same household can usually go longer between pump-outs. But use matters as much as size, and Minnesota's rules require the tank to be checked at least every three years regardless. The sludge and scum measurement taken at each pump-out is the real guide to your next date.
Does a garbage disposal change the septic tank size I need?
In Minnesota, yes: if a garbage disposal is installed or planned, the tank must be at least 50 percent larger than the base requirement and have multiple compartments or tanks. North Dakota's tank table already includes an allowance for a food waste disposal unit. Either way, a disposal adds solids, so expect to pump more often.