Pick the tank too small and you overwork the system and shorten the life of your drainfield. Pick it too big for the site and you have paid for concrete and dig you did not need. The right concrete septic tank size is set by how many bedrooms the house has and what your county requires, and the dimensions decide how the tank fits the hole you dig.
We pour these tanks every week in Kalispell, so here are the real numbers, not a generic chart: concrete septic tank sizes, actual dimensions, and how to match a tank to your house.
Concrete septic tank sizes are rated by liquid capacity in gallons, most commonly 1000, 1250, and 1500 gallons for residential use, with the required size set by the number of bedrooms and local code. Below are the dimensions and the sizing rules.
Concrete septic tank sizes and dimensions
These are the outside dimensions of our two most common residential tanks. Both share the same 68-inch width and 64-inch height; the tank simply gets longer to hold more.
| Tank size | Width | Length | Height | Typical use |
|---|---|---|---|---|
| 1000 gallon | 68 in | 99 in | 64 in | Homes up to about 3 bedrooms |
| 1500 gallon | 68 in | 144 in | 64 in | Larger homes, about 4 to 5 bedrooms |
Because the width and height stay the same, the excavation is a similar depth for both; the 1500-gallon tank just needs a longer hole. That makes planning the dig simpler when you are deciding between the two.
Concrete septic tank dimensions matter for one more reason: weight. A tank this size is several thousand pounds of reinforced concrete, which is why it ships and sets on a crane truck rather than a trailer and a couple of people. When you compare concrete septic tank sizes, you are also comparing how each one gets delivered and placed, and heavier is not a drawback when the crew setting it does this every day.
How to size a septic tank by bedrooms
Septic tanks are sized by bedroom count, not square footage, because bedrooms are the standard proxy for how many people live in a home and how much wastewater it generates. The more bedrooms, the larger the required tank.
As a general rule, a 1000-gallon tank serves a home of up to about three bedrooms, and a 1500-gallon tank covers roughly four to five. Your county and Montana DEQ set the actual minimum for your bedroom count, and that minimum is what the permit enforces. Always confirm the required size against your approved design before you order.
Two things can push you up a size within the same bedroom count. A garbage disposal adds solids and shortens settling time. A home that regularly hosts more people than the bedrooms suggest, or a finished basement with a future bedroom in it, is worth planning for now. When you are on the line between two concrete septic tank sizes, it is usually cheaper to move up at install than to dig the old tank out and retrofit later.
Septic tank size is the working liquid volume a tank holds, measured in gallons, and it determines how long wastewater is retained so solids can settle before the effluent moves to the drainfield. A tank that is too small shortens that retention time and sends solids downstream.
Single-compartment vs. two-compartment tanks
Many modern septic tanks use two compartments instead of one. The first compartment does most of the settling, and the second gives the effluent a quieter second stage before it leaves the tank. Two-compartment tanks generally improve the quality of the effluent reaching the drainfield.
Whether your design calls for one or two compartments depends on local code and the system type. It does not change the outside dimensions in the table above; it changes the baffling inside the tank.
Tank size and drainfield size go together
Tank size is only half of the sizing question. The drainfield has to be sized to the same design flow, and the soil on your site determines how much drainfield that takes. A bigger tank does not make up for an undersized field, and vice versa.
If you are still working out the whole system, our guide to septic system design covers how the tank and drainfield get sized together from soil and flow. Once the tank is spec’d, the septic tank installation guide walks the set and connections.
Sizes above residential
Not every project fits a residential tank. Commercial buildings, multi-home systems, and high-flow sites need larger capacity and, where vehicles cross the tank, a traffic-rated build. Those systems get sized to measured or estimated commercial flow rather than bedrooms.
We build tanks and communal systems well beyond the residential range. For anything carrying traffic loads or serving a commercial site, see traffic-rated and commercial septic systems.
Do bigger concrete septic tank sizes cost more?
Yes, but less than most people expect, and the tank is rarely the biggest line on the invoice. Stepping from a 1000 to a 1500-gallon tank adds concrete and a slightly longer excavation, not a new category of work. The larger cost drivers are the excavation, the drainfield, and site access, which are set by your soil and layout rather than by the tank alone.
The practical takeaway: do not undersize the tank to save a little upfront. The gap between concrete septic tank sizes is modest next to the cost of reworking an overloaded system later. Size it right the first time and the tank becomes the cheapest long-term component in the whole system.
Frequently asked questions
What are the dimensions of a 1000 gallon concrete septic tank?
A Glacier Precast 1000-gallon concrete septic tank measures 68 inches wide, 99 inches long, and 64 inches tall. Dimensions vary by manufacturer, so confirm the exact measurements against the tank you are buying before you excavate, since the hole has to match the tank footprint plus working room on each side.
What are the dimensions of a 1500 gallon concrete septic tank?
A Glacier Precast 1500-gallon concrete septic tank measures 68 inches wide, 144 inches long, and 64 inches tall. It shares the same width and height as the 1000-gallon tank and is longer to hold the extra volume, so the excavation depth is similar but the hole is longer.
What size septic tank do I need for a 3 bedroom house?
A three-bedroom home commonly requires a 1000-gallon septic tank, though the minimum is set by your county and Montana DEQ, not by a national rule. Larger homes step up to 1250 or 1500 gallons. Confirm the required capacity on your approved septic design before ordering, because the permit enforces that minimum.
Is a bigger septic tank better?
A larger tank gives wastewater more time to settle, which can help, but oversizing beyond the design flow mostly adds cost without added benefit and still requires a correctly sized drainfield. The right size is the one matched to your bedroom count and design flow. Undersizing is the real risk, because it shortens retention time and can push solids to the field.
What is the difference between a single and two-compartment septic tank?
A single-compartment tank does all its settling in one chamber. A two-compartment tank splits that into a larger settling chamber and a smaller second chamber, which generally improves the clarity of the effluent leaving the tank. Both hold the same rated volume for a given size; the difference is internal baffling, not outside dimensions.
Do concrete septic tank sizes affect installation cost?
Larger concrete septic tank sizes cost somewhat more for the tank and a longer excavation, but the tank is usually a small part of total cost. Excavation, the drainfield, and site access drive most of the price and are set by soil and layout, not tank size. Sizing correctly for your bedroom count matters more than choosing the smallest tank to save money.
Match the tank to your house, then dig once
Sizing a septic tank is not complicated once you have two numbers: your bedroom count and your county’s required minimum. Match those, confirm the dimensions against the hole you plan to dig, and decide single or two-compartment with your designer. A tank sized right and set right is one you can forget about for decades.
Get the exact tank spec for your site
We build the common concrete septic tank sizes in stock, from 1000 and 1500-gallon tanks to larger and custom capacities for bigger homes and commercial sites, all in our NPCA-certified plant. Send us your bedroom count and county and we will confirm the required size, hand you the exact drawing with dimensions, and deliver and set the tank on our own crane trucks.
There is one dimension on our drawings that decides whether your riser lids finish flush or proud of the grass, and most homeowners never ask about it until the backfill is done.
Request your septic tank spec and quote, or start with the complete guide to concrete septic tanks for sizing, lifespan, and cost in one place. Product drawings live on the concrete septic systems page.