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Septic Tank Size Calculator

Size your septic tank by bedrooms or by daily flow — then get the drainfield area, trench length, pump-out interval, and a full installed-cost estimate with U.S. pricing, a bedroom comparison table, and a downloadable PDF.

Septic systems are governed by state and county health codes and require a percolation test, a permit, and a licensed installer. These are 2024-2025 U.S. average figures. For planning purposes only — not a design.

Household, Sizing & Cost
1 Household
bed
people
Codes size by bedrooms (assume 2 people per bedroom), not by who lives there today
gal / day
U.S. typical 50-75 gal per person per day for indoor use
Disposals, whirlpool tubs and water softeners add solids and flow — many codes require extra capacity
2 Sizing Basis & Soil
Add 250 gal for each bedroom above 3 — the near-universal U.S. rule
days
Tank = daily flow × retention days. Most codes require at least 24-48 hours.
gal
We will check it against the bedroom code minimum and flag it if it is undersized
Only a licensed perc test settles this — the number drives your drainfield size
20%
Headroom for guests, a future bedroom, or a home office. Sizing up is far cheaper now than later.
3 Tank Material & Cost
$/ gal
A 1,000-gal concrete tank typically runs $1,000 - $1,500 delivered
$/ gal
Light and cheap to set, but can shift or float in high water tables
$/ gal
Rust- and crack-proof, easy to place in tight lots, priciest per gallon
$/ sq ft
Conventional trenches ~$8 - $20/sq ft installed; mound systems run far higher
$total
$total
$total
Needed when the drainfield sits uphill of the tank or gravity flow is not possible
Septic Sizing
Recommended Tank Size
—
Enter your household to begin
Tank size —
Daily flow —
Drainfield area —
Full System Cost Breakdown
Code minimum —
Septic tank —
Drainfield —
Excavation & installation—
Perc test, design & permits—
TOTAL INSTALLED ESTIMATE—
Cost / gal capacity —
Trench length (3 ft wide) —
Pump-out every —
System Cost Chart
Estimated cost by category ($)
Tank Size by Bedrooms — Your Soil & Pricing
BedroomsDaily FlowMin TankDrainfieldTotal Estimate

Code minimums at 1,000 gal base, 2 people per bedroom, 60 gal/person/day, and your soil rate of 0.60 gal/sq ft/day.