Methodology

Every figure the tools use, imported directly from the same code they run. If you disagree with an assumption, you can see exactly which number to change.

Tank size

Minimum liquid capacity for one- and two-family dwellings from the International Private Sewage Disposal Code, Table 802.7.1. Beyond eight bedrooms the calculator adds 225 gallons per bedroom, continuing the table’s step. With a garbage disposal the minimum is multiplied by 1.5, the allowance many state and county codes apply. The result is rounded up to a stock size: 750, 1,000, 1,250, 1,500, 2,000, 2,500, 3,000 gallons.

Bedrooms	Minimum (gallons)
1	750
2	750
3	1,000
4	1,200
5	1,425
6	1,650
7	1,875
8	2,100

Design flow is 150 gallons per bedroom per day, the figure most state codes use to size drain fields.

States and counties often set their own minimums, usually higher. The county permit is the final word.

Pumping interval

Years between pump-outs from the Penn State College of Agricultural Sciences Cooperative Extension table (A. R. Jarrett), which assumes year-round occupancy, at least 24 hours of retention and 50% digestion of retained solids. The 1,250-gallon row is interpolated between the published 1,000 and 1,500-gallon rows; other tank sizes are interpolated linearly. A garbage disposal multiplies the interval by 0.67. Whatever the interval, the tool never suggests more than 3 years between inspections, per EPA guidance.

Tank	1	2	3	4	5	6	7	8	9	10
500 gal	5.8	2.6	1.5	1	0.7	0.4	0.3	0.2	0.1	—
750 gal	9.1	4.2	2.6	1.8	1.3	1	0.7	0.6	0.4	0.3
1,000 gal	12.4	5.9	3.7	2.6	2	1.5	1.2	1	0.8	0.7
1,250 gal	15.6	7.5	4.8	3.4	2.6	2	1.7	1.4	1.2	1
1,500 gal	18.9	9.1	5.9	4.2	3.3	2.6	2.1	1.8	1.5	1.3
2,000 gal	25.4	12.4	8	5.9	4.5	3.7	3.1	2.6	2.2	2
2,500 gal	30.9	15.6	10.2	7.5	5.9	4.8	4	3.5	3	2.6

New or replacement system cost

The system estimate adds a tank, a field sized per bedroom, any treatment unit and pump chamber the system type needs, and fixed items. Figures are planning ranges calibrated against published 2025–26 cost data — NerdWallet, the Texas Septic Guide installer survey, SepticCalculator and HomeGuide and Angi summaries — so a 3-bedroom system at typical soil and average labour lands inside the published installed range for its type.

System type	Field per bedroom	Treatment unit	Pump chamber	Engineered design
Conventional gravity (gravel trenches)	$1,300 – $2,700	—	No	No
Chamber system (gravelless trenches)	$1,200 – $2,500	—	No	No
Low-pressure dosed field (pump to field)	$1,600 – $3,200	—	Yes	Yes
Mound system	$2,500 – $5,000	—	Yes	Yes
Aerobic treatment unit (ATU)	$700 – $1,400	$4,000 – $8,000	No	Yes
Drip distribution	$1,500 – $2,800	$3,500 – $7,000	No	Yes
Sand or media filter	$1,200 – $2,500	$5,000 – $10,000	Yes	Yes

Tank, supplied and set	Planning range
750 gallons (concrete)	$1,000 – $2,200
1,000 gallons (concrete)	$1,400 – $3,200
1,250 gallons (concrete)	$1,900 – $3,800
1,500 gallons (concrete)	$2,300 – $4,500
2,000 gallons (concrete)	$3,200 – $6,000
2,500 gallons (concrete)	$4,000 – $7,500
3,000 gallons (concrete)	$4,800 – $9,000

Factor	Applied to	Multiplier
Concrete (precast)	Tank	×1
Plastic (polyethylene)	Tank	×0.9
Fiberglass	Tank	×1.1
Good — sandy loam, deep to water	Field	×0.9
Typical / not sure	Field	×1
Poor — clay, shallow rock or high water table	Field	×1.3
Lower-cost area (rural South, Midwest)	Everything except the permit	×0.85
Average	Everything except the permit	×1
High-cost area (Northeast, West Coast, resort areas)	Everything except the permit	×1.3

Fixed item	Planning range
Perc test / soil evaluation	$600 – $2,000
Engineered design	$500 – $2,500
Conventional design	$0 – $800
Permit and inspections	$200 – $1,500
Pump chamber, pump and alarm	$1,500 – $4,000
Abandon the old tank	$800 – $3,000
Risers and lids	$300 – $800
Difficult site	$1,000 – $4,000

The quote check

A quote is placed against the planning range (more than 25% outside counts as well outside), and flagged when:

the quoted tank is smaller than the code minimum for the bedrooms and disposal;
no perc test or soil evaluation has been done;
the permit isn’t included;
a replacement is quoted without evidence the field has failed;
an aerobic system is quoted without the service contract;
a pumped system is priced well below the range, suggesting the pump chamber is missing;
the price is well above the range.

Pumping, repair, inspection and perc test costs

Pump-out	Planning range
Up to 750 gallons	$250 – $450
Up to 1,000 gallons	$300 – $550
Up to 1,250 gallons	$350 – $650
Up to 1,500 gallons	$400 – $750
Up to 2,000 gallons	$500 – $950

Pumping extra	Planning range
Locating the tank	$100 – $400
Digging to a buried lid (each)	$50 – $200
Emergency or weekend	$100 – $350
Effluent filter cleaning	$50 – $150

Repair	Planning range
Clear a clogged line from the house to the tank	$150 – $500
Clean or replace an effluent filter	$50 – $350
Replace an inlet or outlet baffle	$250 – $900
Replace a cracked tank lid	$150 – $700
Install a riser and lid	$300 – $800
Replace a float switch	$150 – $400
Replace the alarm or control panel	$300 – $900
Replace an effluent or dosing pump	$800 – $2,500
Replace an aerobic unit air pump (aerator)	$300 – $900
Repair or replace the distribution box	$500 – $2,000
Jet the drain field lines	$300 – $900
Repair a damaged pipe between tank and field	$500 – $2,500

Inspection	Planning range
Visual inspection	$100 – $250
Full functional inspection	$300 – $650
Full inspection with pump-out	$550 – $1,100
Camera of the lines	$200 – $600
Hydraulic load / dye test	$150 – $400
Aerobic unit inspection	$150 – $400

Perc test	Planning range
Perc test only	$300 – $1,500
Soil evaluation (profile holes)	$500 – $1,800
Perc test plus system design	$1,000 – $3,500
Excavator for test pits	$200 – $600

The problem checker

Each cause carries a weight from 1 to 5 for each symptom it explains. A cause’s score is the sum of its weights for the ticked symptoms, divided by five times the number ticked. The weights are set so that cheap causes — a full tank, a clogged filter or line — outrank a failed drain field unless the evidence is over the field.

Cause	Symptoms and weights
A clog in one fixture's drain	Only one drain is slow (5); Toilets or drains gurgle (1)
A dry trap, failed wax ring or blocked vent	Sewage smell inside the house (4); Toilets or drains gurgle (2)
A clog in the line from the house to the tank	Several drains are slow at once (3); Water backs up at the lowest drain (3); It was pumped recently and it's still happening (5); Toilets or drains gurgle (2)
The tank is overdue for pumping	The tank hasn't been pumped in 3+ years (5); Several drains are slow at once (3); Water backs up at the lowest drain (2); Toilets or drains gurgle (2); Smell outside near the tank (2); Sewage smell inside the house (1)
A blocked inlet baffle or clogged effluent filter	Several drains are slow at once (2); It was pumped recently and it's still happening (2); Water backs up at the lowest drain (2); Smell outside near the tank (1)
A failed effluent pump or float switch	The septic alarm is going off (5); Water backs up at the lowest drain (1); Several drains are slow at once (1)
An aerobic unit's air pump has failed	An aerobic unit has gone quiet or smells (5); The septic alarm is going off (2); Smell outside near the tank (2); Smell over the drain field (1)
The drain field is failing	Soggy ground or standing water over the field (5); Smell over the drain field (4); Strips of greener grass over the field (2); Sewage or water is surfacing at the tank (3); Water backs up at the lowest drain (1); It was pumped recently and it's still happening (2)
Rain or a high water table has saturated the field	It started after heavy rain or snowmelt (5); Soggy ground or standing water over the field (2); The septic alarm is going off (1); Toilets or drains gurgle (1)
Too much water at once	It happens after laundry or heavy water use (5); Toilets or drains gurgle (1); Several drains are slow at once (1)