Drain-field / leach-field replacement cost calculator

The drain field is the part that most often wears out. This tool turns the required absorption area into trench linear feet, then prices it at your $/ft and adds the gravel and pipe.

Planning estimate: this is a planning estimate from the numbers you enter and standard reference quantities — not a bid or a contract. Get itemized written quotes from licensed contractors and confirm measurements before you commit.
Designer, driller, health department & permit: Septic design, drain-field sizing and well siting must be verified by a licensed septic designer or professional well driller and approved by your local health department. Sizing rules, setbacks and perc / soil-loading requirements vary by jurisdiction. Confirm the design, pull the required permit and get the required inspection before you dig or drill.

Calculator

sq ft
$/ft
ft
Typical planning default 3 ft — confirm with your local health department.
ft
Depth of drainage gravel in each trench.
Estimated total$3,600.00
Trench length300 LF (900 sq ft ÷ 3.0 ft)
Trench cost$3,600.00 (× $12.00/ft)
Gravel volume66.67 cu yd (300 × 3.0 × 2.0 ÷ 27)
Perforated pipe300 linear ft

A 900 sq ft field at 3.0 ft effective width needs 300 LF of trench ≈ $3,600.00 at $12.00/ft, plus ≈ 66.67 cu yd of gravel. Drain-field sizing and setbacks must be verified by a licensed designer and your local health department, with a permit.

Formula

Area becomes trench, then trench becomes cost, gravel and pipe:

trench LF = ceil(absorption area ÷ effective trench width)
trench cost = trench LF × your $/ft
gravel (cu yd) = trench LF × width × gravel depth ÷ 27
perforated pipe = trench LF

Get the required absorption area from your soil and design flow with the absorption-area calculator. The effective trench width and the $/ft are yours to set — the tool carries no price.

Worked example

A field needing 900 sq ft of absorption at a 3 ft effective width is ceil(900 ÷ 3) = 300 linear feet of trench. At $12/ft that is $3,600 of trench. Gravel is 300 × 3 × 2 ÷ 27 ≈ 66.67 cubic yards, and you need about 300 ft of perforated pipe.

Sizing and rebuilding a leach field

A drain field (leach field) fails when the soil below the trenches clogs with biomat and can no longer absorb effluent at the design rate — effluent then surfaces or backs up. A replacement usually means new trenches, fresh drainage gravel and new perforated pipe, sized for the same design flow the house produces. Because the size is driven by soil, a replacement in slow (clay) soil needs far more trench than the same house in fast (sandy) soil.

Work the sizing chain in order: daily flow with the daily-flow tool, loading rate from your perc test with the perc lookup, absorption area with the absorption-area tool, and the number of laterals with the trench-length calculator. Bring the area here to price it.

Basis: geometry (area ÷ width, rounded up to whole linear feet) plus the gravel identity (LF × width × depth ÷ 27 cubic feet per cubic yard). Trench width and gravel depth are labeled planning defaults. Drain-field sizing, setbacks and layout must be verified by a licensed septic designer and your local health department, and a replacement needs a permit and inspection.

Frequently asked questions

How big a drain field do I need?
Divide your design daily flow (gpd) by the soil loading rate (gpd per sq ft) from your perc test to get the absorption area, then bring that area here. The absorption-area and perc-rate calculators do the first step; slower soil needs more area.
What is effective trench width?
The width of trench that actually absorbs effluent, used to turn absorption area into linear feet. Three feet is a common planning default, but your local health department sets the real value — adjust the field to match.
How much gravel will I need?
Multiply the trench linear feet by the width and the gravel depth, then divide by 27 to get cubic yards. The tool does this for you; in the example, 300 LF of 3 ft trench with 2 ft of gravel is about 66.67 cubic yards.
Can I just extend the old field?
Sometimes, if there is suitable soil and setback room and the health department allows it. Often a full replacement or a new field location is required. A licensed designer and your local health department decide what your site permits.
Is a permit needed to replace a leach field?
Yes, in nearly every jurisdiction, along with an inspection and usually a fresh perc test and design. Confirm the requirements before you dig — this tool estimates quantities and cost, not the approved design.

Related calculators

Drain-Field Absorption AreaDrain-field absorption area calculator: design daily flow divided by soil loading rate gives the required square feet. Loading rate comes from your perc test.Trench Length & LateralsTrench length and laterals calculator: absorption area divided by effective trench width gives linear feet, then the number of laterals by max run.New Septic System CostEstimate the cost of a new septic system from your own line items — tank, drain field, excavation, inspection and permit — plus labor and a contingency buffer.