Drain Field Size Calculator
Drain field sizing depends on your state's code, your soil type, and your household's wastewater flow. This calculator uses the official tables from Texas (30 TAC 285), North Carolina (15A NCAC 18E), and Florida (62-6 FAC) to show the required absorption area and trench length.
Enter your home's details and soil class (from a perc test or soil report) to get a sizing estimate and cost range. If you don't know your soil type, the calculator shows the range across standard-suitable classes and recommends a soil evaluation.
How this works
The calculator applies each state's own sizing rule. In Texas, required absorptive area equals the daily wastewater usage rate from Table III of 30 TAC 285 divided by the soil's application rate from Table I. Trench length is that area divided by the trench width plus 2 ft, because the rule counts the bottom plus one foot of sidewall on each side. North Carolina sizes on 120 gallons per day per bedroom divided by the long-term acceptance rate for the soil group. Florida uses Table I flows (minimum 200 gallons per day) divided by the Table III loading rate.
Costs are the published ranges from HomeGuide, theseptic.guide, and HomeAdvisor (retrieved Oct 2026), shown by field type and never multiplied by your computed length. The planning total adds the published low ends and the published high ends of the lines shown; it is an estimate, not a quote. Sizes are planning numbers. Your county or a licensed designer sets the final design.
Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-10.
Sources
- TCEQ RG-472 — On-Site Sewage Facility Rules Compilation (May 2017)
- NC 15A NCAC 18E .0401 — System Design
- FL 62-6.008 FAC — Standards for OSTDS Installation
- HomeGuide — Drain / Leach Field Replacement Cost (Dec 2025) (third-party estimate)
- theseptic.guide — Drainfield Replacement Cost (third-party estimate)
- HomeAdvisor — Septic Tank Installation Cost (third-party estimate)
Frequently Asked Questions
How big a drain field does a 3-bedroom house need?
It depends on the state and the soil. In Texas, a 3-bedroom home under 2,500 sq ft needs 300 gpd (240 with water-saving fixtures), divided by the soil rate: 789–1,500 sq ft. In Florida, 300 gpd ÷ 0.35–0.80 = 375–857 sq ft for trenches. In North Carolina, 360 gpd ÷ 0.1–1.2 = 300–3,600 sq ft depending on soil group.
Can I use a perc test result?
Yes. Pick the soil class your report lists. Florida's Table III also lists perc-rate bands (e.g., 5–10 min/inch = moderately limited). The perc test tells you what loading rate applies to your site.
Why won't the tool size my soil?
Some soils (Texas Class Ia/IV, Florida severely limited) aren't allowed for a standard drain field. Those sites need a designed system — usually an ATU with spray or drip irrigation, or a mound.
Does a replacement field have to be bigger?
Often, because current codes apply. Replacement fields must usually meet today's tables, not the code the original field was built under. Florida adds 50% to the size after a hydraulic failure on post-1983 systems.
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