Drain Field Size Calculator
ANA›Life Services Authority›National Calculator Authority›Drain Field Size Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Drain Field Size Calculator
Estimate the required drain field (leach field) area for a septic system based on household size, daily wastewater flow, and soil percolation rate.
Number of Bedrooms
bedrooms
Daily Wastewater Flow
gallons/day
Soil Percolation Rate
min/inch
Trench Width
feet
Trench Center-to-Center Spacing
feet
Calculate
function draUpdateFlow() { var beds = parseFloat(document.getElementById('dra-bedrooms').value); if (!isNaN(beds) && beds >= 1) { // EPA guideline: 75 gallons per person per day, 2 people per bedroom var flow = Math.round(beds * 2 * 75); document.getElementById('dra-flow').value = flow; } }
function draGetLoadingRate(perc) { // EPA/standard loading rates (gallons/day/sq ft) based on perc rate (min/inch) // Source: EPA Design Manual for Onsite Wastewater Treatment and Disposal Systems if (perc 60 min/inch: unsuitable }
function draCalc() { var errEl = document.getElementById('dra-error'); var resEl = document.getElementById('dra-result'); errEl.style.display = 'none'; resEl.style.display = 'none';
var flow = parseFloat(document.getElementById('dra-flow').value); var perc = parseFloat(document.getElementById('dra-perc').value); var trenchW = parseFloat(document.getElementById('dra-trench-width').value); var trenchS = parseFloat(document.getElementById('dra-trench-spacing').value);
// --- Validation --- var errors = []; if (isNaN(flow) || flow 60) errors.push("Percolation rate > 60 min/inch: soil is unsuitable for a conventional drain field.");
if (errors.length > 0) { errEl.innerHTML = errors.join(''); errEl.style.display = 'block'; return; }
// --- Core Formula --- // Loading Rate (R) from perc table (gal/day/ft²) var R = draGetLoadingRate(perc);
// Required Absorption Area (A) = Daily Flow (Q) / Loading Rate (R) var A = flow / R; // sq ft
// Trench length = Absorption Area / Trench Width var trenchLen = A / trenchW; // linear feet
// Number of trenches (assume standard 100 ft max trench length) var maxTrenchLen = 100; // ft var numTrenches = Math.ceil(trenchLen / maxTrenchLen); var actualTrenchLen = trenchLen / numTrenches;
// Total field dimensions var fieldLength = actualTrenchLen; // ft var fieldWidth = numTrenches * trenchS; // ft var totalArea = fieldLength * fieldWidth; // sq ft (gross)
// Percolation rate category var percCategory = ""; if (perc ' + 'Soil Loading Rate' + R.toFixed(2) + ' gal/day/ft²' + 'Required Absorption Area' + A.toFixed(0) + ' ft²' + 'Total Trench Length Needed' + trenchLen.toFixed(0) + ' linear ft' + 'Number of Trenches' + numTrenches + '' + 'Length per Trench' + actualTrenchLen.toFixed(1) + ' ft' + 'Field Length × Width' + fieldLength.toFixed(1) + ' ft × ' + fieldWidth.toFixed(1) + ' ft' + 'Gross Field Area' + totalArea.toFixed(0) + ' ft²' + 'Soil Type (Perc)' + percCategory + '' + '' + '⚠️ Add a 100% reserve area of equal size adjacent to the field as required by most codes.
'; resEl.style.display = 'block'; }
#### Formulas Used
Required Absorption Area:
A (ft²) = Q (gal/day) ÷ R (gal/day/ft²)
Where Q = daily wastewater flow and R = soil loading rate determined from the percolation rate table below.
Daily Flow Estimate:
Q = Bedrooms × 2 persons/bedroom × 75 gal/person/day
Trench Length:
L (ft) = A ÷ Trench Width
Soil Loading Rate Table (EPA):
Perc Rate (min/in)Loading Rate (gal/day/ft²) ≤ 51.20 6 – 100.80 11 – 150.72 16 – 200.64 21 – 250.56 26 – 300.48 31 – 450.40 46 – 600.32
60Unsuitable
#### Assumptions & References
- Daily flow estimated at 75 gallons per person per day with 2 persons per bedroom (EPA standard).
- Soil loading rates are taken from the EPA Design Manual: Onsite Wastewater Treatment and Disposal Systems (EPA 625/1-80-012).
- Maximum recommended individual trench length is 100 feet to ensure uniform distribution.
- A 100% reserve area of equal size is required by most state and local codes — not included in the calculated area.
- This calculator provides an estimate only. A licensed engineer or soil scientist must conduct a site-specific percolation test and design per local regulations.
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