Drain Slope & Fall Calculator

ANA›Life Services Authority›National Calculator Authority›Drain Slope & Fall 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 Slope & Fall Calculator

Calculate the slope (gradient), fall (drop), or run (horizontal length) of a drain pipe. Enter any two values to find the third.

Run / Horizontal Length (m)

Fall / Drop (mm)

Slope Ratio (1 in X)

Slope (%)

Slope (degrees)

Calculate Clear

(function() {

function draGetVal(id) { var v = document.getElementById(id).value.trim(); return v === "" ? null : parseFloat(v); }

function draSetVal(id, val) { document.getElementById(id).value = val; }

function draShowError(msg) { var r = document.getElementById("dra-result"); r.style.display = "block"; r.className = "calc-result calc-error"; r.innerHTML = "Error: " + msg; }

function draShowResult(rows) { var r = document.getElementById("dra-result"); r.style.display = "block"; r.className = "calc-result"; var html = ""; rows.forEach(function(row) { html += "" + row[0] + "" + "" + row[1] + ""; }); html += ""; r.innerHTML = html; }

window.draCalc = function() { var run_m = draGetVal("dra-run"); var fall_mm = draGetVal("dra-fall"); var ratio = draGetVal("dra-slope-ratio"); var pct = draGetVal("dra-slope-pct"); var deg = draGetVal("dra-slope-deg");

// Validate non-negative var inputs = [run_m, fall_mm, ratio, pct, deg]; for (var i = 0; i 1) { draShowError("Please enter only ONE slope representation (ratio, %, or degrees) at a time."); return; }

if (ratio !== null) { if (ratio === 0) { draShowError("Slope ratio (1 in X) cannot be zero."); return; } slope_frac = 1.0 / ratio; } else if (pct !== null) { slope_frac = pct / 100.0; } else if (deg !== null) { slope_frac = Math.tan(deg * Math.PI / 180.0); }

// Now we have: run_m, fall_mm (convert to m: fall_m = fall_mm/1000), slope_frac var fall_m = (fall_mm !== null) ? fall_mm / 1000.0 : null;

// Count known values (run and fall as one group, slope as another) var knownRun = (run_m !== null); var knownFall = (fall_m !== null); var knownSlope = (slope_frac !== null);

var totalKnown = (knownRun ? 1 : 0) + (knownFall ? 1 : 0) + (knownSlope ? 1 : 0);

if (totalKnown 0) ? (1.0 / slope_frac) : Infinity; var calc_pct = slope_frac * 100.0; var calc_deg = Math.atan(slope_frac) * 180.0 / Math.PI; var calc_fall_mm = fall_m * 1000.0;

// Populate fields draSetVal("dra-run", run_m.toFixed(3)); draSetVal("dra-fall", calc_fall_mm.toFixed(1)); draSetVal("dra-slope-ratio", isFinite(calc_ratio) ? calc_ratio.toFixed(2) : "∞"); draSetVal("dra-slope-pct", calc_pct.toFixed(3)); draSetVal("dra-slope-deg", calc_deg.toFixed(4));

// Compliance check (AS/NZS 3500 & UK Building Regs: min 1:80 = 1.25%, max 1:12 ≈ 8.33%) var minSlope = 1.0 / 80.0; // 1:80 var maxSlope = 1.0 / 12.0; // 1:12 var compliance = ""; if (slope_frac maxSlope) { compliance = "⚠️ Above maximum recommended slope (1:12 / 8.33%). Risk of self-siphonage."; } else { compliance = "✅ Within recommended range (1:80 to 1:12)."; }

draShowResult([ ["Run (Horizontal Length)", run_m.toFixed(3) + " m"], ["Fall (Vertical Drop)", calc_fall_mm.toFixed(1) + " mm (" + fall_m.toFixed(4) + " m)"], ["Slope Ratio", isFinite(calc_ratio) ? "1 : " + calc_ratio.toFixed(1) : "Flat (no slope)"], ["Slope (%)", calc_pct.toFixed(3) + " %"], ["Slope (degrees)", calc_deg.toFixed(4) + "°"], ["Compliance", compliance] ]); };

window.draClear = function() { ["dra-run","dra-fall","dra-slope-ratio","dra-slope-pct","dra-slope-deg"].forEach(function(id){ document.getElementById(id).value = ""; }); var r = document.getElementById("dra-result"); r.style.display = "none"; r.innerHTML = ""; };

})();

#### Formulas Used

Core relationship:

Slope (fraction) = Fall (m) ÷ Run (m)

Conversions:

Example: A 6 m drain at 1:60 slope → Fall = 6 ÷ 60 = 0.1 m = 100 mm

#### Assumptions & References

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References

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