Drain Flow Rate Calculator

ANA›Life Services Authority›National Calculator Authority›Drain Flow Rate 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 Flow Rate Calculator

Calculate the volumetric flow rate through a circular drain pipe using Manning's equation for open-channel flow, or through a drain orifice using the orifice flow equation.

Calculation Method

Manning's Equation (Pipe / Open Channel) Orifice Flow Equation (Drain Hole / Grate)

Pipe Diameter (m)

Internal diameter of the circular pipe

Pipe Slope (m/m)

Rise over run (e.g. 0.01 = 1%)

Manning's n

0.010 – PVC / smooth plastic 0.012 – Concrete (smooth) 0.013 – Cast iron / ductile iron 0.015 – Corrugated metal (light) 0.024 – Corrugated metal (heavy) Custom value…

Custom Manning's n

Fill Ratio (0–1)

Depth of flow ÷ diameter (1.0 = full pipe)

Orifice Area (m²)

Total open area of the drain / grate

Water Head (m)

Depth of water above the drain opening

Discharge Coefficient (Cd)

0.61 – Sharp-edged orifice (standard) 0.82 – Well-rounded entrance 0.98 – Short tube / re-entrant Custom value…

Custom Cd

Calculate Flow Rate

#### Formulas Used

Manning's Equation (full or partial pipe):

Q = (1/n) × A × R2/3 × S1/2

For a full circular pipe: A = π D² / 4, R = D / 4

For a partial circular pipe at fill ratio y/D = r:

θ = 2 × arccos(1 − 2r)  [radians] A = (D²/8)(θ − sin θ) P = D × θ / 2 R = A / P

Orifice Flow Equation:

Q = Cd × A × √(2 × g × h)

#### Assumptions & References

(function(){ // Toggle method panels window.draToggleMethod = function(){ var m = document.getElementById('dra-method').value; document.getElementById('dra-manning-inputs').style.display = m === 'manning' ? '' : 'none'; document.getElementById('dra-orifice-inputs').style.display = m === 'orifice' ? '' : 'none'; document.getElementById('dra-result').style.display = 'none'; };

// Enable/disable custom n document.getElementById('dra-manning-n').addEventListener('change', function(){ document.getElementById('dra-manning-n-custom').disabled = this.value !== 'custom'; }); // Enable/disable custom Cd document.getElementById('dra-cd').addEventListener('change', function(){ document.getElementById('dra-cd-custom').disabled = this.value !== 'custom'; });

function showError(msg){ var r = document.getElementById('dra-result'); r.style.display = 'block'; r.innerHTML = '⚠ ' + msg + ''; }

function fmt(v, dp){ return v.toFixed(dp); }

window.draCalc = function(){ var method = document.getElementById('dra-method').value; var r = document.getElementById('dra-result');

if(method === 'manning'){
var D = parseFloat(document.getElementById('dra-diameter').value);
var S = parseFloat(document.getElementById('dra-slope').value);
var nSel = document.getElementById('dra-manning-n').value;
var n = nSel === 'custom'
? parseFloat(document.getElementById('dra-manning-n-custom').value)
parseFloat(nSel); var fillRatio = parseFloat(document.getElementById('dra-fill-ratio').value);

if(isNaN(D) || D 0.'); if(isNaN(S) || S 0.'); if(isNaN(n) || n 0.'); if(isNaN(fillRatio) || fillRatio 1.0) return showError('Fill ratio must be between 0.01 and 1.0.');

var A, P, R, Q, theta;

if(fillRatio >= 0.9999){ // Full pipe A = Math.PI * D * D / 4; P = Math.PI * D; R = D / 4; } else { // Partial pipe – circular segment theta = 2 * Math.acos(1 - 2 * fillRatio); // radians A = (D * D / 8) * (theta - Math.sin(theta)); P = D * theta / 2; R = A / P; }

Q = (1 / n) * A * Math.pow(R, 2/3) * Math.pow(S, 0.5); var V = Q / A; // mean velocity

r.style.display = 'block'; r.innerHTML = '### Results – Manning's Equation ' + '' + 'Flow Rate Q' + '' + fmt(Q,6) + ' m³/s' + '' + fmt(Q1000,4) + ' L/s' + '' + fmt(Q3600,3) + ' m³/hr' + 'Mean Velocity' + '' + fmt(V,4) + ' m/s' + 'Flow Area A' + '' + fmt(A,6) + ' m²' + 'Hydraulic Radius R' + '' + fmt(R,5) + ' m' + 'Wetted Perimeter P' + '' + fmt(P,5) + ' m' + 'Fill Ratio (y/D)' + '' + fmt(fillRatio*100,1) + '%' + '';

} else {
// Orifice
var Ao = parseFloat(document.getElementById('dra-orifice-area').value);
var h = parseFloat(document.getElementById('dra-head').value);
var cdSel = document.getElementById('dra-cd').value;
var Cd = cdSel === 'custom'
? parseFloat(document.getElementById('dra-cd-custom').value)
parseFloat(cdSel);

if(isNaN(Ao) || Ao 0.'); if(isNaN(h) || h 0.'); if(isNaN(Cd) || Cd 1.0) return showError('Discharge coefficient must be between 0 and 1.');

var g = 9.81; var Qo = Cd * Ao * Math.sqrt(2 * g * h); var Vo = Qo / Ao;

r.style.display = 'block'; r.innerHTML = '### Results – Orifice Flow Equation ' + '' + 'Flow Rate Q' + '' + fmt(Qo,6) + ' m³/s' + '' + fmt(Qo1000,4) + ' L/s' + '' + fmt(Qo3600,3) + ' m³/hr' + 'Flow Velocity' + '' + fmt(Vo,4) + ' m/s' + 'Orifice Area' + '' + fmt(Ao,6) + ' m²' + 'Water Head' + '' + fmt(h,3) + ' m' + 'Discharge Coeff. Cd' + '' + fmt(Cd,3) + '' + ''; } }; })();

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References

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