.flow-calc-container {
max-width: 800px;
margin: 0 auto;
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
background: #f9f9f9;
border: 1px solid #e0e0e0;
border-radius: 8px;
padding: 25px;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.flow-calc-header {
text-align: center;
margin-bottom: 25px;
color: #0056b3;
}
.flow-calc-row {
display: flex;
flex-wrap: wrap;
gap: 20px;
margin-bottom: 20px;
}
.flow-calc-input-group {
flex: 1;
min-width: 250px;
display: flex;
flex-direction: column;
}
.flow-calc-label {
font-weight: 600;
margin-bottom: 8px;
color: #333;
}
.flow-calc-input {
padding: 12px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 16px;
transition: border-color 0.3s;
}
.flow-calc-input:focus {
border-color: #0056b3;
outline: none;
}
.flow-calc-hint {
font-size: 12px;
color: #666;
margin-top: 5px;
}
.flow-calc-btn {
width: 100%;
background-color: #0056b3;
color: white;
padding: 15px;
border: none;
border-radius: 4px;
font-size: 18px;
font-weight: bold;
cursor: pointer;
transition: background-color 0.2s;
}
.flow-calc-btn:hover {
background-color: #004494;
}
.flow-calc-results {
margin-top: 30px;
background: #fff;
border: 1px solid #ddd;
border-radius: 4px;
padding: 20px;
display: none;
}
.result-item {
display: flex;
justify-content: space-between;
align-items: center;
padding: 12px 0;
border-bottom: 1px solid #eee;
}
.result-item:last-child {
border-bottom: none;
}
.result-label {
font-weight: 500;
color: #555;
}
.result-value {
font-weight: 700;
font-size: 18px;
color: #0056b3;
}
.content-section {
max-width: 800px;
margin: 40px auto;
font-family: inherit;
line-height: 1.6;
color: #333;
}
.content-section h2 {
color: #0056b3;
margin-top: 30px;
}
.content-section ul {
margin-bottom: 20px;
}
.content-section p {
margin-bottom: 15px;
}
function calculateFlowRate() {
// 1. Get Input Values
var diameterInput = document.getElementById('pipeDiameter');
var velocityInput = document.getElementById('flowVelocity');
var d_mm = parseFloat(diameterInput.value); // Diameter in mm
var v_ms = parseFloat(velocityInput.value); // Velocity in m/s
// 2. Validate Inputs
if (isNaN(d_mm) || d_mm <= 0) {
alert("Please enter a valid positive number for Pipe Diameter.");
return;
}
if (isNaN(v_ms) || v_ms < 0) {
alert("Please enter a valid positive number for Water Velocity.");
return;
}
// 3. Perform Calculations
// Convert diameter mm to meters
var d_m = d_mm / 1000;
// Calculate Radius in meters
var r_m = d_m / 2;
// Calculate Cross-Sectional Area in Square Meters (A = pi * r^2)
var area_m2 = Math.PI * Math.pow(r_m, 2);
// Area in mm2 for display
var area_mm2 = Math.PI * Math.pow((d_mm / 2), 2);
// Calculate Volumetric Flow Rate in Cubic Meters per Second (Q = A * v)
var q_m3s = area_m2 * v_ms;
// Convert to Target Units
var q_lps = q_m3s * 1000; // Litres per second
var q_lpm = q_lps * 60; // Litres per minute
var q_m3h = q_m3s * 3600; // Cubic metres per hour
// 4. Update UI
document.getElementById('resLpm').innerHTML = q_lpm.toFixed(2) + " l/min";
document.getElementById('resLps').innerHTML = q_lps.toFixed(3) + " l/s";
document.getElementById('resM3h').innerHTML = q_m3h.toFixed(3) + " m³/h";
document.getElementById('resArea').innerHTML = area_mm2.toFixed(1) + " mm²";
// Show results container
document.getElementById('resultsArea').style.display = "block";
}
Flow Rate Calculator UK
Whether you are designing a domestic heating system, sizing pipework for a new bathroom, or calculating pump requirements for an industrial application, understanding flow rate is essential. This Flow Rate Calculator is designed specifically for UK standards, utilizing metric units (millimetres for pipe size and metres per second for velocity) to give you accurate flow capacities in Litres per Minute (l/min), Litres per Second (l/s), and Cubic Metres per Hour (m³/h).
How to Calculate Water Flow Rate in Pipes
The volumetric flow rate ($Q$) of water moving through a pipe is determined by the relationship between the cross-sectional area of the pipe ($A$) and the average velocity of the water ($v$). The fundamental formula used by hydraulic engineers is:
Q = A × v
Where:
- Q is the Flow Rate (m³/s)
- A is the Cross-sectional Area of the pipe (m²)
- v is the Velocity of the fluid (m/s)
Standard UK Pipe Sizes and Flow Rates
In the United Kingdom, copper and plastic pipework comes in standard metric sizes. It is important to note that the calculation requires the internal diameter, whereas pipes are often sold by their external diameter. For accurate results, subtract the wall thickness from the nominal size.
Typical Copper Pipe Internal Diameters (approximate):
- 15mm Pipe: ~13.6mm internal diameter
- 22mm Pipe: ~20.2mm internal diameter
- 28mm Pipe: ~26.2mm internal diameter
Recommended Water Velocities
When designing a system, simply increasing the pressure to get more flow isn't always the solution. High water velocity can cause noise (water hammer), erosion corrosion in copper pipes, and increased pressure loss due to friction.
- General Water Supply: 1.0 to 1.5 m/s
- Hot Water Heating Systems: 0.75 to 1.5 m/s
- Maximum Recommended (Copper): Generally kept below 3.0 m/s to prevent erosion.
Why is Flow Rate Important?
Combi Boilers: Understanding your mains water flow rate (l/min) helps determine if a combi boiler is suitable for your property. Most modern high-performance combis require a dynamic flow rate of at least 10-15 l/min to operate two showers simultaneously.
Pump Sizing: When selecting a central heating pump or a booster pump, you must know the required flow rate to ensure the pump curve matches the system resistance.
Underfloor Heating: Calculating the mass flow rate is critical to ensure the correct amount of heat energy is delivered to the floor manifold.
How to Use This Calculator
- Enter the Internal Diameter of your pipe in millimetres (mm).
- Enter the estimated Water Velocity in metres per second (m/s). If unsure, 1.5 m/s is a standard design velocity for domestic supply.
- Click "Calculate Flow Rate" to see the capacity of that pipe size in various metric units.