Pipe Size Calculator
Calculate required diameter based on flow rate and pressure drop
L/min
GPM
m³/h
Meters
Feet
Bar
PSI
kPa
PVC / Plastic (C=150)
Copper (C=140)
Steel – New (C=120)
Cast Iron (C=100)
Galvanized Iron (C=110)
Please enter valid positive numbers for all fields. Pressure drop cannot be zero.
Calculation Results
Minimum Internal Diameter:
— mm
Minimum Internal Diameter (Imperial):
— inches
Fluid Velocity (at calculated size):
— m/s
Note: Select the next standard commercial pipe size larger than the minimum calculated above.
How to Calculate Pipe Size from Flow Rate and Pressure
Calculating the correct pipe size is a fundamental task in fluid dynamics and engineering. Whether you are designing an irrigation system, plumbing for a residential building, or an industrial process piping network, selecting the right diameter is crucial for efficiency and safety. Undersized pipes lead to excessive pressure loss and high energy costs, while oversized pipes result in unnecessary material expenses and potential sedimentation issues.
To calculate pipe size from flow rate and available pressure, engineers typically rely on friction loss formulas. The most common empirical formula for water flow is the Hazen-Williams equation. For more viscous fluids or high-precision requirements, the Darcy-Weisbach equation is preferred. The calculator above utilizes the Hazen-Williams method, which is standard for water distribution systems.
The Relationship: Flow, Diameter, and Pressure
The core physics of pipe sizing involves balancing three variables:
- Flow Rate (Q): The volume of fluid moving through the pipe per unit of time (e.g., Liters per minute or Gallons per minute).
- Pipe Diameter (d): The internal width of the pipe. A small change in diameter has a massive impact on pressure drop because resistance is inversely proportional to the diameter raised to the roughly 5th power.
- Pressure Drop (P or hf): The loss of energy due to friction as fluid moves along the pipe length.
The Calculation Formula
Using the Hazen-Williams equation, we can rearrange the terms to solve for the Internal Diameter ($d$) when the Flow Rate ($Q$), Pipe Length ($L$), Roughness Coefficient ($C$), and Allowable Pressure Drop ($P$) are known.
$$ d = \left( \frac{4.52 \times L \times Q^{1.85}}{C^{1.85} \times P} \right)^{0.2055} $$
Where:
- d: Pipe Internal Diameter (inches)
- L: Length of pipe (feet)
- Q: Flow rate (GPM)
- P: Pressure drop (PSI)
- C: Hazen-Williams Roughness Coefficient (e.g., 150 for PVC, 120 for new steel)
Step-by-Step Calculation Guide
If you are performing this calculation manually without the tool above, follow these steps:
- Determine your Flow Rate: Identify how much water needs to reach the destination (e.g., 100 liters per minute).
- Measure the Distance: Calculate the total length of the piping run. Add equivalent lengths for fittings (elbows, valves) if precision is required.
- Set Allowable Pressure Drop: Determine how much pressure you can afford to lose. For example, if your source is 4 Bar and your equipment needs 3 Bar, your allowable drop is 1 Bar.
- Choose Material: Select your pipe material to determine the 'C' factor. Smoother pipes like PVC allow for smaller diameters than rougher pipes like concrete or old iron.
- Apply the Formula: Input these values into the equation to find the minimum internal diameter.
- Select Standard Size: The calculated diameter will rarely match a standard pipe size exactly. Always round up to the next commercially available nominal pipe size to ensure the pressure drop remains within your limit.
Why Velocity Matters
While pressure drop is critical, you must also check the fluid velocity. If the calculated pipe size results in a velocity higher than 2.5 to 3 meters per second (approx 8-10 ft/s), you risk water hammer, noise, and accelerated pipe erosion. Conversely, velocities below 0.6 m/s may allow sediment to settle. Our calculator provides a velocity check to help you ensure your system remains within safe operating limits.