Enter pipe specs and supply pressure to get instant flow results
Pipe dimensions
Inner Diameter
Pipe Length
Supply pressure
Supply Pressure
Flow Units
At the far end optional
Pressure Needed at Outlet
psi
Height Rise to Outlet
Pipe Material (Hazen-Williams C coefficient)
⚠ Please enter valid values (greater than zero) for diameter, length, and pressure.
WaterTankCalculator.com — Pipe Flow Rate Results
Your Results
Flow Rate
—
US Gallons per Minute (GPM)
GPM
— gal/min
LPM
— L/min
m³/hour
— m³/h
Velocity
— m/s
Velocity (ft/s)
— ft/s
Friction Loss
— psi
Flow Velocity Zone
SlowOptimalFast
📖 How To Use
How to Use This Pipe Size Flow Rate Calculator
Calculate how much water your pipe can deliver in under 30 seconds:
Enter the pipe inner diameter
Use the internal (bore) diameter — not the outer diameter. Choose your unit (inches, mm, cm, or ft). For standard schedule 40 PVC, ½" pipe has an inner diameter of roughly 0.622 in; 1" pipe is about 1.049 in.
Enter the pipe length
Measure the total run from the supply to the outlet in feet or metres. Bends, valves and fittings add friction too — add their equivalent length (from the fitting maker's tables), or as a rough allowance, 10–20% to the length.
Enter your supply pressure
Enter the pressure at the start of the pipe in psi, bar, kPa or metres of water column (mWC) — read it with a gauge on a nearby tap while no water is running. For a gravity tank, enter the height of the water above the outlet in mWC.
Say what happens at the far end (optional)
Enter the pressure you need left at the outlet (for a sprinkler, valve or fixture) and how far the outlet sits above the supply. Both use up pressure, leaving less to push water through the pipe. Leave them at zero for an open pipe discharging at the same level.
Select pipe material
Material sets the Hazen-Williams C coefficient — a smoothness factor (higher is smoother). Typical design values: PVC/HDPE 150, copper 140, new cast iron 130, galvanised steel 120, old steel 100. Pick the closest match.
Hit Calculate
You get the flow the pipe can carry in GPM, LPM and m³/h, the velocity (m/s and ft/s), and the friction loss along the run. The note flags velocities outside the normal range.
Tip: For best accuracy, always use the pipe's internal diameter. Wall thickness on ½" schedule 40 PVC is about 0.109 in — using the outer diameter (0.840 in) instead of the inner diameter (0.622 in) would more than double the calculated flow (about +120%, because flow scales with diameter to the power 2.63).
📐 The Formula
Pipe Flow Rate Formula — Hazen-Williams
This calculator uses the Hazen-Williams equation, widely used for water in full pipes under turbulent flow at ordinary temperatures. In SI units:
Q = 0.2785 × C × D2.63 × S0.54
Q = flow rate (m³/s)
C = Hazen-Williams coefficient
D = pipe inner diameter (m)
S = friction head loss ÷ pipe length (m/m)
Friction head = supply head − head needed at the outlet − height rise
In US units: Q (GPM) = 0.2813 × C × d (in)2.63 × S0.54; 1 psi = 2.31 ft of water
Velocity is derived from the continuity equation:
v = Q / A
v = flow velocity (m/s)
Q = volumetric flow rate (m³/s)
A = cross-sectional area = π × (d/2)²
Hazen-Williams C Coefficients by Material
Pipe Material
C Value
Condition
PVC / HDPE
150
New or smooth plastic
Copper
140
New copper tubing
Galvanised Steel
120
Typical design value
Cast Iron
130
New, unlined; falls with age
Aged Steel / Iron
100
10–20 years old
Badly Corroded
60–80
Heavy tuberculation
Recommended Flow Velocities
Application
Min Velocity
Optimal Range
Max Velocity
Residential supply
0.6 m/s
0.9 – 1.5 m/s
2.4 m/s
Irrigation mains
0.5 m/s
0.9 – 1.8 m/s
3.0 m/s
Fire mains
0.6 m/s
1.5 – 3.0 m/s
4.5 m/s
Industrial process
0.9 m/s
1.5 – 3.0 m/s
4.5 m/s
These are common design ranges, not code limits. Very low velocities can let sediment settle; high velocities cause noise, erosion and a bigger risk of water hammer. Copper tube makers, for example, commonly recommend no more than about 8 ft/s (2.4 m/s) in cold-water lines. Your local plumbing code governs.
⚙️ Use Cases
When to Use a Pipe Size Flow Rate Calculator
🏠
Home Plumbing Design
Size supply lines for bathrooms, kitchens, and outdoor taps to ensure adequate pressure at fixtures.
🌿
Irrigation Systems
Confirm your header pipe delivers enough flow to run all drip emitters or sprinklers simultaneously.
🏗️
Tank Fill & Drainage
Find how much a pipe can deliver at a known pressure, then use the refill or drainage time calculators to see how long a tank takes.
🔥
Fire Suppression
Get a first estimate for supply lines. Sprinkler systems are designed to fire codes such as NFPA 13 by qualified designers — use this only as a rough check.
🏭
Industrial Process
Size coolant or process water lines to meet flow demands without exceeding velocity limits.
📉
Troubleshooting Low Flow
Identify if low pressure at a fixture is caused by pipe diameter restriction or actual supply pressure loss.
🧮 Worked Example
Worked example: a gravity line from a hill tank
A tank sits 15 m above a garden outlet at the end of 200 m of 2-inch schedule 40 PVC (inner diameter 2.067 in, C = 150). The outlet discharges into a trough, so no pressure is needed there. Enter the 15 m as the supply pressure (15 mWC), 200 m as the length, and leave the outlet fields at zero.
Velocity: about 2.05 m/s (6.7 ft/s) — inside the usual range, toward its upper end. Friction uses up all 15 m of head (21.3 psi) along the run.
With 1-inch pipe (inner diameter 1.049 in) on the same run, the flow drops to about 45 L/min (11.8 GPM) — roughly one-sixth, because flow scales with diameter to the power 2.63.
❓ FAQ
Frequently Asked Questions
How do I calculate flow rate from pipe size and pressure?
Use the Hazen-Williams equation. In US units: Q (GPM) = 0.2813 × C × d2.63 × S0.54, where d is the inner diameter in inches, C is the pipe material coefficient, and S is the friction head loss divided by the pipe length (1 psi = 2.31 ft of water). The friction head is what is left of the supply pressure after the pressure needed at the outlet and any height rise. This calculator does the maths and checks the velocity.
What pipe size do I need for a specific flow rate in GPM?
Size it so the velocity stays in a normal range — about 5 ft/s (1.5 m/s) is a common design target for cold water. At that velocity, schedule 40 PVC carries roughly 4.7 GPM in ½" pipe (0.622 in bore), 8.3 GPM in ¾" (0.824 in), 13.5 GPM in 1" (1.049 in) and 31.7 GPM in 1½" (1.610 in). Then check with this calculator that your supply pressure can push that flow along your pipe length.
What is the difference between GPM and LPM?
GPM (US gallons per minute) and LPM (litres per minute) are both volumetric flow rate units. 1 GPM = 3.785 LPM. GPM is common in North America; LPM is standard in Europe, Australia, and most of Asia. This calculator displays both simultaneously.
What is a good pipe flow velocity for residential water supply?
For residential water supply, the ideal velocity is 0.9 to 1.5 m/s (about 3 to 5 ft/s). Below 0.6 m/s, sediment can accumulate. Above 2.4 m/s (8 ft/s), you risk noise, water hammer, and accelerated pipe erosion. The velocity indicator in this calculator shows you whether your result falls in the safe range.
Does pipe material affect flow rate?
Yes — the pipe wall roughness affects friction losses. PVC and HDPE are the smoothest (C = 150) and therefore allow the highest flow for a given pressure. Older or corroded steel pipes have much higher roughness (C = 60–100), reducing flow significantly. Because flow is proportional to C, a PVC pipe (C = 150) carries about 50% more than an old steel pipe (C = 100) of the same bore under the same conditions.
How does pipe length affect flow rate?
Longer pipes create more friction, which reduces flow rate. In the Hazen-Williams equation flow varies with length to the power −0.54, so doubling the pipe length cuts the flow by about 31%, not 50%. For long runs, you typically need to increase pipe diameter or supply pressure to maintain target flow rates at the end of the line.
What is pressure drop and why does it matter?
Pressure drop is the loss of static pressure as water flows through a pipe due to friction with the pipe walls. It matters because your outlet pressure equals supply pressure minus pressure drop — if the drop is too large, fixtures at the end of the run won't have enough pressure to operate properly. Each fixture, valve or sprinkler has a minimum working pressure set by its maker and local code — enter that as "pressure needed at outlet" so the calculator leaves it in reserve.
Can I use this calculator for gas pipes?
No — the Hazen-Williams equation applies only to incompressible fluids like water. Gas flow calculations require different equations (AGA or Weymouth formula for natural gas) and must account for compressibility. For gas pipe sizing, consult a licensed gas engineer and use dedicated gas flow calculators.
✅ Sources & Review
Sources and review
The Hazen-Williams equation is a standard empirical method for water pipes; the figures on this page come from these references:
Hazen-Williams equation in SI form, Q = 0.2785 C D2.63 S0.54, as given in standard water-supply hydraulics texts; the US form with 0.2813 is the same equation in gallons per minute and inches.
Typical C values (PVC 150, copper 140, new cast iron 130, galvanised steel 120, old steel 100) are common published design values; older or corroded pipes can be much lower.
Schedule 40 PVC inner diameters per ASTM D1785 dimensions (½" 0.622 in, ¾" 0.824 in, 1" 1.049 in, 1½" 1.610 in, 2" 2.067 in).
NIST SP 811 — unit conversions (1 psi = 0.703 m = 2.31 ft of water).
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
