Flow Rate Calculator

1. Pick the question: velocity in a pipe, checked against a limit, or smallest inside diameter for a target flow.
2. For a check, enter the flow in gallons per minute and the inside diameter in inches.
3. For a target, enter the target flow the pipe must carry.
4. Choose the material whose limit applies. Pick custom if your manufacturer or code gives another number.
5. Read the speed against the limit, or the minimum inside diameter.

A flow rate calculator for pipes connects three quantities: how much water moves, how big the pipe is inside, and how fast the water travels. The speed is what most design rules limit, because a fast flow can make noise and wear the pipe and its fittings.

The EngineeringToolbox copper table gives three cold and hot limits, and says the limits exist to avoid erosion, corrosion and noise. This calculator does the arithmetic behind those limits. It does not decide which limit your system must meet.

For the flow itself, a gallons per minute calculator turns a timed fill or a pipe size into gallons per minute. This page does not link to it, because that page is not yet published.

The flow must be in cubic feet per second and the area in square feet. The flow in gallons per minute is divided by 448.8 to get cubic feet per second. The inside area of a round pipe is 0.7854 times its diameter squared, with the diameter in feet.

1. Convert the flow from gallons per minute to cubic feet per second.
2. Find the speed by dividing the flow by the inside area, or find the area needed by dividing the flow by the limit.
3. Compare the speed with the limit for your material, or convert the minimum area to an inside diameter.

Take 100 gpm in a pipe with an inside diameter of 2 inches, cold copper, limit 8 ft/s.

Flow: 100 / 448.8 = 0.223 cfs. Area: 0.7854 x (2 / 12) squared = 0.0218 sq ft. Velocity: 0.223 / 0.0218 = 10.2 ft/s, which is above the 8 ft/s limit.

For the same flow with a limit of 8 ft/s, the minimum inside diameter is sqrt((0.223 / 8) / 0.7854) x 12 = 2.4 inches. Round up to the next standard size, and confirm it against your code.

Inside Diameter

Speed falls with the square of the diameter, so a small change in inside size changes the speed a lot. Use the inside measurement.

Water Temperature

For copper, the hot-water limit drops from 5 ft/s to 2 ft/s above 140 F, according to the EngineeringToolbox table.

Material

PVC Schedule 40 is listed at 5 ft/s. PEX has no limit in the sources read here, so use the manufacturer guide.

Flow Demand

Fixtures open together raise the flow. Plan for the peak flow, not one fixture at a time.

Limits below come from published design tables. They are guides, not a substitute for the code that applies to your project.

Checking an Existing Line

See whether a supply line is running faster than its material limit.

Choosing a Pipe Size

Find the smallest inside diameter that keeps the speed within the limit for a target flow.

Comparing Materials

See how the same flow sits against a copper limit and a PVC limit.

1. Using Nominal Size

The speed depends on the inside diameter, which is smaller than the nominal size.

2. Forgetting the Hot-water Limit

Copper hot water above 140 F has a much lower limit than cold water.

3. Treating a Limit as a Guarantee

A speed under the limit can still be noisy at fixtures or at bends.

4. Skipping the Code

Your adopted plumbing code and a licensed plumber set the requirement for your system.

The arithmetic is exact to the precision shown. The result depends on the flow, the inside diameter and the limit you choose.

Published tables give limits by material: copper cold water up to 8 ft/s, copper hot water below 140 F up to 5 ft/s, and PVC Schedule 40 up to 5 ft/s. Your code sets the limit that applies.

Convert gallons per minute to cubic feet per second by dividing by 448.8, then divide by the inside area. A 100 gpm flow in a 2 inch pipe gives about 10.2 ft/s.

The EngineeringToolbox copper table gives 2 ft/s above 140 F and says the limits exist to avoid erosion, corrosion and noise.

About 2.4 inches inside diameter, from sqrt((0.223 / 8) / 0.7854) x 12. Round up to the next standard size.

No readable primary source for PEX was found. Use the manufacturer guide or select custom with the limit they give.

No. The gallons per minute calculator converts a timed fill or pipe flow to gpm. This tool checks the speed in that pipe against a limit.

No. The limit depends on the material and service. A larger inside diameter lowers the speed for the same flow, which is how a pipe is brought under the limit.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere.

• EngineeringToolbox, Copper Tubes – Maximum Water Velocities (cold 8 ft/s; hot below 140 F 5 ft/s; hot above 140 F 2 ft/s; reasons given).
• EngineeringToolbox, PVC Pipes Schedule 40 – Friction Loss vs. Water Flow (“velocities should not exceed 5 feet per second”; engineering reference site).
• EngineeringToolbox, Water Systems – Maximum Flow Velocities (general service 3 to 8 ft/s; low-noise tap water 1.6 to 2.3 ft/s).
• Workforce LibreTexts, Water 131: Advanced Water Mathematics (Alvord), 8.3 Flow Rate (Q = A x V; 448.8; typical 2 to 7 ft/s).