Water Velocity and Pipe Size: The 5 and 8 ft/s Limits

Velocity from gpm and inside diameter

Water moving through a full pipe follows one formula:

Velocity (ft/s) = 0.4085 × gpm ÷ ID² (ID in inches)

The 0.4085 comes from 231 cubic inches per gallon and the circle area: one gallon a minute through a 1-inch bore moves 0.4085 feet per second. Because the ID is squared, small changes in bore matter a lot. The same 10 gpm in a 3/4-inch pipe is 9.07 ft/s in PEX (ID 0.671 in) but 6.63 ft/s in Type L copper (ID 0.785 in).

Inside diameters come from the Copper Development Association's Copper Tube Handbook (Table 14.2) and Table 7.11 of the Plastics Pipe Institute's PEX design guide. Our gallons in a pipe guide explains why nominal size can't stand in for ID.

What the published limits say

The documents don't agree on one number, and none of them is a law by itself.

• Copper, cold water: CDA says the designer should not exceed 8 ft/s, to avoid noise and erosion-corrosion.
• Copper, hot water up to about 140 °F: 5 ft/s. Where water routinely runs hotter than 140 °F, CDA says 2 to 3 ft/s.
• Small copper tube: for 1/2-inch and smaller, CDA says to consider lower velocities, because burrs left by poor reaming and many sharp turns create local turbulence. Its general statement is 5 to 8 ft/s, with the lower end for smaller tube.
• Circulating hot water: CDA says supply and return should both stay under the hot-water limit, and warns against an oversized pump or an undersized return.
• PEX: PPI reports that in the tests it cites, no detectable erosion of the pipe surface occurred above 12 ft/s, and that design velocities are generally restricted by factors other than the pipe, such as valves and fittings.

Read that last one carefully. It says the tubing is not the weak point. It does not say 12 ft/s is a good design velocity for a house. The brass fittings, valves and fixtures in the run are still there.

The model International Plumbing Code (2018 edition, as shown on UpCodes) says in Section 604.9 that flow velocity shall be controlled to reduce the possibility of water hammer, and that arrestors are required where quick-closing valves are used. It gives no ft/s figure there, so the code text handles velocity only indirectly. Other editions may differ, and adopted editions and amendments vary across the 50 states and D.C. Check what your jurisdiction adopted with your local building department.

Flow that fits under each limit

Rearranged, the same formula gives the most flow a pipe carries at a chosen velocity: gpm = velocity × ID² ÷ 0.4085. Computed from the IDs above. CDA's 5 and 8 ft/s are copper guidelines; the PEX rows use them only for comparison:

A 2.5 gpm shower branch in 1/2-inch Type L runs 3.44 ft/s. Five gpm through the same tube runs 6.88, past the hot-water line, while 3/4-inch Type L takes 7.54 gpm before it reaches 5 ft/s.

These are flow ceilings against a guideline, not a verdict on any pipe. The water flow and velocity calculator does the same math for your IDs and shows a flag against the limit it selects. For PEX it flags 8 ft/s, the common design limit it cites from one state's rule, not a PPI limit.

What a faster pipe costs in pressure

Velocity also drives friction. The Hazen-Williams equation, as CDA prints it, is psi per foot = 4.52 × gpm^1.85 ÷ (C^1.85 × ID^4.87), with C = 150 for copper. For new plastic, EPA's EPANET manual lists C of 140 to 150; we use 150 for PEX. EPA also notes that the equation applies to water only and was originally developed for turbulent flow, and that C can change considerably as a pipe ages. Treat the result as an estimate for clean water.

Take 10 gpm through a 100-foot run, starting at 60 psi, with the outlet 20 feet above the start. Lifting water costs 0.433 psi per foot (CDA uses 0.434), or 8.66 psi. Friction is on top of that, and fittings, meters and valves add more:

The 3/4-inch PEX line loses 2.15 times what 3/4-inch Type L loses, the same flow in a smaller bore. Stepping 3/4-inch PEX up to 1-inch PEX cuts the loss from 21.05 to 6.22 psi, and the pipe is under 8 ft/s as well. If a fixture needs a minimum pressure, the extra size is worth pricing against a callback for low flow.

CDA's friction table is for copper only. It says other materials need extra allowances for corrosion, scaling and caking, so an aged line can be rougher than the C = 150 above. Moving from C = 150 to 120 multiplies the friction loss by 1.51.

Using it on a job

1. Write down the flow you must deliver at the farthest fixture, from the manufacturer's data or your code's method, not from a guess.
2. Pick the pipe and look up its ID, not its nominal size.
3. Compute velocity and compare it to the limit for the material and for hot or cold water.
4. Compute friction for the full run, adding an allowance for fittings and valves. CDA publishes equivalent lengths for fittings; we don't reproduce its table.
5. Subtract friction, elevation and device losses from the pressure you have. If what's left is below what the fixture needs, go up a size.

Static pressure matters too. The model IPC (Section 604.8, same 2018 text) calls for a pressure-reducing valve where building pressure exceeds 80 psi static, with exceptions. That is a code topic, not a sizing shortcut.

This is general information, not engineering or code advice. Your local plumbing code, as adopted in your state or city, and the building department decide what size a job needs.

Bottom line

Velocity is 0.4085 × gpm ÷ ID². Use CDA's 8 ft/s cold and 5 ft/s hot as guideline limits for copper, remember that PEX tubing tolerates more but valves and fittings may set the limit, and check the pressure left after friction and lift. Run your own flow and pipe through the water flow and velocity calculator, and see the water heater sizing guide for the demand side of the same question.