How is pipe size derived from flow?

The continuity equation Q = V × A relates flow, velocity, and cross-sectional area. For a chosen velocity limit the tool finds the smallest standard pipe whose inside area keeps the actual velocity at or below that limit, which fixes the diameter.

Why does ASHRAE cap mains at about 4 ft/s?

Above roughly 4 ft/s, water flow in occupied-space mains becomes audible and erosion-corrosion accelerates at fittings. Risers and short non-critical runs tolerate higher velocity, up to about 8 to 10 ft/s, because noise matters less there.

How is head loss calculated?

The tool uses the Darcy-Weisbach equation with a Swamee-Jain friction factor and a roughness of 0.00015 ft for commercial steel. The result is feet of head lost per 100 ft of straight pipe at the actual velocity for that size.

Does this include fittings and valves?

No. The head loss shown is for straight pipe only. Add equivalent-length or velocity-head allowances for elbows, tees, valves, and the coil or chiller to get the full circuit head for pump selection.

What flow rate should I use for a given tonnage?

At the standard 10°F chilled-water differential, flow is about 2.4 GPM per ton of cooling. For a custom differential, GPM equals the load in BTU/h divided by 500 times the temperature rise in °F.

Chilled Water Pipe Sizing Calculator

Oversized chilled-water pipe wastes copper or steel and money; undersized pipe is noisy and pumps hard. This calculator applies the continuity equation against ASHRAE velocity limits to pick the smallest Schedule 40 steel pipe that stays quiet, then reports the friction head you will pay for that choice.

How it works

Sizing is driven by velocity, and head loss is computed separately for the chosen size:

Q (cfs)   = GPM × 0.002228
A (ft²)   = Q / V_limit          (continuity Q = V × A)
diameter  = smallest standard ID whose actual V ≤ V_limit
head loss = f × (100/D) × V² / (2g)   (Darcy-Weisbach, per 100 ft)

The friction factor f comes from the Swamee-Jain approximation using a steel roughness of 0.00015 ft, and Reynolds number is based on chilled water near 50 °F. The tool also lists every standard size so you can see the velocity and head trade-off across the range.

Example and notes

A 120 GPM main held to 4 ft/s lands on roughly 3-inch Schedule 40 pipe, running near 3.3 ft/s with about 2 ft of head loss per 100 ft. Bumping the velocity limit to 8 ft/s would allow 2-inch pipe but more than triples the friction head, so the pump and pipe trade against each other. Always add fitting and equipment losses on top of the straight-pipe figure before selecting a circulator.