Pipe volume and flow rate calculator

A pipe is a cylinder, so its volume is the inside area times the length:

Volume (cu in) = π ÷ 4 × inside diameter² × length in inches

Gallons = cubic inches ÷ 231

One U.S. gallon is 231 cubic inches by definition. Water weighs about 8.34 lb per gallon, so the same math tells you how much weight a run of full pipe adds to a hanger or a joist.

The step people get wrong is the diameter. A "3/4 inch" pipe is not 0.75 in inside. Copper and PEX are copper tube size, with an outside diameter 1/8 in larger than the nominal size, and the wall comes off that. PVC is iron pipe size, a different set of diameters again. Using the nominal size instead of the real inside diameter is off by 5% to 30% on common copper and PEX sizes.

These are the inside diameters the calculator uses, in inches. Copper is the ASTM B88 outside diameter minus two walls for each type. PEX is the ASTM F876 SDR-9 outside diameter minus two minimum walls; real walls run a little thicker, so true PEX volume is slightly lower. PVC is the Schedule 40 average inside diameter.

Type K has the thickest wall and the smallest bore, Type M the thinnest wall and the largest. PEX has the smallest bore of all at the same nominal size, because its wall is thick for its pressure rating. That is why a PEX system often needs to step up a size to carry the same flow as copper.

Worked from the inside diameters above:

Where this comes up on the job:

• Hot water wait. A 50 ft run of 3/4 in Type L holds 1.26 gallons. At a 1.5 GPM faucet, that is about 50 seconds of cold water before the hot arrives. In 1/2 in it is 0.6 gallons, about 24 seconds. Volume is the case for smaller branch lines, a home run manifold, or a recirculation pump.
• Hydronic systems. Add the pipe volume to the boiler and emitters to size an expansion tank and work out how much glycol to buy.
• Flushing and disinfection. Know how many gallons to push through a new line, or how much disinfectant a section needs.
• Draining and filling. How much water comes out when you open a line, and how long it takes to refill.

Flow is area times velocity. In plumbing units:

GPM = 2.448 × velocity (ft/s) × inside diameter² (in)

The 2.448 rolls up the unit changes: square inches to square feet (÷ 144), seconds to minutes (× 60) and cubic feet to gallons (× 7.48).

A common rule of thumb for copper is to keep water at or under about 8 ft/s for cold lines and about 5 ft/s for hot. Faster water is noisy, causes water hammer when valves close, and wears the inside of fittings, especially on hot lines. When a run needs more flow than its size can carry at those speeds, step up a size. Code sizing methods (fixture units and pressure loss) set the final pipe size; velocity is the sanity check.

Hydronic heating is where pipe volume matters most. The system volume sets the expansion tank size and how much glycol to buy, and both are worked from the total gallons in the boiler, the emitters and every foot of pipe.

Example. A radiant floor with 1,500 ft of 1/2 in PEX, 80 ft of 1 in Type L copper supply and return mains, and a boiler that holds 3 gallons:

For a glycol mix, multiply the system gallons by the glycol share the maker's chart calls for at your lowest expected temperature. At a 30% mix, this system needs about 6.2 gallons of glycol and 14.6 gallons of water. Pre-mix or measure carefully: too little glycol risks a freeze, too much costs heat transfer and pump power.

Expansion tank makers size tanks from the system volume, the fill pressure, the relief valve setting and the hottest water temperature. Their sizing tables start from the gallon figure this calculator gives you.

Full pipe is heavier than it looks, and the water often weighs more than the pipe. At 8.34 lb per gallon:

Add the weight of the pipe itself from the maker's data and space hangers to code and to the pipe maker's guidance. Large mains, storage loops and long horizontal runs of plastic pipe, which sags when warm, need the closest support.

Weight also matters when you drain down. A 2 in main that is 100 ft long dumps about 16 gallons when opened, so have the hose or the bucket count ready.

The simplest flow test needs a bucket of known size and a stopwatch. Open the fixture or hose bib all the way, start timing when the stream hits the bucket, and stop at the fill line.

GPM = gallons ÷ seconds × 60

Do it twice and average. Test at the hose bib nearest the meter to see what the service delivers, then at a problem fixture to see what is being lost along the way. The calculator also shows how fast that flow moves through each size of copper and PEX, which tells you whether a line is undersized for what it is being asked to carry.

For a well, a bucket test at the pressure tank tells you whether the pump keeps up. For a tankless water heater, it tells you whether the flow will reach the heater's minimum and how many fixtures it can serve at once.

When the numbers point to a repipe, a recirculation pump or a bigger service line, put it in a Koira estimate with the pipe, fittings and labor as separate lines. The plumbing cost estimator helps price the job.