Plumbing

Hydronic system volume calculator (gallons)

US gallons from copper, PEX, baseboard, boiler, and glycol mix. Runs in your browser. Nothing is uploaded.

Pipe runs

Volume uses inside diameter, not the nominal size as ID. Generic 3/4″ charts treat 0.75 in as ID; 3/4″ Type L is 0.785 in, 3/4″ PEX is 0.671 in.

Emitters

Finned-tube baseboard (ft)

Baseboard gal/ft

Typical 0.30; override from the manual.

Cast-iron sections

Gallons per section

Typical 1.5; large sections can be ~2.

Panel rads / other emitters (gal)

Plant

Boiler / heat exchanger water volume (gal)

Buffer / hydro-separator (gal)

Underfloor (UFH) loops

Uses the PEX SDR-9 ID table. Gallons = gal/ft × loop length × number of loops.

PEX size

Loop length (ft)

Number of loops

Glycol mixed fill

Mix percent (volume) 0% (water) 30% 50% Custom

Custom mix %

Concentrate and water below are for mixing a fill of this percent, not for pouring concentrate into a system already full of water.

System gallons

—

Water weight (~60°F)—

Concentrate in mixed fill—

Volume breakdown in US gallons
Component	Gallons
Total gallons	—

How the numbers work

Pipe volume is the inside cylinder: cubic inches ÷ 231 in³ per US gallon. Per foot of pipe, gal/ft = π × (ID ÷ 2)² × 12 ÷ 231. Nominal size is not the ID — Type L and Type M copper (ASTM B88) and PEX SDR-9 CTS (ASTM F876) use the table IDs unless you type a measured ID on the row.

Baseboard 0.30 gal/ft and cast-iron 1.5 gal/section are typical takeoff figures, not a survey of every product. Override them from the emitter or boiler manual. Panel rads and other emitters are an explicit gallons field.

Glycol gallons are concentrate in a mixed fill (total × percent ÷ 100) plus the water that makes up the rest. That is not an amount to pour into a full water system.

Not engineering, code, or manufacturer advice. The expansion-tank line is a coarse Extrol-style ballpark for water only; glycol expands more. Size tanks from the manufacturer’s charts.

Compare gal/ft with the Taco water-volume chart (PDF) if you want a manufacturer table. This page does not download that file.

FAQ

Why isn’t 3/4″ pipe 0.75 inches inside?

Nominal size is not the inside diameter. 3/4″ Type L copper is 0.785 in ID; 3/4″ PEX SDR-9 is 0.671 in. That is why 100 ft of 3/4 Type L is about 2.514 gal and the same length of 3/4 PEX is less.

Can I pour the concentrate gallons into a full system?

No. The concentrate figure is for mixing a fill of that percent (concentrate + water = system volume). Pouring that amount into a system already full of water over-concentrates and overfills.

Is the expansion-tank size exact?

No. It is a coarse water-system ballpark (Extrol-style #15 / #30 / #60 bands). Glycol expands more. Size tanks from the manufacturer’s charts.