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Hydronic pipe size calculator
A hydronic circuit is sized from its flow. The load and the temperature drop give the GPM, and the GPM gives the smallest copper tube whose water speed stays at or under your limit. Caleffi's example is 80,000 BTU/h at a 20°F drop, which is 8 GPM. In Type M copper that is a 1 inch tube at 2.93 ft/s with a 4 ft/s limit, losing 3.55 ft of head per 100 ft, and the 3/4 inch tube is over the limit at 4.96 ft/s. The page works in water only. To size the boiler that feeds the loop, use the boiler size calculator. For the gas line to that boiler, use the gas pipe sizing calculator.
- Take the circuit load in BTU/h from the heat loss or the emitter rating, and choose the design temperature drop. Caleffi calls 20°F the traditional value.
- Choose copper Type L or Type M and the highest velocity you accept. Caleffi quotes 4 ft/s for tube 2 inch and smaller.
- The result gives the GPM, the smallest tube at or under the limit, its velocity and its head loss per 100 ft, with the neighboring sizes and the load each carries. Add the equivalent length to get the head loss of the whole run.
Example result for the default inputs
80,000 BTU/h at 20°F drop, copper Type M, velocity at or under 4 ft/s.
Flow
8.00 GPM
Smallest tube
1 inch Type M
Velocity
2.93 ft/s
Head loss per 100 ft
3.55 ft of head
| Load | 80,000 BTU/h |
|---|---|
| Temperature drop | 20°F |
| Velocity limit | 4 ft/s |
| Flow | 8.00 GPM |
| Inside diameter | 1.055 inch |
| Friction | 0.0154 psi per ft |
| Flow at the limit | 10.91 GPM |
| Load at the limit | 109,100 BTU/h |
| 3/4 inch Type M | 4.96 ft/s, 12.77 ft of head per 100 ft, up to 64,400 BTU/h, over the limit |
| 1 inch Type M | 2.93 ft/s, 3.55 ft of head per 100 ft, up to 109,100 BTU/h |
| 1-1/4 inch Type M | 1.96 ft/s, 1.33 ft of head per 100 ft, up to 163,400 BTU/h |
| 1-1/2 inch Type M | 1.40 ft/s, 0.59 ft of head per 100 ft, up to 228,600 BTU/h |
- Water only. The 500 in the flow formula, the density and the friction all change with antifreeze, and Caleffi says its Formula 3-1 with the fluid's own density and specific heat should be used for a glycol solution.
- The 500 is rounded from 8.33 x 60 and fits water near 60°F. On hotter water the flow it gives is a little low, and the difference grows with the temperature.
- Friction is the Hazen-Williams figure for cold water that the Copper Tube Handbook table uses. Hot water is thinner, so the head loss in a heating loop is somewhat lower and this figure errs high.
- Head loss is for straight tube. Elbows, tees and valves add equivalent length, which the handbook gives in its Table 14.7, and the heat emitters and boiler add their own loss.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the tube size is calculated
A hydronic circuit carries heat in moving water, so the load and the temperature drop set the flow, and the flow sets the tube. This page turns a load in BTU/h and a design drop into GPM, picks the smallest copper tube whose velocity stays at or under a limit you set, and shows the velocity and head loss of that tube and its neighbors. Every size also lists the load it carries at the limit, so a tube you already have can be read the other way. The page covers water in Type L and Type M copper. Antifreeze changes the flow formula and the friction, so it is out of scope.
- Flow
GPM = load / (500 x temperature drop)The 500 is the 8.33 pounds in a gallon of water times 60 minutes, with a specific heat of 1. Caleffi's example is 80,000 BTU/h at 8 GPM and a 20°F drop, and the same load at 10°F is 16 GPM. It fits cold water best, near 60°F, so hotter water needs a little more flow than the formula gives.- Velocity
ft/s = 0.408 x GPM / (inside diameter in inches)^2Caleffi's Formula 3-3. The inside diameter is the exact bore from the Copper Tube Handbook, not the nominal size. At 8.00 GPM a 1 inch Type M tube, bore 1.055 inch, runs at 2.93 ft/s.- Friction
psi per ft = 4.52 x GPM^1.85 / (150^1.85 x inside diameter^4.87)This is the Hazen-Williams formula that the Copper Tube Handbook says its Table 14.6 is based on, with C = 150. Tests reproduce the printed values for Type L and Type M from 1/2 inch to 3 inch. Caleffi's 3/4 inch Type L case at 6 GPM gives 0.0381 psi per foot and 5.72 psi over 150 ft, as printed.- Head loss
ft of head per 100 ft = psi per ft x 100 x 144 / 62.4Caleffi turns head into pressure with the density of the water, 62.4 lb/ft3 at 50°F, so 1 psi is 2.31 ft of head. The 1 inch tube above loses 3.55 ft of head per 100 ft.- Smallest tube
first size, from 1/2 inch up, with velocity at or under the limitSizes are compared unrounded. At 8.00 GPM and a 4 ft/s limit the 3/4 inch tube runs at 4.96 ft/s and fails, and the 1 inch tube passes at 2.93. Caleffi quotes an ASHRAE guideline of 4 ft/s for tube 2 inch and smaller.- Load a tube carries
load = limit x (inside diameter)^2 / 0.408 x 500 x temperature dropThese are the same formulas run the other way. A 3/4 inch Type M tube carries up to 64,400 BTU/h at 20°F and 4 ft/s, and a 1 inch tube carries up to 109,100. Every size in the result shows its own figure.
Worked examples from published hydronic data
The default case uses Caleffi's own arithmetic, 500 x 8 GPM x 20°F = 80,000 BTU/h. At 4 ft/s the 3/4 inch Type M tube is over the limit at 4.96 ft/s and 12.77 ft of head per 100 ft, and the 1 inch tube runs at 2.93 ft/s and 3.55 ft of head. In Type L the same load still takes a 1 inch tube, at 3.11 ft/s and 4.08 ft of head. At a 10°F drop the flow doubles to 16 GPM and the tube grows to 1-1/4 inch.
The table below sets 8 printed values from Table 14.6 of the Copper Tube Handbook next to the friction formula on this page. All 8 agree to the third decimal, which is how the formula was checked. The handbook stops each column at 8 ft/s, so every row is under that.
| Tube | GPM | Velocity, ft/s | Printed, psi per ft | Formula, psi per ft | Agrees |
|---|---|---|---|---|---|
| 1/2 inch Type L | 5 | 6.87 | 0.161 | 0.161 | Yes |
| 1/2 inch Type M | 5 | 6.30 | 0.130 | 0.130 | Yes |
| 3/4 inch Type L | 10 | 6.62 | 0.098 | 0.098 | Yes |
| 3/4 inch Type M | 10 | 6.20 | 0.084 | 0.084 | Yes |
| 1 inch Type L | 20 | 7.77 | 0.096 | 0.096 | Yes |
| 1-1/4 inch Type M | 30 | 7.34 | 0.066 | 0.066 | Yes |
| 2 inch Type L | 70 | 7.25 | 0.039 | 0.039 | Yes |
| 2-1/2 inch Type L | 100 | 6.71 | 0.026 | 0.026 | Yes |
Eight cases below run through the same functions the form uses. Four of them share the 80,000 BTU/h load and change one input at a time. A boiler loop and two mains show how the tube grows with the flow. The 12,000 BTU/h zone lands on the 1/2 inch tube at 1.51 ft/s, under the 2 ft/s that Caleffi gives for carrying air to the separator, and the page says so.
| Case | Setup | Result |
|---|---|---|
| Caleffi example load, Type M | 80,000 BTU/h, 20°F drop, Type M, 4 ft/s limit | 8.00 GPM, 1 inch tube, 2.93 ft/s, 3.55 ft of head per 100 ft |
| Same load at a 10°F drop | 80,000 BTU/h, 10°F drop, Type M, 4 ft/s limit | 16.00 GPM, 1-1/4 inch tube, 3.92 ft/s, 4.79 ft of head per 100 ft |
| Same load in Type L | 80,000 BTU/h, 20°F drop, Type L, 4 ft/s limit | 8.00 GPM, 1 inch tube, 3.11 ft/s, 4.08 ft of head per 100 ft |
| Same load with a 5 ft/s limit | 80,000 BTU/h, 20°F drop, Type M, 5 ft/s limit | 8.00 GPM, 3/4 inch tube, 4.96 ft/s, 12.77 ft of head per 100 ft |
| Boiler loop, 150,000 BTU/h | 150,000 BTU/h, 20°F drop, Type M, 4 ft/s limit | 15.00 GPM, 1-1/4 inch tube, 3.67 ft/s, 4.25 ft of head per 100 ft |
| Single small zone, 12,000 BTU/h | 12,000 BTU/h, 20°F drop, Type M, 4 ft/s limit | 1.20 GPM, 1/2 inch tube, 1.51 ft/s, 2.15 ft of head per 100 ft |
| Light commercial main, 600,000 BTU/h | 600,000 BTU/h, 20°F drop, Type M, 4 ft/s limit | 60.00 GPM, 2-1/2 inch tube, 3.93 ft/s, 2.23 ft of head per 100 ft |
| Large main, 1,200,000 BTU/h | 1,200,000 BTU/h, 20°F drop, Type M, 4 ft/s limit | 120.00 GPM, 4 inch tube, 3.16 ft/s, 0.87 ft of head per 100 ft |
Standards and data sources
- Copper Development Association, Copper Tube Handbook (CDA Publication A4015-26/24) - Tables 14.2b and 14.2c for the Type L and Type M dimensions, the water velocity limits of 8 ft/s for cold water and 5 ft/s for hot water up to about 140°F with 2 to 3 ft/s where the water routinely runs hotter (page 15), and Table 14.6 for friction loss, which the handbook says is based on the Hazen-Williams formula with C = 150. Tests reproduce its printed values for Type L and Type M tube.
- Caleffi, idronics issue 16, Circulation in hydronic systems - Formula 3-2 for the flow, with the 500 rounded from 8.33 x 60 and valid for cold water, and Formula 3-3 for velocity. It quotes an ASHRAE guideline of 4 ft/s for tube 2 inch and smaller and 4 ft of head per 100 ft above that, a minimum of 2 ft/s to carry air to the separator, and the 12.7 and 2.6 ft per 100 ft examples in 120°F water.
- Caleffi, idronics issue 31, Appendix B, Pressure drop for copper tubing - The Hazen-Williams formula for copper tube in psi per foot, and the example of 150 ft of 3/4 inch Type L at 6 GPM that gives 0.0381 psi per foot and 5.72 psi. Tests reproduce it.
- Caleffi, idronics issue 23 - Erosion corrosion in smaller copper tube is avoided by keeping the flow velocity at or under 5 ft/s, which agrees with the handbook's figure for hot water.
Limitations of this calculator
- Water only. Antifreeze changes the 500 in the flow formula, the density and the friction, and Caleffi says its fuller Formula 3-1, with the fluid's own density and specific heat, should be used for a glycol solution. Glycol sizing needs the fluid maker's tables.
- The friction is for cold water. Table 14.6 in the handbook is a Hazen-Williams table, and a heating loop runs hotter, where water is thinner. In Caleffi's 120°F examples this page reads 4 percent high for a 1/2 inch tube and 14 percent high for a 2 inch tube. Treat the head loss as a ceiling for a hot loop.
- The 500 fits water near 60°F. Caleffi's worked case at 101°F differs by about 0.7 percent between the 500 and its fuller formula, and the difference grows with the temperature.
- Copper only. Type L and Type M from 1/2 inch to 6 inch are in the table. PEX, Type K copper, steel and plastic pipe have other bores and other friction and are not included.
- Straight tube only. Elbows, tees, valves, the boiler and the heat emitters add head loss that this page leaves out. The handbook's Table 14.7 gives equivalent lengths for fittings, and the optional length field takes the total equivalent length you work out.
- The velocity limit is yours. Caleffi quotes 4 ft/s for tube 2 inch and smaller, the Copper Tube Handbook gives 5 ft/s for hot water up to about 140°F and 2 to 3 ft/s above that, and 8 ft/s for cold water. The field takes 1 to 8 ft/s.
- Larger tube is sized differently by ASHRAE. For tube above 2 inch, Caleffi quotes a limit of 4 ft of head per 100 ft. This page picks by velocity and warns when a tube above 2 inch is over that head loss.
- The largest tube in the table, 6 inch Type L, carries about 335 GPM at 4 ft/s, which is 3,349,400 BTU/h at 20°F. A larger load is refused. Split it into parallel circuits or size the pipe from the pipe maker's tables.
- One circuit at a time. The page sizes a single pipe for the flow you give. A header or a main that serves several branches takes the sum of their loads, and each branch takes its own drop.
- This is a planning estimate, not a design drawing. A licensed contractor or engineer, the manufacturer's data and the authority having jurisdiction decide the installation.
Common hydronic pipe sizing mistakes
- Using the 500 for antifreeze. The 500 comes from the density and specific heat of water. A glycol mix has other values, and Caleffi says to use its fuller Formula 3-1 for an antifreeze solution. Friction also changes with the mix, so a glycol loop needs the fluid maker's tables for both.
- Rounding the tube down when the velocity is close. At 8.00 GPM the 3/4 inch Type M tube runs at 4.96 ft/s against a 4 ft/s limit, 24 percent over. Its head loss is 12.77 ft per 100 ft, 3.6 times the 3.55 ft of the 1 inch tube. Friction rises faster than velocity, so a tube that is a little over costs a lot of head.
- Taking the drop from the wrong circuit. Each circuit gets its own drop. The same 80,000 BTU/h needs 8.00 GPM at 20°F and 16.00 GPM at 10°F, and the tube grows from 1 inch to 1-1/4 inch. Caleffi gives 10 to 15°F for radiant floor circuits and 25 to 35°F for panel radiators and fan coils.
- Mixing up Type L and Type M. Type L has a thicker wall and a smaller bore. At 8.00 GPM the 1 inch tube runs at 2.93 ft/s in Type M and 3.11 ft/s in Type L, and loses 3.55 and 4.08 ft per 100 ft, 15 percent more. A 3/4 inch tube carries 64,400 BTU/h at the limit in Type M and 60,400 in Type L.
- Leaving fittings out of the length. The head loss per 100 ft is for straight tube. Elbows, tees and valves add equivalent length, and the handbook gives those lengths in Table 14.7. On the 1 inch tube a run of 100 ft of tube and 50 ft of fittings loses 5.32 ft of head, not 3.55.
- Sizing tube above 2 inch by velocity alone. Caleffi quotes ASHRAE sizing tube larger than 2 inch by head loss, at no more than 4 ft per 100 ft. At 100.00 GPM and a limit of 8 ft/s, a 2-1/2 inch Type M tube passes the velocity test at 6.55 ft/s and still loses 5.74 ft per 100 ft. The page warns in that case.
Common questions about hydronic pipe size
How do you find the GPM for a hydronic loop?
Divide the load in BTU/h by 500 times the temperature drop in °F. The 500 is 8.33 pounds per gallon times 60 minutes per hour. Caleffi's example is 80,000 BTU/h at a 20°F drop, which is 8 GPM, and the same load at 10°F is 16 GPM. The factor fits cold water best, so a hot loop needs a little more flow.
What size copper tube carries 80,000 BTU/h?
At a 20°F drop that is 8 GPM. With a 4 ft/s limit the smallest Type M tube is 1 inch at 2.93 ft/s and 3.55 ft of head per 100 ft, and in Type L it is 1 inch at 3.11 ft/s and 4.08 ft of head. The 3/4 inch tube is over the limit at 4.96 ft/s. At a 10°F drop the same load needs 1-1/4 inch Type M.
What velocity limit should I use for hydronic copper tube?
Caleffi quotes an ASHRAE guideline of 4 ft/s for tube 2 inch and smaller, based on flow noise, and says that speed is low enough that erosion of copper is not an issue. The Copper Tube Handbook allows 5 ft/s for hot water up to about 140°F and 2 to 3 ft/s where the water routinely runs hotter. Caleffi also asks for at least 2 ft/s so the water carries air bubbles to the separator. This page opens on 4 ft/s and takes 1 to 8.
How does the page work out head loss?
It uses the Hazen-Williams formula with C = 150, the one the Copper Tube Handbook says its Table 14.6 is based on, with the exact bore of the tube. Tests reproduce that table's printed values for Type L and Type M. Caleffi's 3/4 inch Type L example at 6 GPM, 0.0381 psi per foot, matches too. The result is in feet of head per 100 ft, using 1 psi = 2.31 ft of head.
Is the head loss right for hot water?
It runs high. The formula is for cold water and hot water is thinner. In Caleffi's 120°F examples, which give 12.7 and 2.6 ft per 100 ft at 4 ft/s, this page gives 13.2 and 3.0. Use the figure as a ceiling when picking a circulator.
Can I use it for a glycol loop?
No. The 500 in the flow formula and the friction both assume water. Caleffi says its fuller formula, with the density and specific heat of the fluid, should be used for an antifreeze solution. Get the fluid maker's flow and friction tables for the mix.
How much does a given copper tube carry?
Each size in the result lists the load it carries at your velocity limit and drop. At a 20°F drop and 4 ft/s, a 3/4 inch Type M tube carries up to 64,400 BTU/h and a 1 inch tube up to 109,100. In Type L the 3/4 inch figure is 60,400.
Does Type L or Type M change the answer?
Slightly. Type L has a thicker wall and a smaller bore, so the same flow runs faster and loses more head. At 8.00 GPM the 1 inch tube runs at 2.93 ft/s in Type M and 3.11 in Type L, and both pass a 4 ft/s limit. Near a size boundary the type can move the answer up a size.
What about tube larger than 2 inch?
Caleffi quotes ASHRAE sizing it by head loss, at no more than 4 ft per 100 ft, and not by velocity. This page picks by velocity and flags any tube above 2 inch that is over that head loss.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
Size the tube, then check the loop
The tube size here is the smallest that meets your velocity limit. Add the equivalent length of the fittings to get the head loss of the run, then add the boiler, the heat emitters and any valves before choosing a circulator. Boiler output comes from the boiler size calculator, and the gas line to it from the gas pipe sizing calculator. You can also ask the assistant to go over a piping layout.
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