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Hydronic Pipe Sizing – Heating & Cooling

Build your heating, cooling or brine system in a flow diagram: pipes are sized automatically from power and ΔT, with pressure drop, noise limits, balancing, heat loss and the pump's operating point.
Water · 30…55 °C · 0.94 l/s

System

Heating
Choosing a system type fills in typical fluid, temperatures and limits; everything can be changed afterwards.
Water
–
Freezing point 0 °C · at 42.4 °C: density 991.3 kg/m³, cp 4.179 kJ/(kg·K), kinematic viscosity 0.63 mm²/s

Pipes and sizing

Copper EN 1057
0 = no limit. Typical 100–250 Pa/m for heating and cooling.
Normal – offices, schools, shops
Max velocity by inner diameter: ≤13 mm 0.7 · ≤16 mm 0.8 · ≤20 mm 1.0 · ≤26 mm 1.2 · ≤33 mm 1.4 · ≤42 mm 1.6 · ≤53 mm 1.8 · >53 mm 2.0 m/s. The lower of this and the max velocity applies.
12×1
Added to each section's length for bends, tees and valves when the pressure drop is calculated.

Pump

Pump head = the pressure drop through the heaviest branch (pipes, components and valves) plus the margin. Wire-to-water efficiency: small circulators 25–40 %, larger pumps 55–75 %.

Heat loss and insulation

Mineral wool (pipe section) – λ 0.035
By GModG (Germany, formerly GEG) and free of condensation
The surroundings and the insulation apply to all pipes; a pipe can have its own values (click the pipe). Warm pipes lose heat, cold pipes take up heat and can gather condensation.

Project

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File
Share and print

The file is a link file: double-click it to open the project in your browser, or choose it with Open. Older .json files can still be opened. Like the project file, it contains all project data – share it only with people who may see it.

Units

°C
l/s
kPa
Pa/m
m/s
m
kW
W
l

Flow diagram

100%

Pumps and pump head

Branches and balancing

The branch with the highest pressure drop (●) sets the pump head. The other branches must be throttled by the difference – that is the job of the balancing valve in the branch.

Components

Pipe sizes and pressure drop

Pipe list

Pipe insulation

Show calculation

Calculation per pipe

PipeMass flow [kg/s]Density [kg/m³]Inner diameter [mm]Velocity [m/s]ReFriction factorLength (+ fittings) [m]Δp [kPa]Δp per metre [Pa/m]Note
Main circuit: P1 → Heat source (54×2)0.9364991500.48138,8000.022210 (+3)0.66351
Main circuit: Heat source → Radiator group 1 (supply) (54×2)0.9315985.8500.48147,1000.021310 (+3)0.63248.6
Main circuit: Radiator group 1 (supply) → Radiators 1 (35×1,5)0.4772985.8320.60237,7000.02245 (+1.5)0.813125
Main circuit: Radiators 1 → Valve 1 (35×1,5)0.4794990.3320.60232,0000.02335 (+1.5)0.848130
Main circuit: Valve 1 → Radiator group 1 (return) (35×1,5)0.4794990.3320.60232,0000.02335 (+1.5)0.848130
Main circuit: Radiator group 1 (supply) → Radiator group 2 (supply) (35×1,5)0.4543985.8320.57335,9000.02275 (+1.5)0.745115
Main circuit: Radiator group 2 (supply) → Radiators 2 (35×1,5)0.3579985.8320.45128,2000.02395 (+1.5)0.48975.2
Main circuit: Radiators 2 → Valve 2 (35×1,5)0.3596990.3320.45124,0000.02495 (+1.5)0.5178.5
Main circuit: Valve 2 → Radiator group 2 (return) (35×1,5)0.3596990.3320.45124,0000.02495 (+1.5)0.5178.5
Main circuit: Radiator group 2 (supply) → Underfloor heating (supply) (22×1)0.09635985.8200.31112,2000.02955 (+1.5)0.45770.3
Main circuit: Underfloor heating (supply) → Shunt Underfloor heating (22×1)0.09635985.9200.31112,1000.02955 (+1.5)0.45770.3
Main circuit: Shunt Underfloor heating → Underfloor heating (return) (22×1)0.09732995.8200.3117,7600.03325 (+1.5)0.51979.9
Main circuit: Radiator group 2 (return) → Radiator group 1 (return) (35×1,5)0.4569991.6320.57328,7000.02395 (+1.5)0.789121
Main circuit: Underfloor heating (return) → Radiator group 2 (return) (22×1)0.09732995.8200.3117,7500.03325 (+1.5)0.5279.9
Main circuit: Radiator group 1 (return) → P1 (54×2)0.9364991500.48138,8000.022210 (+3)0.66351
Underfloor circuit: P2 → Loop 1 (supply) (35×1,5)0.4782994.1320.59826,5000.024310 (+3)1.76135
Underfloor circuit: Loop 1 (supply) → Floor loop 1 (35×1,5)0.2869994.1320.35915,9000.02755 (+1.5)0.35855
Underfloor circuit: Floor loop 1 → Valve 3 (35×1,5)0.2874995.7320.35914,3000.02825 (+1.5)0.36756.5
Underfloor circuit: Valve 3 → Loop 1 (return) (35×1,5)0.2874995.8320.35914,3000.02825 (+1.5)0.36756.5
Underfloor circuit: Loop 1 (supply) → Loop 2 (supply) (28×1,2)0.1913994.125.60.37413,2000.02885 (+1.5)0.50878.2
Underfloor circuit: Loop 2 (supply) → Floor loop 2 (28×1,2)0.1913994.125.60.37413,2000.02885 (+1.5)0.50878.2
Underfloor circuit: Floor loop 2 → Valve 4 (28×1,2)0.1916995.825.60.37411,9000.02965 (+1.5)0.52380.4
Underfloor circuit: Valve 4 → Loop 2 (return) (28×1,2)0.1916995.825.60.37411,9000.02965 (+1.5)0.52380.4
Underfloor circuit: Loop 2 (return) → Loop 1 (return) (28×1,2)0.1916995.825.60.37411,9000.02965 (+1.5)0.52380.4
Underfloor circuit: Loop 1 (return) → P2 (35×1,5)0.479995.8320.59823,9000.024910 (+3)1.8139

Basis of the calculation

Method
Each pipe: flow from the heat load and ΔT (or as given), the smallest size within the velocity, noise and pressure drop limits, pressure drop by Darcy–Weisbach at the pipe's own temperature. The branch with the largest pressure drop sets the pump head; balancing valves take the difference in the other branches.
Equations
  • Q = P / (ρ · cp · ΔT)
  • v = Q / (π d² / 4)
  • Re = ρ v d / μ
  • Δp = f · (L_eq / d) · ρ v² / 2
  • L_eq = L · (1 + 30 %)
  • H = Δp_index · (1 + 10 %)
  • T_out = T_amb + (T_in − T_amb) · e^(−U′L / (ṁ cp))
Friction factor
Colebrook–White (turbulent, iterated); 64/Re below Re 2300, blended between 2300 and 4000
Fluid data
IAPWS-95 (water), Melinder / IIR 2010 (secondary coolants), via CoolProp 8.0.0
Standards
EN 1057 (k = 0.0015 mm) · EN ISO 12241 · GModG, Anlage 8
Notes
  • Fittings are counted as an addition to the pipe length (equivalent length). Components (heat exchangers, valves) are listed with their own pressure drop in the component table.

Regulations and standards by country

The calculation is the same everywhere; these are the rules and standards to check where you build. "Checked" = the official text was read on that date. "Check manually" = the source could not be read from here (standard for sale or site not reachable) – open the link and check it yourself.
TopicRule or standardWhat it coversSource checked
System designCIBSE Guide CReference data for pipe pressure drop in heating and chilled water systems.Check manually
InsulationApproved Document L, Vol. 1 & 2, 2021 edition incorporating 2023 amendments (England)Energy efficiency of building services, including insulation of heating and cooling pipes.Checked 2026-10-25
Glycol and environmentThe Water Supply (Water Fittings) Regulations 1999, Schedule 1Ethylene glycol makes the water fluid category 3 (slight health hazard): the filling point from the mains needs backflow protection for that category.Checked 2026-10-25

Hydronic pipe sizing – frequently asked questions

How do I size heating and chilled water pipes?

Draw the circuit in the flow diagram and give each heat source, coil or radiator group its power and ΔT. The flow is power / (ρ·cp·ΔT), and every pipe gets the smallest size that keeps the maximum velocity, the noise limit and the maximum pressure drop per metre. Typical design values for heating and chilled water are 100–250 Pa/m and 0.5–1.5 m/s.

Calculate in the tool above ↑

How is the pressure drop in a water or glycol pipe calculated?

The pressure drop uses Darcy–Weisbach with the Colebrook–White friction factor and the fluid's density and viscosity at the pipe's own temperature. Glycol and other brines are more viscous than water, especially when cold, so they give a higher pressure drop and often need one size bigger.

Calculate in the tool above ↑

How much pump head do I need?

The pump head is the pressure drop through the heaviest branch – pipes, fittings, coils, heat exchangers and valves – plus a margin. Enter two points of the pump curve and the calculator shows the operating point and warns if the pump is too small or too big.

Calculate in the tool above ↑

What glycol concentration does a brine system need?

Choose a concentration whose freezing point is at least 3 K below the lowest temperature in the system, including cold pipes outdoors. The calculator shows the freezing point of every concentration of ethylene glycol, propylene glycol, ethanol and other brines (Melinder/IIR) and works out how much concentrate you need.

Calculate in the tool above ↑

How do I balance the branches of a heating or cooling system?

The branch with the highest pressure drop sets the pump; the other branches are throttled by the difference in their balancing valves. The calculator shows the throttling and the Kv each valve needs. With reverse return (Tichelmann) the paths are almost equally long and need less throttling.

Calculate in the tool above ↑

Which pipes can I size?

Copper to EN 1057 and ASTM B88, PEX, multilayer PEX-AL-PEX, PE pipe, PP-R, PB, CPVC, PVC-U, steel and stainless press pipe – with inside diameters from each standard. The pipes most used in your country come first in the list.

Calculate in the tool above ↑

What rules apply to a secondary cooling system with glycol or brine?

In an indirect (secondary) system the refrigerant stays in the chiller or machinery room, and water, glycol or brine carries the cold to the users. F-gas leak checks and certificates follow the refrigerant circuit, not the secondary loop. EN 378 covers the whole installation, and a glycol or brine loop only falls under the Pressure Equipment Directive above 10 bar and certain sizes. This calculator sizes the secondary loop: pipes, pump, freeze protection and pipe materials.

Read the rules for secondary cooling systems

Assumptions: Darcy–Weisbach with the Colebrook–White friction factor; fluid properties from CoolProp (IAPWS-95 for water, Melinder/IIR data for glycols and brines) at the mean temperature. Inner diameters from each standard's common wall series – check against your supplier. The noise limits are guidance values; follow the project's own requirements.

Disclaimer: This tool does not guarantee correct results. Always verify pipe dimensions, pressure ratings and other values manually before installation.

New feature – please report any errors: Report a bug

Help – hydronic pipe sizing (heating & cooling)

The calculator sizes the pipes of a water, glycol or brine system that you build in a flow diagram, and works out pumps, balancing and valves.

1. System and fluid

  • Choose the system type first: it fills in a typical fluid, limits and an example system that you then change.
  • For glycols and brines choose the concentration; the list shows the freezing point of each. Keep the lowest temperature at least 3 K above it.
  • Each circuit can have its own fluid (click the pump), for example brine on one side of a heat exchanger and water on the other. A circuit behind a shunt group mixes with its primary circuit and has the same fluid.

2. Pipes and noise

  • The pipe list starts with the systems most used in your country, then every system by material with its standard.
  • Each pipe gets the smallest size that keeps both the maximum velocity and the noise limit for that size, and (if set) the maximum pressure drop per metre.
  • The noise class lowers the allowed velocity in small pipes, where flow noise is heard most. Strict is for homes, bedrooms, hotels and hospitals.

3. Flow diagram

  • The diagram shows each circuit as a loop: the pump on the left, the supply along the top, the branches between the supply and return, and the return along the bottom. Pipes are coloured by temperature.
  • Click a pipe to set its length or pick its size by hand (✎). Right-click a pipe, or select it and use the button, to insert a heat exchanger, a coil, a chiller, a valve and so on exactly there.
  • Drag a component onto another pipe to move it there, also into another circuit. “↶ Undo” undoes the last change (Ctrl+Z), Delete removes what is selected.

4. Components and flows

  • A component is given by its power (the temperature after it is calculated) or by its outlet temperature (the power is calculated). A component with an outlet temperature in the main circuit sets the circuit's temperatures; without one, the calculator warns if the powers do not add up.
  • The pump flow comes from the largest power and the design ΔT, or is given directly. With branches the flow is the sum of the branch flows, each from the branch's power and ΔT.
  • A heat exchanger can be linked to one in another circuit: it then moves the same heat the other way, and the calculator warns if the temperatures cross.

5. Balancing, valves and shunts

  • The branch with the highest pressure drop sets the pump head. The balancing valve in the other branches takes up the difference; the table shows the throttling and the Kv it needs.
  • A control valve is given by its Kvs. Its authority β is the valve's pressure drop divided by the pressure the branch has available; aim for 0.5 and at least 0.25. The calculator suggests a Kvs from the standard series.
  • A shunt group is a branch with its own secondary circuit and pump. The primary flow follows from the secondary flow, the set point and the temperatures; the authority of the shunt valve is shown too.
  • With reverse return (Tichelmann) the return runs on to the last branch and back in a pipe of its own, so every branch gets an almost equally long path and needs less throttling.

6. Heat loss

  • Each pipe loses (or picks up) heat to its surroundings according to its size, material and insulation. The temperature after the pipe is calculated and carried on through the circuit.
  • The surrounding temperature and insulation thickness apply to every pipe; a pipe can have its own, for example outdoors or in a duct. Switch the calculation off to size without losses.

7. Pump and volume

  • Pump head = the pressure drop through the heaviest branch plus the margin. The pump power is the hydraulic power divided by the efficiency.
  • Enter two points from the pump's data sheet and the pump curve is drawn against the system curve and the operating point is found; the calculator warns if the pump is too small or clearly too big.
  • The fluid volume is the pipes and components plus the extra volume you enter; for glycols and brines the amount of concentrate is shown too.

8. Save and print

  • “Save” downloads the project as a file, “Open” opens it again, “Print” prints the results with the flow diagram on a page of its own. Older projects with pipe sections are turned into a flow diagram when opened.