Underfloor Heating Pipe Calculator

How to calculate underfloor heating pipe length

The calculator separates pipe in the floor, manifold tails and buying allowance

Enter the room size

Use the internal room length and width. The calculator represents the full rectangular area; fixed exclusions must be marked separately on the installation drawing.

Choose pipe and spacing

Closer pipe centres require more pipe. Diameter also affects loop length, bend radius and hydraulic performance.

Enter the manifold distance

The route from the manifold to the heated zone and back is included in each loop.

Review and print the plan

Check the number and length of loops and the proportional layout. Confirm the final design against a room-by-room heat-loss calculation.

How much underfloor heating pipe is needed per m²?

The main quantity is determined by the distance between adjacent pipe runs

Simple estimate

Heated area ÷ spacing in metres

For example, an 18 m² heated area with 150 mm (0.15 m) pipe centres needs about 18 ÷ 0.15 = 120 m of pipe in the floor. Add the flow and return tails for every loop, then add the buying allowance.

L_floor ≈ A_heated ÷ s

Pipe centres	Approximate pipe quantity	Typical context
100 mm	10 m/m²	Higher heat demand, bathrooms or perimeter zones
150 mm	6.7 m/m²	Common close spacing in living areas
200 mm	5 m/m²	Well-insulated rooms where the design permits
250 mm	4 m/m²	Lower heat-demand areas
300 mm	3.3 m/m²	Wide spacing only where system calculations allow

This is a geometric estimate. Bends and the selected fixing system can make a manufacturer’s design quantity slightly higher or lower.

Underfloor heating pipe layout: spiral or serpentine?

Choose a pattern to suit the room shape, colder external surfaces and the required temperature uniformity

Spiral layout

Spiral for a more even floor temperature

The warmer flow and cooler return pipes run next to each other. The pipe travels inward at twice the final spacing, turns through a U-bend and returns through the gaps. A separate zigzag in the centre is normally unnecessary; a narrow zone may be better suited to a serpentine run.

Even heat distribution
The centre gap remains no greater than the selected spacing
Fewer tight 180° bends

Serpentine layout

Serpentine for narrow or perimeter zones

The pipe runs back and forth from one side of the room to the other. The flow can start beside the coldest external wall or glazing, but the water cools along the route so the floor temperature may be less even.

Simple in long, narrow areas
Can start at the coldest wall
More 180° bends

Which underfloor heating pipe diameter and loop length should you use?

Diameter barely changes metres per square metre, but it changes pressure loss and the practical loop length

Outside diameter	Conservative starting point	Planning note
12 mm	60 m	Often used in thin retrofit systems; the smaller bore usually means shorter loops.
16 mm	80 m	A very common domestic size. Some systems permit more, subject to a hydraulic calculation.
17–18 mm	90 m	Used by particular fixing systems; the practical limit depends on the true internal bore.
20 mm	100 m	Lower resistance but a larger bend radius; manufacturer limits can be longer.

There is no single maximum length for every pipe. The practical limit depends on internal diameter, flow rate, pressure loss, pump duty and permitted temperature drop. Include both manifold tails in the limit and check the manufacturer’s design tables.

Where should underfloor heating pipe not be laid?

The calculator uses the full rectangular room; mark permanent obstacles and unheated areas on the installation plan

Fixed kitchen units and appliances

Pipe is commonly omitted beneath floor-standing food-storage cabinets, kitchen islands, fridges and freezers. An external-wall strip may be an exception where the system design requires it.

Baths, showers and sanitary fittings

Practice varies by system. Avoid wastes, services and floor fixings, and agree any heated area beneath a bath or shower tray before laying pipe.

Fireplaces, stairs, partitions and fixings

Keep clear of heavy permanent structures and places where the floor will be drilled. Do not cross movement joints without the protection specified by the design.

What should be checked before installation?

Pipe quantity is only one part of a wet underfloor heating design. A room-by-room heat-loss calculation should confirm the required output, pipe centres and water temperatures. The designer should also check floor-covering resistance, flow rate, pressure loss, controls and hydraulic balancing. Closer spacing may support lower water temperatures, but does not by itself guarantee sufficient output.

✓ Calculate the heat loss for every room✓ Choose an accessible manifold position✓ Protect pipe where movement joints are crossed✓ Avoid joints within the floor build-up✓ Pressure-test before covering the pipe✓ Photograph the pipe layout before it is concealed

Technical sources: Wavin UK installation guidance, Wavin UK pipe-spacing guidance, Uponor UK installation guide.

Frequently asked questions about underfloor heating pipe

Practical answers about pipe quantity, spacing, loop length, diameter and layout

How much underfloor heating pipe do I need per m²?

As a geometric guide, allow about 10 m/m² at 100 mm centres, 6.7 m/m² at 150 mm, 5 m/m² at 200 mm, 4 m/m² at 250 mm and 3.3 m/m² at 300 mm. Add the flow and return tails and a buying allowance.

What pipe spacing should I use for underfloor heating?

100–200 mm centres are common design choices, but the correct spacing depends on room heat loss, floor build-up, covering and water temperature. Perimeter areas beside glazing may need closer spacing.

What is the maximum underfloor heating loop length?

There is no universal figure. This calculator starts with conservative editable assumptions of about 60 m for 12 mm, 80 m for 16 mm, 90 m for 17–18 mm and 100 m for 20 mm pipe, including manifold tails. Check the pipe manufacturer’s pressure-loss data.

How many underfloor heating loops are needed for one room?

The estimated installed length is divided so the average loop stays within the selected limit. For example, 145 m against a 100 m limit requires at least two loops. Loops should normally be reasonably similar in length and balanced at the manifold.

Which pipe size should I choose: 16, 17, 18 or 20 mm?

16 mm is common in domestic wet systems, while 17, 18 and 20 mm are used by particular systems and larger applications. Internal bore, wall thickness, bend radius and hydraulic design matter as much as outside diameter.

Is a spiral or serpentine underfloor heating layout better?

A spiral places warmer flow beside cooler return pipe and usually produces a more even surface temperature. A serpentine pattern can suit long narrow zones and can start at a cold external wall, but temperature tends to fall along the run.

Should underfloor heating pipe go under kitchen units?

Pipe is usually omitted below fixed floor-standing cupboards, islands, fridges and freezers. Mark furniture and floor-fixing positions before the final pipe plan; an external-wall strip can be a project-specific exception.

Can underfloor heating pipe go beneath a bath or shower?

It depends on the system and bathroom layout. Some wet systems permit it, but wastes, services and fixing points must remain clear. This calculator does not model obstacles, so show exclusions on the installation plan.

How much spare underfloor heating pipe should I buy?

A 5% allowance is a useful preliminary assumption when dimensions and manifold position are known. The allowance is not for hidden joints: each loop should be one continuous length from the manifold and back.

Why does the calculator not show heat output in W/m²?

Pipe diameter and spacing alone are not enough. Reliable output needs room heat loss, flow and return temperatures, floor layers, covering resistance and flow rate. A number without those inputs would be misleading.

Are my room measurements uploaded?

No. The calculation and drawing run in your browser; the room dimensions are not sent to the server.

Underfloor Heating Pipe Calculator

Underfloor heating pipe layout

Loop distribution

How would pipe spacing change the result?

How to calculate underfloor heating pipe length

Enter the room size

Choose pipe and spacing

Enter the manifold distance

Review and print the plan

How much underfloor heating pipe is needed per m²?

Heated area ÷ spacing in metres

Underfloor heating pipe layout: spiral or serpentine?

Spiral for a more even floor temperature

Serpentine for narrow or perimeter zones

Which underfloor heating pipe diameter and loop length should you use?

Where should underfloor heating pipe not be laid?

Fixed kitchen units and appliances

Baths, showers and sanitary fittings

Fireplaces, stairs, partitions and fixings

What should be checked before installation?

Frequently asked questions about underfloor heating pipe

Underfloor heating pipe layout

Loop distribution

How would pipe spacing change the result?

How to calculate underfloor heating pipe length

Enter the room size

Choose pipe and spacing

Enter the manifold distance

Review and print the plan

How much underfloor heating pipe is needed per m²?

Heated area ÷ spacing in metres

Underfloor heating pipe layout: spiral or serpentine?

Spiral for a more even floor temperature

Serpentine for narrow or perimeter zones

Which underfloor heating pipe diameter and loop length should you use?

Where should underfloor heating pipe not be laid?

Fixed kitchen units and appliances

Baths, showers and sanitary fittings

Fireplaces, stairs, partitions and fixings

What should be checked before installation?

Frequently asked questions about underfloor heating pipe

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