How to calculate the pipe length needed for underfloor heating

We are still preparing products.

But you can look at other categories.

How to calculate the length of pipe needed for floor heating

One of the most common practical questions people face when planning floor heating is a simple but key issue: how many meters of pipe should you actually buy? Too little and you have to interrupt the installation, make a connection or order express with an extra charge. Too much and you unnecessarily waste money on the remainder that will remain in the warehouse. In practice, I have seen both – and both are unnecessary if the calculation is done correctly from the beginning.

This article will walk you through the entire calculation process step by step – from the basic formula through the influence of spacing, room shape, adding a reserve, to practical examples from real projects. If you do not yet know which pipe diameter to choose, I recommend first reading the article Which pipe diameter to choose for floor heating (16, 17, 18, 20, 25 mm) in this Knowledge Center – the pipe dimension directly affects the hydraulic design, but has only an indirect influence on the calculation of the loop length.

Basic principle: area, spacing, length

Calculating the length of pipe for floor heating is essentially simple math. When you know the area you want to heat and you know in which spacing you will lay the pipe, you get an almost exact number. Add the supply and return routes from the manifold to the room and you're done.

The basic formula looks like this:

Let's break it down with a concrete example. You have a living room with an area of 20 m². The manifold is located in the hallway, 4 meters from the center of the room. You will lay the pipe with a spacing of 15 cm (0.15 m).

Area ÷ spacing: 20 ÷ 0.15 = 133.3 m
Route to the manifold: 2 × 4 = 8 m (supply + return)
Total: 133.3 + 8 = 141.3 m
With a 10% reserve: approx. 155 m

In this case, you would go for PEX 17×2 pipe in a 120 m pack (not enough) or for PEX 17×2 pipe in a 240 m pack, where you will have a comfortable reserve for one loop.

Pipe spacing and its impact on consumption

Spacing is one of the two main factors that determine the total length of pipe. The smaller the spacing, the more pipe per square meter. The relationship is linear – with half the spacing, you will use twice as much pipe for the same area.

In practice, the following spacings are most commonly used for floor heating:

10 cm – maximum performance, typically in edge zones near windows or in bathrooms with high heat loss; consumption approx. 10 m of pipe per 1 m² of area
12.5 cm – a compromise between performance and consumption; approx. 8 m/m²
15 cm – the most popular spacing for living rooms with standard insulation; approx. 6.7 m/m²
20 cm – suitable for very well-insulated new buildings or rooms with low loss; approx. 5 m/m²
25 cm – exceptionally, only in passive houses or as supplementary heating; approx. 4 m/m²

Important: you do not choose the spacing arbitrarily. It should be the result of a thermal calculation – if you have correctly calculated the heat loss of the room and choose a temperature pair (supply/return, e.g. 40/30 °C), the spacing is the result, not the input. Therefore, if you are calculating the length of pipe without project documentation, keep in mind that the final pipe consumption will be confirmed only by a specific hydraulic design.

What is counted in the area and what is not

This is the point where people make the most mistakes. Only the effectively heated area is taken into account in the calculation – that is, the area under which the pipe will actually be laid. Deduct:

Area under fixed built-in cabinets, kitchen units, a shower enclosure (if built-in) or a bathtub
Area in the so-called edge strip near the walls – standard 15–20 cm from the wall, where the pipe is not laid (depends on the project)
Area of columns, stairs and other fixed structures

In practice, this can reduce the effective area by 5 to 15 % compared to the total floor area of the room. In a living room with dimensions 5 × 4 m (20 m²), the effective area after deducting furniture and edge strips can be realistically only 16–17 m².

These details are described in detail in the article Installation of pipe for floor heating: spacing, laying and fastening, where you will also find schemes for the placement of edge insulation.

Distance from the manifold: supply and return

Each underfloor heating loop must physically cover a distance from the manifold to the heated room – and then back. This route is counted in both directions, i.e. multiplied by 2. If the manifold is in a technical room in the middle of the house and the loop goes to a distant bedroom 8 m away in a straight line, the actual route through the floor and corridors may be 10–12 m. In both directions, this is an additional 20–24 m for one loop – a number that should never be underestimated.

For an accurate calculation, draw a floor plan and measure the actual routes – not just the distance in a straight line, but the actual routing through the spaces (corridors, expansion joints through walls, etc.).

Maximum length of one loop – why it has a limit

Technically important point: one underfloor heating loop cannot be infinitely long. The reason is hydraulic resistance – the longer the loop, the higher the resistance and the harder the pump must overcome the pressure difference. With an overly long loop, uneven flow or insufficient heat to distant parts of the loop may occur.

Therefore, larger rooms are divided into multiple loops. For example, a living room with an area of 35 m², with a diameter of 17 mm and a spacing of 15 cm, will be divided into two loops of approximately 120–130 m (including routes to the manifold). Each loop has its own circuit on the manifold. This division must be planned so that both loops have as similar a length as possible – otherwise balancing the hydraulics becomes much more difficult.

Calculation for the whole house: step-by-step procedure

For a larger project (a family house, an apartment with multiple rooms), the best approach is to handle each room separately and then add up the results. Here is a specific procedure:

Step 1: List of rooms and their effective areas

Create a table: room, gross area, effective area (after subtracting furniture and edge strip), distance from the manifold.

Step 2: Determine the spacing for each room

Bathroom and entrance corridor can have a spacing of 10–12.5 cm (higher performance, cold floors), living rooms 15–20 cm, storage or technical rooms 20–25 cm.

Step 3: Calculate the pipe length for each loop

For each room, calculate: (effective area ÷ spacing) + (2 × distance from the manifold). If the result exceeds the maximum loop length for the given diameter, divide the room into 2 loops and repeat the calculation.

Step 4: Sum and reserve

Add up all the loops. Add a 10 % reserve for unexpected routes, bends, and possible adjustments during installation.

Step 5: Selection of packaging

Choose pipe packaging that matches your needs. Ideally, buy one larger reel for the whole house (fewer joints = higher reliability) or divide according to loops. Never connect the pipe in the middle of a loop under the concrete – joints must be accessible.

Practical example: three-room apartment

I will show a real case. You have an apartment with the following rooms, the manifold is in the corridor:

Room	Gross area (m²)	Effective area (m²)	Spacing (m)	Distance from manifold (m)	Loop length (m)
Living room	28	23	0.15	5	163
Bedroom	14	11	0.20	8	71
Children's room	12	10	0.15	7	81
Bathroom	6	5	0.10	3	56
Corridor	5	4.5	0.15	1	32
Total without reserve					403 m
Total with 10 % reserve					443 m

Warning: the living room is 163 m. This is 73 m over the safe limit for an average 17 mm (max. approx. 100 m). Solution: divide the living room into two loops – each approx. 80–85 m. The total length of pipe remains the same, only it is distributed into two circuits on the manifold.

The total requirement for this apartment is therefore approx. 443–450 m of pipe. To cover the entire apartment with one type of diameter (e.g. 17 mm PEX), you would reach for a combination, for example, two packages of PEX 17×2 with 240 m each (total 480 m, reserve approx. 30 m), or a single wholesale package of PEX 17×2 – 600 m, which will cover the entire apartment with a significant reserve and is usually more cost-effective per meter.

Influence of room shape on the actual length of pipe

So far we have assumed ideal rectangular rooms. In practice, it is rarely the case. Slanted roofs, recesses, columns, L-shaped layouts – all of these add to the length of the pipe due to necessary bends and changes in direction. That is why a 10 % reserve is a minimum, not a luxury.

Moreover, different patterns are used when laying the pipe. The most common are:

Snake (serpentine / meander) – the pipe goes back and forth in parallel; a simpler pattern, but it can have uneven temperature distribution (supply is always on one side, return on the other)
Spiral (spiral / snail) – the pipe unwinds in a spiral from the center or from the edge; better thermal balance, but more demanding in laying and planning the route
Dual snake (bifilar) – a combination of snake + spiral; the hot supply and cold return are next to each other, which results in a more even temperature distribution

From the point of view of pipe length, there is not a significant difference between these patterns – with the same spacing and the same area, the total length is almost identical. The difference is in the temperature distribution and in how demanding it is to lay the pipe. For calculating the length, the laying pattern does not play a role, only the way you bend the pipe around corners can add a few centimeters here and there.

Edge zones and intensified spacing near windows

Heat loss is greater near windows and external walls. Therefore, in the so-called edge zone (typically 0.5–1.0 m from the window or external wall), the pipe is laid with a smaller spacing – for example, 10 cm, while in the central zone of the room it is 20 cm. This intensification adds to the total length of the pipe. When calculating, you need to take it into account by calculating the edge and central zones separately:

Edge zone: width 0.8 m, window length 3 m → area 2.4 m², spacing 0.10 m → 24 m of pipe
Rest of the room: 20 m² – 2.4 m² = 17.6 m², spacing 0.20 m → 88 m of pipe
Total: 112 m (+ routes to the manifold)

In comparison with a uniform spacing of 0.15 m throughout the room (approx. 133 m), this combination is more economical in terms of pipe and at the same time more effective in performance near the windows.

Calculator vs. manual calculation: when an estimate is enough and when a project is needed

For a simple room with a regular shape, a manual calculation or an online calculator is sufficient. For an entire house, complicated layouts, or when energy certification is required, a proper thermal and hydraulic project is necessary. This not only determines the length of the pipe, but also the diameter, the temperature of the medium, the setting of the valves on the manifold, and the type of pump.

If you are doing the calculation yourself and want to be sure, always add at least 10 % reserve to your manual estimate – regardless of how carefully you measured. From personal experience, I know that during installation, something always turns out differently – the manifold is moved by a meter, the route goes through another room, or it turns out that the recess near the fireplace needs to be heated separately.

More about choosing the right type of pipe (PEX vs. multilayer) can be read in the article How to choose pipe for underfloor heating: PEX vs. multilayer. If you are dealing with the question of pressure classes PN6 vs. PN10, see the article What pressure and temperature (PN6, PN10) suit my underfloor heating system.

How to choose the right pipe package

Pipe for underfloor heating is sold in reels of different lengths. Basic rule: one loop = one reel. Never connect the pipe in the middle of a loop under the concrete screed – even if you use a crimped joint, it is a weak point that is not accessible and can be a source of leakage for the next decade.

For small projects (one room, bathroom, additional loop), it is suitable to use a package of PEX 17×2 – 120 m, which will cover one loop of a medium-sized room. This package is ideal for a standard bedroom or children's room.

For larger rooms or two loops, it is suitable to use a package of PEX 17×2 – 240 m. From one reel, two loops can be cut, or one longer loop for a larger room – but they must go directly one after the other without joints.

For whole-house projects or larger buildings, the economic solution is a large package of PEX 17×2 – 600 m. The price per meter is significantly lower in such a package and you do not have to deal with the logistics of multiple smaller reels.

For rooms with higher hydraulic resistance or longer routes (e.g. in a family house, where the manifold is far away), a 20 mm diameter is suitable – here reach for pipe PEX 20×2 – 200 m. The internal diameter of this pipe is larger, the resistance is lower, and the loop can be longer.

Most Common Mistakes in Calculating Pipe Length

Over the years of practice, I've seen these typical mistakes:

Forgetting the routing to the manifold – an underestimated item, can represent 10–30 m per loop
Counting gross area instead of effective – overestimating the need, or incorrect manifold setup
Ignoring the maximum loop length – too long a loop = weak flow = cold floor at the end of the circuit
No reserve – during installation, you almost always need a few extra meters
Purchasing pipe by the meter from several small spools with joints – saving money in the wrong place
Same spacing throughout the house without considering heat loss – bathroom and living room have different requirements

Most Frequently Asked Questions (FAQ)

How many meters of pipe do I need per 1 m² of floor?

It depends on the laying spacing. At 10 cm spacing, it's 10 m/m², at 15 cm approximately 6.7 m/m², at 20 cm approximately 5 m/m². To these numbers, you always need to add the routing from the manifold to the room (supply and return).

Can I connect the pipe in the middle of the loop if the spool length is not enough?

Technically it is possible using crimped joints, but it is strongly not recommended. A joint under the screed is a permanent weak point – it is not accessible, not visually inspectable, and in case of a leak, it can cause extensive damage. Always buy a spool whose length covers the entire loop without connecting.

Should I have the same length for all loops?

Ideally yes – similar lengths make hydraulic balancing easier. In practice, a deviation of up to 10–15 % between the longest and shortest loop is acceptable. Larger differences are corrected by adjusting the valves on the manifold, which requires more precise balancing.

Is it worth buying a larger spool (e.g., 600 m) even for a small project?

It depends on the total project requirement. If your project is small (one loop of 80–100 m), a large spool is not necessary – the rest will not be used. However, if you are covering an entire house with five to six loops, a 600 m spool is more economical and practical.

What if I make a mistake in the calculation and buy less pipe?

If it is discovered before the screed is poured, it can be reordered. If it is discovered only during installation and there is no time to wait for delivery, you are in trouble – that's why you should always calculate with at least a 10 % reserve. Screeding cannot be "paused" for a few days without the risk of damaging the partially assembled structure.

Does the calculation apply the same to PEX and multi-layer pipe?

Yes, the length calculation is the same – it depends only on the area, spacing, and routing to the manifold. The type of material (PEX, PEX-Al-PEX, multi-layer) does not change the meterage. The difference is in hydraulic resistance (and thus the maximum loop length) and in the laying method – multi-layer pipe is stiffer and harder to bend, which at tight spacing may require a larger bend radius. More about the comparison of both types can be found in the article PEX pipe vs. multi-layer pipe: differences, advantages and disadvantages.

Conclusion

Calculating the pipe length for underfloor heating is not rocket science – it just requires order, accurate dimensions, and a few proven rules. Effective area, spacing, routing to the manifold, maximum loop length, and a 10 % reserve – these are all the ingredients you need. If you consider all of them, the result will be reliable, the installation smooth, and the installation free of unpleasant surprises during screeding.

If you are dealing with a more complex project with multiple rooms, different spacings, and long routes, don't forget to handle each room separately, check the maximum length of each loop, and round up the total pipe requirement – it is always better to have an extra meter than to miss one after the floor has been concreted.

Do you have a question about this topic?

Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.