How To Measure for Underfloor Heating

Brrr! It’s definitely getting colder out there and as such, a lot of you may be thinking about installing underfloor heating (UFH) during your next renovation project. There’s lots of advantages to be had from UFH – whereas conventional central heating systems warm up rooms by heading the air at the side of the room (usually vai radiators) causing warm air to rise and pushing the cold air down, often creating a stuffy room; UFH provides a form of centralised radiant heat that allows more evenly distributed heat with no ‘hot spots’, not to mention helping reduce drafts and airborn allergens. Did we also mention that once installed, UFH requires virtually no maintenance or upkeep?

Sounds like a no-brainer we think you’ll agree, but one big question that DIYers often have about UFH is how to measure out and determine how much UFH Cable of Mat you’ll need. Before we show you how to do that though, there are a couple of small but important differences between Mat Kits and Loose Cable kits to understand:

  • The output wattage per m² on a mat system is determined by how powerful the cable is. Both 150 watt & 200 watt mats have the same cable spacing, however, the 200 watt mat will feature more powerful cable.
  • Loose cable kits, on the other hand, use the same power cable – the output wattage per m² is determined based on the spacing between the cables. Minimum & maximum spacing is necessary to avoid hot/cold spots.

Whichever underfloor heating type you decide to install in your property, please remember that it is always best to be a little short than have any cable left over. If there is excess cable which the installer decides to cut, the underfloor heating cable/mat will not work. In these circumstances, it is recommended to purchase a smaller mat. Hopefully, you can avoid that scenario by using the below measuring methods to work out the maximum heated area of you room:

Measuring Out Your Room for Underfloor Heating

There are three common methods you can use for measuring out the maximum heated area of your room space before buying your UFH kit:

*In the case of the above, you would generally need to purchase a 3.5m² underfloor heating mat kit

*In the case of the above, you would generally need to purchase a 3.5m² underfloor heating mat kit

*In the case of the above, you would generally need to purchase a 9.5m² underfloor heating mat kit

Prefer to Use a Loose Cable System?

For smaller complicated areas or areas where there are many obstacles, many professional installers prefer the use of a Loose Cable UFH kit. Although this may take a little more time, effort and planning to install, the loose cable kit is more convenient in situations where it’s necessary go around obstacles, provided the minimum cable distance outlined by the manufacturer is always maintained.

Follow these steps to determine which cable kit you need for your UFH installation:

Step 1

Using measuring methods 1, 2 or 3, first calculate the Maximum Heated Area for your room. For the purposes of this example we’re going to set the Maximum Heated Area as 3.62m²

Step 2 

Decide the wattage you would like your underfloor heating to output using the below table. This will determine the spacing you need between the loose cables and ultimately the length of cable required to cover that area in order to achieve your desired wattage output.

Please note that the cable kits are manufactured and advertised based on 200 watts per m² at a 65mm spacing. Typically the higher the wattage the quicker the floor will achieve the target temperature and vice versa.

Loose Cable Kits

65mm Spacing = 200 watts pm² 75mm Spacing = 175 watts pm² 85mm Spacing = 150 watts pm² 95mm Spacing = 135 watts pm²
Area Covered m² Area Covered m² Area Covered m² Area Covered m²
1 1.1 1.3 1.4
1.5 1.7 2 2.2
2 2.3 2.4 3
3 3.4 4 4.4
4 4.6 5.3 5.9
5 5.7 6.7 7.4

Step 3 

Let’s say you would like to achieve a 150 watt output in a Maximum Heated Area of 3.62m². 

Based on the above table, when the cable is spaced at 85mm (150 watts p/m²) instead of 65mm (200 watts p/m²), the same cable will achieve an area of 1.3m² + 2m² = 3.3m². Both cables can be connected to a single thermostat via a junction box.

Therefore you would need to purchase a 1m² & a 1.5m² 200 watt p/m² loose cable kit. You can use the table as a guide and work out the spacing and length of cable applicable to your own Maximum Heated Area and desired wattage output and then purchase the relevant kit to achieve it.

Make sense? If you’re still confused then you can always give our friendly customer services team a call on 01782 223822 for some further advice or use our live chat function on the website. It’s important to remember that whilst it’s fine to measure and lay your UFH kits yourself, you should always enlist the help of a registered Part P qualified electrician when it comes to connecting the system to the electricity supply and thermostat.

You might also enjoy these posts on the Tile Mountain blog…

How to Lay and Install Underfloor Heating Mats

Best Tiles For Underfloor Heating

How to Choose Underfloor Heating

Dean heads up the content team here at Tile Mountain, commissioning, editing and sometimes even writing some of the incredibly useful and entertaining content we bring to you on a regular basis. With a background in consumer journalism and a raft of content marketing experience gained from working with huge brands spanning mobile tech and automotive, right through to professional sports and lifestyle sectors, he knows a thing or two about creating content that people love to engage with. Dean also holds the mantle of being our resident grammar pedant, but when he’s not getting angry about dangling participles he spends his spare time going to gigs, watching his beloved Stoke City FC, and adding to his collection of deadstock adidas trainers.