Question 01
What Is the Subfloor?
Confirm whether the base is concrete, screed, tile backer board, timber or another approved construction.
Electric underfloor heating sizing
How to Choose the Right Size Electric Underfloor Heating System
Choose the system for the subfloor and finish, calculate the free heated area, confirm the required output, then select a mat or cable that fits the real layout.
Do not match a mat to the room's total area or buy an oversized cable with the intention of shortening it.
START HERE
Start With the Right Questions
System type, output and coverage are separate decisions. Work through them in this order.
Question 02
What Is the Final Floor Finish?
The heating system, cover layer, floor sensor and temperature limits must suit the complete finished build-up.
Question 03
How Much Free Area Can Be Heated?
Deduct permanent exclusions and clearances, then prove that the mat or cable route fits the available shape.
Question 04
What Heating Duty Is Required?
Decide whether the system provides background comfort or must meet the room's calculated heat loss as the main emitter.
STEP 1
Match the System to the Floor
Obtain confirmation that the complete build-up is suitable. Approval of the finish alone does not automatically approve the adhesive, underlay, compound or substrate.
| Final finish or project | Common system route | Checks before selection |
|---|---|---|
| Tile or stone | Heating mat, loose cable or compatible cable/decoupling system | Subfloor stability, insulation, adhesive, output and minimum cable cover |
| Engineered wood or laminate | Foil system or another build-up approved for the finish | Floor-temperature limit, underlay, floating-floor instructions and moisture conditions |
| Vinyl or linoleum | Approved system beneath a smoothing layer or specified dry build-up | Manufacturer approval, temperature limit, flatness and minimum cover |
| Carpet | System approved for carpet and underlay | Combined thermal resistance or TOG, floor-temperature limit and output |
| Awkward or irregular room | Loose cable or cable/decoupling system may fit better | Cable spacing, perimeter clearances, fixed objects and layout drawing |
Tile or stone
- Common system route
- Heating mat, loose cable or compatible cable/decoupling system
- Checks before selection
- Subfloor stability, insulation, adhesive, output and minimum cable cover
Engineered wood or laminate
- Common system route
- Foil system or another build-up approved for the finish
- Checks before selection
- Floor-temperature limit, underlay, floating-floor instructions and moisture conditions
Vinyl or linoleum
- Common system route
- Approved system beneath a smoothing layer or specified dry build-up
- Checks before selection
- Manufacturer approval, temperature limit, flatness and minimum cover
Carpet
- Common system route
- System approved for carpet and underlay
- Checks before selection
- Combined thermal resistance or TOG, floor-temperature limit and output
Awkward or irregular room
- Common system route
- Loose cable or cable/decoupling system may fit better
- Checks before selection
- Cable spacing, perimeter clearances, fixed objects and layout drawing
STEP 2
Calculate the Free Heated Area
Measure from the route that can safely contain heating—not simply the external room dimensions.
Measure the Gross Room Area
Break an irregular room into rectangles, calculate each area and add them. A dimensioned sketch is more reliable than one length and width.
Mark Permanent Exclusions
Deduct floor-level units, baths, trays, toilet pedestals, fitted cabinets, solid-base appliances, fireplaces and fixed objects that block heat dissipation.
Apply Clearances
Use the manual's clearances around walls, drains, heat sources and movement joints. Not every remaining centimetre is usable.
Draw the Actual Route
Prove that the mat can turn and pass between exclusions without crossing or bunching. Confirm the largest size that fits comfortably.
WORKED EXAMPLE
Sizing a 2.4m × 2.0m Bathroom
The 4.8m² gross area contains a 1.7m² bath plus 0.55m² of vanity and toilet exclusions. Layout clearances reduce the usable heating area further.
| Calculation | Result |
|---|---|
| Gross room area | 4.8m² |
| Fixed exclusions | 2.25m² |
| Remaining before clearances | 2.55m² |
| Practical free heated area after layout checks | Approximately 2.35m² |
Gross room area
- Result
- 4.8m²
Fixed exclusions
- Result
- 2.25m²
Remaining before clearances
- Result
- 2.55m²
Practical free heated area after layout checks
- Result
- Approximately 2.35m²
STEP 3
Select the Right Heat Output
Mat area describes coverage; watts per square metre describes installed output. Heat loss, insulation, floor construction, finish and room use determine the right choice.
Use only outputs approved for the final finish and build-up
Do not use the highest output to compensate for poor insulation
Check the heated area can meet heat loss for primary heating
Use a floor sensor and the required finish temperature limit
Confirm electrical load, thermostat rating and switching
STEP 4
Decide Between a Mat and Loose Cable
A mat is quick in open rectangular areas. Loose cable provides more freedom around islands, sanitaryware and angled walls but requires accurate designed spacing.
| Choose a mat when… | Choose loose cable when… |
|---|---|
| The free heated area has broad, regular runs | The room has narrow passages or many exclusions |
| Speed and preset spacing are priorities | The layout needs flexible turns with approved spacing |
| A stocked mat size fits comfortably | No practical mat size fits without overlap |
The free heated area has broad, regular runs
- Choose loose cable when…
- The room has narrow passages or many exclusions
Speed and preset spacing are priorities
- Choose loose cable when…
- The layout needs flexible turns with approved spacing
A stocked mat size fits comfortably
- Choose loose cable when…
- No practical mat size fits without overlap
FINAL CHECK
Before You Order
Moveable furniture on legs may be treated differently by each system. Keep the as-built cable layout so future owners know where heating is installed and where drilling is unsafe.
Complete floor build-up confirmed compatible
Dimensioned plan with exclusions and route
Free heated area calculated from the layout
Heat loss checked for primary heating
Mat or cable size and output chosen from current data
Thermostat, sensor, load and circuit agreed
Insulation, preparation materials and cure times planned
Electric system sizing
Frequently Asked Questions
Answers about free area, oversized mats, multiple mats and output selection.
Should I buy a mat the same size as the room?
No. Measure only the free heated area and confirm the mat can be routed through it. Permanent units, sanitaryware and clearances reduce usable coverage.
Can I cut an oversized mat to make it fit?
You may cut carrier mesh to turn the mat if instructions allow it, but the heating cable must never be cut or shortened. Choose a smaller mat or more flexible cable system.
Can I use more than one mat?
Yes in some designs, but each mat must fit safely and an electrician must verify combined load, thermostat rating, circuit and switching before installation.
Does a higher W/m² mat always heat the room better?
No. Output must suit the finish, heat loss and control strategy. The highest output is not a substitute for insulation or room design.