Heat Output Calculator

Choosing the right size stove matters. If it's too small, your room never quite gets warm. If it's too big, you waste fuel and the stove runs inefficiently. Before you buy during the Heat & Flame Event, use our free Heat Output Calculator to find the kW output that suits your room.

It takes less than a minute. Enter your room's length, width and height in metres or feet, then tell us how well insulated it is, whether it opens onto another room or a staircase, and whether you like it warmer than 20°C. The calculator gives you a recommended kW output and suggests suitable multifuel stoves and pellet stoves from our range.

As a general guide, a compact 5kW stove suits most average Irish sitting rooms, while larger or open-plan spaces may need 8kW or more. To heat your radiators and hot water as well, consider a boiler stove. The calculator is a guide only, as room shape, chimney draw and fuel type all affect real-world output. For advice tailored to your home, talk to our Heating Centre experts or call (0504) 21900.

What Is Heat Output (kW)?

Heat output is the amount of heat a stove produces, measured in kilowatts (kW). The higher the kW, the larger the space it can heat. As a general rule of thumb, you need about 1kW for every 14m³ of room space.

Example: a living room measuring 5m × 4m with a 2.4m ceiling is 48m³. Divide by 14 and you get roughly 3.5kW, so a 4–5kW stove would be a good fit.

The calculator refines this rule of thumb using your insulation, room layout and preferred temperature, so you get a more accurate recommendation.

How to Use the Heat Output Calculator?

Step 1: Measure your room.
Measure the length, width and ceiling height of the room you want to heat, in metres or feet. For an L-shaped room, measure each section separately and add the results together.

Step 2: Choose your insulation level.
Select Good for a modern build or upgraded home, Average for homes built before 1990 or if you're unsure, and Poor for older or draughty homes. Better insulation means you need less heat output.

Step 3: Tell us if the room is open to another room or a staircase.
Open-plan spaces and rooms with an open staircase lose heat to other areas, so you'll need a slightly higher output.

Step 4: Choose your preferred temperature.
If you like your room warmer than 20°C, select Yes and the calculator will adjust the result upwards.

Step 5: Get your result.
Click Calculate kW needed to see your recommended heat output, along with stoves from our range that match it.

Why Choosing the Right Stove Size Matters?

• If its too small the stove runs flat out to keep up, so your room never gets quite warm enough.
• If its too big you'll overheat the room or have to keep the stove turned down, which makes it burn inefficiently, waste fuel and blacken the glass.
• Just right you get steady, comfortable heat, lower fuel costs and a cleaner burn.

The calculator sizes a stove for a single room. If you want a boiler stove to run your central heating too, you'll also need to account for the number and size of your radiators and your hot water needs. Boiler stove sizing is best done with an expert, so talk to our Heating Centre team or call (0504) 21900.

Frquently Asked Question 

Is the Heat Output Calculator accurate?
It gives a reliable guide based on your room size, insulation and layout. Room shape, window size, chimney draw and fuel type can all affect real-world output, so we recommend confirming your choice with our team before you buy.

Should I round up or down?
If your result falls between two stove sizes, choose the slightly larger one only if your room is draughty or open-plan. Otherwise, the smaller size usually burns more efficiently.

Does the calculator work for electric fires?
The calculator is designed for multifuel and pellet stoves. Most electric fires produce 1–2kW of heat, which makes them best suited to supplementary heating. Browse our electric fires.

Which is better for my room, a pellet or multifuel stove?
Both come in a wide range of outputs. Read our guide, Wood Pellet or Multi-Fuel Stove?, to compare running costs, efficiency and installation.