If you're in the market for an oil-filled radiator, it's important to consider the size before making your purchase. They can be one of the best radiators if you're searching for a portable option, but choosing the wrong one can negatively affect their efficiency and how comfortable your room feels.
These portable appliances come in a variety of sizes, ranging from small units with around 80W of power to larger models reaching up to 2500W. When selecting the right one for your home, you'll need to consider both the room size and the level of insulation.
To help you determine which size oil-filled radiator you should buy, we called in the experts to explain everything you need to know. Below, you'll find the calculations to make, how to tell if yours is the wrong size, and what can happen if it is.
How to size an oil-filled radiator
With a heat calculator:
To get the most accurate results, Nick Duggan, Managing Director of The Radiator Centre, recommends using an online heat calculator (like this one by B&Q) to determine your specific requirements. After entering your room’s dimensions and type of insulation, windows, and outside wall, the results will be shown in either watts or BTUs (British Thermal Units).
Nick says, "All radiators will state an output in either watts, BTUs or both. By knowing how much heat each radiator will emit, you can now find the best one to efficiently heat each room based on your earlier heat calculation."
Without a heat calculator:
If you prefer to work it out yourself, Nicholas Aukland, Managing Director of Trade Radiators, says to multiply the room’s square meterage by 100 watts for a standard ceiling height with average insulation.
For example: 20 m² room = 2,000 watts
However, if your room is poorly insulated or has particularly high ceilings, you’ll need to add around 10 to 20% on top of that figure. So, the example above would become 2,200 to 2,400 watts.
Nicholas adds, "Most radiator manufacturers will provide sizing guides and all models should list their output in watts so you can match the heater to the space with fairly little trouble."
Shop these top-rated oil-filled radiators
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Perfect for small spaces, this 800W radiator features an adjustable thermostat to keep you toasty this winter.
With three heat settings and 1500W of power, this option is ideal for medium-sized rooms.
This 2500W radiator is an excellent choice for large rooms and includes a temperature control dial.
Nick has over 20 years of experience in central heating and radiators, leading the team at one of the UK's largest independent suppliers.
Nicholas Auckland brings 30 years of heating-industry experience, energy-efficiency expertise and oversight of one of the UK’s largest home-heating ranges, making him ideally placed to explain oil-filled radiators.
How does the size of an oil-filled radiator affect efficiency?
If your radiator is undersized...
Nicholas Aukland explains that by choosing a radiator that’s too small for your room, you’ll end up running it on the highest setting for long stretches of time – after all that, your room will still feel cold!
Nick Duggan adds, "An undersized radiator leads to uneven heat, cold patches and higher electricity use because the appliance never gets a break."
If your radiator is oversized...
Bigger isn’t always better when it comes to radiator size; an oversized model can make the room heat up too quickly and feel uncomfortably stuffy. Nicholas explains that the thermostat will cut in and out repeatedly, and you’ll just end up switching it off because it feels too overwhelming.
Nick Duggan says, "An oversized one short cycles, which is inefficient and uncomfortable, and can create a humid, heavy feeling in the room that makes the space feel smaller. Getting the output right for the room avoids these issues and keeps the temperature stable without wasting energy."
Signs your oil-filled radiator is the wrong size
If you think your radiator is the wrong size, look out for these tell-tale signs, explained by Nicholas Aukland:
It's undersized if...
- You feel heat pooling right beside the unit while the rest of the room stays chilly
- The unit runs flat out for long stretches
- The thermostat rarely clicks off
- You feel you need to sit close to stay warm
- The fins feel very hot but the far wall or corners remain cool, which is often a mix of too little surface area and poor airflow
It's oversized if...
- You get rapid temperature rises and frequent thermostat clicks followed by short, repeated on‑off cycles
- The room can feel stuffy, then cool again a few minutes later
- You want to crack open a window to dump heat
- Overheat cut‑outs trip out
However, Nicholas adds: "Some of the same symptoms can come from placement, so check that fins are not blocked by furniture or drying clothes, doors are closed to stop heat escaping, and draughts are managed before deciding the size is wrong."
To prevent wasting energy, money, and time, it's important to choose the correct size before installing a radiator. By following these calculation tips and watching for the signs mentioned above, you can ensure your heater is the right fit.
If you're unclear whether an oil-filled radiator is the best choice for your home, explore the different heat emitters available, as explained by the experts.
