BTU Calculator

Work out your heating needs in a few easy steps

Trade Radiators Heating Calculator...

To help make choosing a radiator suitable for your home easier, the Trade Radiators heating calculator will predict the necessary heat output (in BTUs and Watts) required to maintain a comfortable room temperature. These values are listed with all the radiators we sell on the site to help shortlist suitable products for you. To help you better understand how, where, why and when you'd use this calculator, we've answered some popular questions about BTU, the heating calculator, how it affects your home and how it works.

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Please note, this is a guide and may not take into account all the factors relating to your particular requirements. Calculations based on Delta – T 50°C (Δ-T50°C)


Let's say you need a new radiator for your kitchen and you're trying to figure out what type to buy. You're thinking of getting the biggest radiator there is because your room seems to get cold all the time. Buying the biggest model though would mean your kitchen could quickly become hot all the time and you'd end up wasting energy (and in turn money) heating up your kitchen. To avoid that, you'd use this calculator.

To help make sure you don't buy a radiator that is the wrong size and also know the watts and BTU required to heat the space the radiator will go in.

BTU stands for British Thermal Units. This is the measurement used to figure out how much energy is needed to heat (and cool) a room based on its size. Simply put; the higher the BTU, the higher the energy output will be. How effective that is though depends on the size of a room and what lies behind the walls, floor and roof.

You wouldn't put a small 600mm by 600mm towel rail in your living room, and you wouldn't put a 1600mm high vertical radiator in your box bedroom. It is possible to pick the wrong sized radiator for your home, especially if you don't understand the energy needed to heat said room and what obstacles may naturally lie in the way. Working out the size and features of a room helps to make an informed purchase.

To effectively calculate what the BTU output for any room is, you have to start by getting a tape measure and measuring the height of the room, the width of the room, the length of the room and then finally the size of the window area (that's the length by width of a window in m²). And to help clear any confusion of what qualifies as width and length, the length of a room (top to bottom) will almost always be longer than the width (side to side).

Letting us know what type of room will help provide a better calculation of how much energy will be needed to heat a space.

Different rooms have different features and items that need to be accounted for. For example, you'd be surprised how many living rooms have a great big radiator but hide it behind a sofa that draws in all the heat. Kitchens are generally tiled or laminated and often a room where doors in and out of the house are constantly being opened and closed, meaning they need a high BTU to stay warm.

The same goes for hallways which will have the highest height, smallest width and most exposure to the cold outside with the front door opening and closing.

Heat doesn't just rise up. It disperses across the entire room in all directions and when you have a floor one storey up or laying over some concrete, it can dramatically affect the heat retention of your room.

For any rooms upstairs, the BTU will usually be lower as the natural heat rising up from the ground floor, combined with insulated floors and carpeting mean that spaces like your bedroom can hold heat pretty well.

When we're on the ground floor, heat efficiency can be altered if you have soil under a wooden floor, the soil under concrete or a suspended floor. What goes on under your floor is very important as a badly ventilated underfloor can cause condensation or contaminated air to build up. Many modern homes will have their own extended underfloor vent to help circulate heat.

If you walk around the outside of your house and see that one of the bricks on the lower wall has a brick-ish coloured plastic grill over it, you have a suspended floor and that is the vent coming from it. If you know that under your wooden floor or carpet is just a large hard grey floor, you have soil under a solid concrete floor.

Unless you're in a bungalow, chances are there will be a room above any ground floor room that heat will want to rise up to. Here is what each of the options in this section of the calculator mean:

- Heated room

If your bedroom or bathroom sits above any used room below, then they're above a heated room. This means that heat coming up can help act as a buffer of sorts for your room.

- A pitched insulated roof

If your attic is above and not insulated, your room may already be very sound heat-wise or need a powerful radiator to keep the space warm.

- Pitched roof insulated 50mm

Thinly insulated roofs are usually a sign that your rooms on the upper floor have good insulation already.

- Pitched roof insulated 100mm

A thickly insulated roof will be a sign that your room has had issues with heat, but this insulation should help it stay warm quite easily.

All walls might look the same but what's inside can be drastically different. It's a general rule of thumb that houses built here in the UK from the 1920s onwards will have cavity walls i.e. walls with a space in the middle. Now some may be uninsulated, but many modern builds with have foam insulation in the cavity that helps a house retain heat.

Figuring out which walls you have can be easy. If the bricks alternate from row to row, then you're likely to have cavity insulation. Now if your wall has been covered over and you can't see the brick, just go to an outside wall with a window in it. If you can confidently say that the wall is thicker than 26cm it will have a cavity in there. If it is much thinner or clearly thicker than that, it could be a solid brick wall. And if you're wondering what a wooden frame wall is, it's just an old wall that is supported in some part by a timber frame, but this is usually only ever-present in very old homes.

Heat loves windows. They're always cold and the easiest way to escape a room. Single glazed windows are the weakest and when in rooms you use a lot, the BTU output will need to be higher to get the space warm.

Many modern homes will have double glazing which does help retain a lot of heat and bring the BTU down.

The number of walls facing the outside does factor how heat retentive a space is. In most houses, the living room will have the two walls closest to the door facing the inside of the house and two facing the outside. Those in the inside will be thinner but be adjoined by other walls with others radiators that give off heat, while the outside walls will just have insulation and then the outside elements.

The outside walls will almost always be where heat has the least trouble escaping. That's why in many homes you'll find that the living room radiator is either against an internal wall where it requires the least resistance to heat up or on the exterior wall underneath your big window where it can force a floor draught upwards towards it. This stops the room from feeling cold all the time through convection (as many people don't realise that radiators convect heat and do not radiate heat).

When you click on any radiator or towel on-site, you’ll notice the first specification we list is the BTU of the product. We use the Delta 50 (also known as T50/ Δ50 ) rating for all our products. This is the industry standard rating, which specifics that flow =75C, return =65C, room =20C. Most home heating systems in the UK follow this rating.

The Delta rating is essential for BTU calculations as it works off an expected ideal room temperature of 20°C. Any reputable store adheres to BS EN 442, and uses Delta 50 as their benchmark. If you see another online retailer listing products as Delta60 (Δ60) or Delta 70 (Δ70), they are providing an inflated BTU output (they’re upping the assumed temperature to make product performance look better).

Don’t get conned by Delta ratings

It is essential to watch out for the use of Delta 60 and Delta 70. A dishonest supplier is essentially lying by stating their radiators provide more heat output when you’d be comparing the same make and model on their site with one you’d find at Trade Radiators.

For example, a radiator listed as Δ60, when it should be Δ50, will have a higher BTU rating by 1.264 (e.g. a Δ50 radiator with a BTU of 1000 would magically now have a BTU rating of 1264). Watch out for this, or else you’ll end up with a radiator which doesn’t heat your room sufficiently.

To help make sure you don't buy a radiator that is the wrong size and also know the watts and BTU required to heat the space the radiator will go in.

Now that you have worked out what type of BTU output you need please browse through our extensive range of designer, convector, aluminium, column, traditional & bathroom radiators where you will find a BTU slider that will help you select the radiator design you want with the correct output.

We are happy to help with any query you may have.

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