BTU Calculator: How To Choose the Right Radiator

Contents:

• BTU meaning
• What size radiator do I need?
• How to estimate BTU for your room
• Choosing the right radiator based on BTU
• Key takeaways
• FAQs

BTU ratings are used with radiators to make it easy to find the right radiator for any particular room.

Understanding how BTU ratings work is actually easy. Radiators are given a BTU rating to tell you how much heat they produce. All you have to do is find out the BTU output you need to heat a room and then find a radiator with the right BTU rating for the room. There are a few calculations involved, but the idea is simple.

In this guide, we’ll tell you everything you need to know about BTU ratings. We’ll explain exactly what the term BTU rating means and how to work out what size radiator you need for a room with a BTU calculation.    

If you’re doing a radiator installation in your home, make sure you understand how BTU ratings work before you choose your radiator.

The meaning of BTU is British thermal unit (BTU).

What is BTU? BTU is a measurement of how much energy it takes to heat something up by a certain amount. To be exact, 1 BTU is the amount of energy it takes to heat up 1lb of water by 1ºF.

With radiators, BTU measurements are used to tell you how much heat a radiator will produce per hour. All plumbed-in radiators are manufactured with a BTU rating.  If, for example, a radiator has a rating of 4000 BTU, this means it can output 4000 BTU worth of thermal energy per hour.

Roughly speaking, 1 BTU is the equivalent of burning a single match.

You use the BTU rating on a radiator to match it with a room. You just need to know what BTU output you need to heat the room, and then you find a radiator with the right BTU rating. We’ll explain how you calculate BTU output for a room next. 

You work out what size radiator you need by working out the BTU output needed to heat a room. This stage does require a few calculations, and we’ll explain how the calculations work.

In reality, you don’t actually need to make the calculations yourself. There are plenty of online BTU calculators you can use. You just input the relevant information into the calculator.

Nonetheless, here’s an explanation of how the calculations work.

The BTU output required to heat a room depends on:

• The temperature you want to heat the room to

• Room size and rate of heat loss

 

Temperature target

You need to know what temperature you want to heat your room to.

Deciding what temperature you want to heat a room to does require some consideration. You might want to keep your living room or your bathroom at a slightly warmer temperature than other rooms, for example. Likewise, you might want to keep rooms like your hallway slightly cooler.

 

Room size and rate of heat loss

The size of the room and the rate at which heat can escape are crucial for working out what BTU output you need to heat your room to your desired temperature.

Room size is an important fundamental factor:

• Room size (width, length and ceiling height) – the bigger the room, the more heat you need to warm it up

The rate of heat loss is slightly more complicated, and it will vary from room to room. However, you can work things out by looking at all the main factors that affect heat loss from a room:  

• Window size, type and quantity – the number of windows, the size and the type (e.g. double or single glazed) all affect heat loss. Windows can be an escape route for heat, although modern windows are much better insulated than older ones. 

• Insulation quality – the quality of insulation has a major effect. You’ll need to consider insulation in the floor, walls and ceiling.

• The number of external walls – external walls are colder, drawing heat out of the room.

• What floor the room is on – if there’s a heated room above, this will help to keep the room warm.

• External doors – external doors can cause heat loss 

You calculate the BTU output requirement by deciding your target temperature, measuring your room and gathering all of the information on the factors that affect heat loss.

Then you can put all of this information into a BTU calculator or use another set BTU formula.

 

Step 1. Measure the room's length, width and height

Get your measuring tools out, and measure the length, width and height of your room. Use this to work out the total volume in cubic metres.

Big rooms often need more than one radiator. Having more than one radiator does more than just provide more power. Often, you need more than one radiator to achieve good heat distribution.

 

Step 2. Count the number of windows, measure them and check the type

Count all your windows, and use your tape measure to measure the height and width of the glass in all the windows. Use this to work out the total square metres of windows in your room.

Also, check whether you have single, double or triple-glazed windows.

 

Step 3. Count external walls and check insulation

Count the number of external walls and find the insulation type in them from the following choices:

• Solid brick

• Insulated cavity wall

• Uninsulated cavity wall

 

Step 4. Check floor insulation in downstairs rooms

If the room is a downstairs room, you need to check the level of insulation in the floor. Choose from the following:

• Suspended timber

• Solid concrete

• Insulated concrete

• Beam and block

 

Step 5. Check ceiling insulation in top floor rooms

If the room is on the top floor, you need to check the insulation in the ceiling to account for heat loss through the ceiling. Choose from the following types of insulation:

• Mineral wool

• Loft insulation boards

• Multi-layer insulation

 

Step 5. Check for external doors

Finally, if the room has an external door, mark this down. 

Once you’ve calculated the BTU requirement for a room, you can buy a radiator.  

The best thing when you’re buying a radiator for a room is to buy one that matches the BTU requirements. If you can’t find an exact match, get one with a slightly higher BTU. You can always turn the heat down slightly.

Be aware that some rooms need more than one radiator. It might be that there isn’t a radiator big enough to heat a large room or one with high heat loss. Some rooms also need more than one radiator to help with heat distribution.

If you have more than one radiator in a room, the BTU of all the radiators should add up to the BTU requirement of the room.

Radiator placement is another important consideration. Here are some of the best places for radiators:

• Under windows

• On the coldest external walls

• Opposite doorways

BTU ratings aren’t easy to understand when you first come across them. However, the idea behind them is simple. BTU ratings are a simple number used to match radiators with rooms according to heating requirements. 

What a BTU rating is exactly is a figure of how much heat a radiator produces per hour. Once you know what the BTU requirements are for a room in a house, you can easily work out what size radiator you need. You just match the BTU output of a radiator to the BTU requirements of a room.