BTU Calculator - Find your perfect radiator with our heating calculator

BTU & Watt Calculator

Find your perfect radiator

Select one or more from the drop downs to get started.


How to use this BTU Calculator?

Simply answer the questions below, then receive your BTU calculation for the room, and explore our personalised recommendations on the radiators that will suit your requirements.

Remember that you can either combine multiple radiators or stick to just one, to reach the BTU requirement.

What is a BTU Calculator?

Our BTU calculator is a simple conversion calculator that will accurately work out the heat output (the rate of energy) in BTU and Watts that is required to heat your home to its optimal potential. On all of the radiators that we have on offer, you’ll find these relevant figures in the specifications section of every product. 

We created this heating conversion calculator to ensure that each room can stay at a comfortable temperature without wasting energy, or money.

How to choose the right radiator?

Before selecting your radiator, it's important to know what the heat output requirements are for that room. How can you find out? Simply use our handy BTU calculator!

To find out the BTU needs of your room, simply enter the dimensions of your room and how many windows you have. The calculator will then tell you what size radiator you need to make the room cosy

Once you have the BTU calculator returns the values that you need, you can then enter them into our Find Your Perfect Radiator form and you’ll see radiators being displayed that meet your requirements.

What Will Our BTU Calculator Tell You?


Unit of power, used by the international system. Watts are used to measuring the power of appliances and calculate energy consumption. The most interesting value is the watt-hour i.e. the consumption of an appliance per hour of use (the most frequently used is the kilowatt hour equal to 1000 Watts/h).

BTU (British Thermal Unit):

A British thermal unit is a unit of measurement for energy, adopted in the United States and the United Kingdom, and currently used instead of Watts (W). A BTU is defined by the quantity of heat required to raise the temperature of 454 grams of water from 60 to 61 degrees Fahrenheit.  BTUs/h are normally used in the definition of the cooling power of air conditioning systems as well as of the heating power of fuel.

For Watts to BTU per hour conversions, 1 W is equal to 3.41 BTU/h. Therefore to convert BTU to Watts you need to divide by 3.41; to convert Watts into BTU/h you need to multiply by 3.41.

How Is A BTU Calculated?

BTUs are a calculation recognised around the world. By considering the dimensions, the windows and the type of room – this measurement can be used to work out how much energy is required to heat the room to a standard temperature (ranging between 18oc to 23oc).

Why Does The Type Of Room Matter For My BTUs?

Different rooms have different needs, which means that you need to consider what's important in each room and the rate of energy needed to heat up each room. For example, if you have a radiator in your living room but it's hidden behind a sofa, it might not be doing its job of keeping your living area warm. Kitchens are often tiled or laminated and are usually the rooms where doors in and out of meaning they’d require a higher BTU. This can also be said for hallways as they have the most exposure to the front door opening and closing letting in cold air from the outside.

Letting us know what type of room you have will help us give you a better and more accurate calculation of what you need to heat your home.

Why Does It Matter What Type Of Windows I Have?

Windows are the easiest ways for heat to escape a room. They’re normally cold as it is because of the glass.

Single glazed windows are the weakest when it comes to letting out heat, if you have single glazed windows the BTU output you require will be higher to keep the area/space warm.

Double glazed windows are definitely stronger and help keep heat in. This means it’ll bring the overall BTU needed down.

Why Does It Matter If I have Patio Doors?

We ask this because Patio doors follow the same rules as windows, where they are notorious for heat loss. Having patio doors can massively increase your BTUs per hour as the glass on these will stay cold and let out heat. 

Your Radiators BTU and Delta Ratings

When you click through to one of our radiators you’ll see a specification for a value called “Delta Ratings”. Delta T or Δt is simply the difference between the heat circulating in the radiator and the desired temperature of the room. 

We strive to have all of our radiators rated in the industry-standard delta rating of T50 although some of our suppliers such as Reina Radiators, will rate their radiators in a delta rating of T70 and this is important to know because the delta rating is essential for a BTU calculation. 

For a full in-depth guide to delta, ratings make sure to check out our blog: Delta Ratings Explained as this has everything you would ever need to know about Delta Ratings.