How to Calculate Heat Requirements for your Home

Somehow a whole fortnight has gone by since my last blog post! How has that happened? Well, I started off wanting to decorate my office (see my post on planning the home office here) and ended up signing up to get my house re-wired! There was so much to be done it made sense to do it all at once. So I’ve been trying to buy a house full of new light fittings, choose the location of every power point and light switch in my house, and had to pack up as  much of the house as possible to give the electricians space to work. That has kept me BUSY! And it’s let to a lot of interesting ‘features’ emerging in my home like this:

Re-wire in progress!

and this:

Channels for the new switches

Yep, whilst I am acquiring lots and lots of lovely power points and lights, I am also acquiring rather a lot of holes in the walls, ceilings and skirtings. And a LOT of dust. But it is a necessary evil and best done in one go if you can face it. Watch out for posts very soon on lessons learned from a re-wire and choosing light bulbs! In the meantime, you can view my post let there be light on the lighting I’ve been considering.

Now, let’s turn our attention to heating, which is actually what this post is all about!

Heating my Home: the Starting Point

I have inherited a mix of radiators in style and age in my home, ranging from this beast:

The biggest radiator in the house, in the Living Room

To this dainty little number:

The smallest radiator in the house, in the home office

I’ve also gained 2 gas fires, and a rather dated electric bar heater in the en-suite:

The en-suite bar heater

The heating works, and the weather has actually been quite mild since I moved in January, so the heating system hasn’t been tested in anger. I have noticed a few interesting things though:

  1. My first gas bill was HUGE and I have no idea where the gas went. I was hardly using the cooker and had turned off a lot of radiators. So what’s going on? I’m still not entirely sure. Maybe the next bill will help shed some light on it – slightly nervously waiting for that!
  2. The downside of having a full height hallway is, that’s a lot of space to heat and you can’t really only heat part of it! This could be part of the answer to point 1 above….
  3. I have an attic space which isn’t heated. I need to decide whether to add some heat up there or whether a plug-in fan heater as and when required is sufficient.
  4. A lot of the radiators don’t have thermostats. This might help explain point 5:
  5. The radiators heat up quite unevenly, and how hot they get is very dependent on how many radiators in the house are on. For example the en-suite radiator only heats up halfway when the other bedroom radiators are turned off. Turn them on, my en-suite radiator gets toasty warm! Then the living room radiator gets fully hot if the dining room is turned off, but if I turn on the dining room, the living room will only heat up half way….I have found similar scenarios around the house as I’ve played with different on/off combinations. This experiment could continue a while and become a minor obsession. (who am I kidding? Major obsession).
  6. Some radiators are just Not Big Enough. My poor guests over the winter had the worst of it, as my guest bedroom is cold and the radiator, although it’s new and gets toasty hot, just isn’t big enough to heat the room.

    A rather small radiator squeezed into the corner of the Guest Bedroom
    A rather small radiator squeezed into the corner of the Guest Bedroom
  7. The main house thermostat is not very responsive to changes in temperature at all.
  8. Because the house is east facing, and the large hallway has north facing windows, the interior of the house is generally pretty cool, even when it’s toasty hot outside. I can be outside in a tshirt, and immediately have to put a sweater on when I come inside. Hmmm.

The Heating Plan

Overall there are a lot of radiators in the house, they work and I haven’t been cold over the winter. The boiler is about 7 years old so should be good for a good number of years as well. But there are improvements to be made. In particular I’m thinking:

  1. As part of the rewire, that old bar heater is getting removed from the en-suite. A digital thermostat is also being added to the house so I can actually have some accurate temperature readings and hopefully a more responsive system. The rewire also caused my first plumbing emergency, as the electrician accidentally put a nail through one of the pipes. These things happen….
  2. Bleed all the radiators and see what difference that makes! An easy DIY job with a radiator bleed key or screwdriver
  3. Speak to a plumber about uneven heating – it may be the system needs flushing and/or balancing. In balancing, basically the radiator settings get tweaked to ensure they all heat evenly. This made a lot of different in my last home. It may also be that some of the older radiators are past their best and need replacing.
  4. Add thermostats to those radiators without them. I’m investigating doing that as a DIY job
  5. Check the sizing of the rooms with small radiators and if necessary, arrange for upgrades to larger models – see more below.

    The radiator in the craft/guest room – on the small side?
  6. Change the gas fire in the living room for a log burner. I’m really keen to do this and moving it up the to-do list. It’ll reduce the gas bill, and whilst I’m getting messy work done, I may as well face into this as well. Has anyone else ever started out with a plan to tackle your home in a nice orderly way and ended up with chaos in the entire house and an entirely different plan?

    The gas fire destined to transform into a log burner
  7. I’m researching underfloor heating with a view to using it in the kitchen, hallway and perhaps even en-suite when I decorate them – this is a bit more long term and will deserve blog posts all on it’s own. There are so many different options of heating, and different types of heating go with different flooring. It will take a LOT of research.
  8. Investigate insulation options. I have an old house which isn’t well insulated, mainly because the options are limited. But there is scope to investigate insulating under the house, and in the roof space I think.

Next Steps on the Heating Plan

So that’s a comprehensive heating plan, which will take some time to implement. Having tackled a couple of points with the rewire already, I thought I’d check the radiator sizes needed to adequately heat the rooms. That gives me a starting point for potential plumbing work.  Here’s my guide to how to check what you need:

How to check the correct radiator size required for a room

The easiest way to check radiator sizing is by using an online calculator. Search ‘BTU calculator’ and you’ll get a list of options. BTU stands for ‘British Thermal Unit’. Radiators in the UK are sized for how many BTUs they output.  Once you know how many BTUs a room requires, you can choose whatever style and shape of radiator you like that provides the right number. Simple!

I’m using the B&Q calculator found here: B&Q BTU Calculator. To use the calculator you need to know:

  • Size of the room: length, width AND height. Most calculators offer metric or imperial measurements
  • Number and type of external windows and doors in the room (not the doors opening into a hallway or landing). Type basically means: are they single or double glazed?
  • Number and size of current radiators. Size being: height, width, whether the radiator is a single or double panel, and whether it has any ‘fins’ behind the flat front panel.
  • What sort of room it is, as different rooms have different heat requirements. A hallway does not need to be as hot as a living room for example.

I’m checking the radiators for Bedrooms 2, 3 and 4 in my house specifically, which are all pictured in this post, and to my eye look potentially too small. So this is what I’m working with:

Room Length m Width m Height m Number of windows Size of current radiators height x width cm
Bedroom 2 (Guest bedroom) 3.6 3.1 2.6 1 60 x 55 single with fins
Bedroom 3 (Craft Room/Guest room) 3.8 3.3 2.6 1 60 x 60 double with fins
Bedroom 4 (Home office) 2.5 2.2 2.6 1 60 x 40 single with fins

I’ve used the measurements provided on the estate agent’s floor plan. In reality the sizes are a little different as the rooms have more angles to take into account. I’m in the process of drawing up a more accurate plan. Especially important is the room height – 2.6m is pretty high and will obviously need more heating!

Calculating BTU required

The next step is simple: input your room sizes into the calculator. This screen shot shows the Bedroom 2 figures:

Calculating heat requirement for the Guest Bedroom

Note l opted to size the rooms as a  lounge, as I expect to use the rooms to work in as well. I did look at the difference between bedroom and lounge sizing though, and there is  not much in it.

Next hit the ‘calculate’ button and you’ll get a BTU result:

BTU Calculator Result


I repeated this exercise for all 3 rooms and got these results:

Room Length m Width m Height m Number of windows BTUs required for room
Bedroom 2 (Guest bedroom) 3.6 3.1 2.6 1 3917
Bedroom 3 (Craft Room/Guest room) 3.8 3.3 2.6 1 4401
Bedroom 4 (Home office) 2.5 2.2 2.6 1 1930

Calculating BTUs of current radiators

Next step: Look at how many BTU the existing radiators output. I did another Google search here of ‘how many BTU does my radiator produce? Again this provides various calculators and tables. I found a suitable table on Here’s an extract from it:

Radiator sizes and heat output

What impressed me about this site was, it provides the tables for different radiator types: single, double, with or without fins… so check what type of radiator you have carefully! Then you can look up the height and width of your radiator and read across the table for a value.It’s important to note that the table provides wattage for the radiator. This needs to be converted to a BTU value. You do this by multiplying the wattage reading by 3.4.

So for my guest room, I have a radiator 60cm high x 55cm wide and it’s a single panel with fins. The table above tells me that a 600×600 radiator of that type will give out 720 watts. Multiply by 3.4 for a BTU value of: 2448! As my radiator is 5cm narrower I know it outputs less than that.

Having looked up my other radiators and done the conversion to BTU, my table now looks like this:

Room Size of current radiators height x width cm BTUs required for room BTUs of current radiator Difference
Bedroom 2 (Guest bedroom) 60 x 55 single with fins 3917 Less than 2448 Over 1469
Bedroom 3 (Craft Room/Guest room) 60 x 60 double with fins 4401 3740 661
Bedroom 4 (Home office) 60 x 40 single with fins 1930 less than 1904 Over 26

So, I now know I have quite a big issue with that cold guest bedroom, which reinforces what my guests have told me! The other rooms are not so bad but could do with upgrading.

Radiator Shopping

Many sites, like B&Q, provide the option to search on BTU requirements. For example, searching through radiators on, I can narrow the option to 2000-4000 BTU, or 4000-6000 BTU.  I’m searching for an alternative for the Guest Bedroom. If I look at the 2000-4000 range, and keep to 60cm high, even a 60×60 double or 60×1000 single aren’t big enough! If I go to a 60cm high by 1100cm wide single radiator, double the existing size, the BTU rating is 3853, which is close to the requirement. Or a 60cm high by 70cm wide double, with BTU of 4272, would be higher. So l need a radiator at least 15cm wider.

Now I have enough info that I can play with radiator sizes and room layouts to decide what will fit best. Clearly I can’t solve the problem in the space where the radiator currently is unless I go for a tall narrow style or remove the furniture fitted up against it:

Current Guest bedroom radiator, in need of beefing up – and another shot of the electrical work in progress!

The most likely solution is to move the radiator elsewhere in the room. Keep watching to see how this one turns out!

Radiator sizes explained!

So that’s how I’ve been investigating what radiator sizes I need in order to develop a heating plan for my home. Does that help you? Or do you have any easier ways to do the maths? Have you over-hauled your heating recently? Let me know and happy DIYing!