Calculating BTU, am I doing something wrong?

24 Jan 2019
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United Kingdom
I'm going to replace all of the old rads in my house. I'm using B&Qs BTU calculator here:

As an example result it says my hall, measuring 4.93m long x 1.91m wide x 2.27m high has a BTU requirement of 1731.

I can find a rad here with 1691 BTU output:

This leads to my question: The rad in the link is 500x400 double panel. The current rad is an 1120x750 double panel! So have radiators improved that much in the last 'X' years, or am I doing the numbers wrong?
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Have a look at the pic and judge for yourself. Sf haven't sold barlo for a while now and the company has now gone bust.
Thanks, but that doesn't tell me whether I've done my calculations right or wrong, or made a mistake somewhere else down the line?
radiator specifications are overly generous in the output they quote. Look at the radiator temperature they use. They also ignore occasional cold snaps when there may be a 30C or 35C difference or more between outdoor and desired indoor temperature.

As you are not a builder trying to boost your profit margin, but a householder wanting to be comfortable, feel free to have longer rads than the calculator suggests. It will enable you to run them warm rather than boiling, and lead to greater comfort. A long radiator also spreads the heat round the room better.

All mine are 50% oversized because the house used to be empty all week and I wanted it to warm up fast. Most of the time they tick along as warm as a teacup.

Proper heating controls and TRV s will prevent any room getting overheated and wasting energy.

BTW 1731BTU is equivalent to a 500W electric heater. Ample for a WC, too small for a bathroom or a tiny bedroom.

My hall has an 1800W radiator in it (BTUs are archaic. Heating salesmen quote them because the numbers sound more impressive that Watts or kW.)
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