This is the million-dollar question. Different names are given to bed sizes around the world so depending on where you are a King-Sized bed might be the same as a Queen-Sized bed or a Super-King-Sized bed. It’s very confusing.
Stick with this as a guide:
In the UK, a ‘King-Sized’ bed (which is known as a ‘Queen-Sized’ bed in America) measures about 150cm (width) x 200cm (length) whilst a ‘Super-King-Sized’ bed (which is sometimes known as a ‘Queen-Sized’ bed in the UK) measures about 180cm (width) x 200cm (length). Beware that if you buy your mattress in a European shop then they may use European measurements to define their ‘King-Size’/’Queen-Size’ beds that will differ to UK measurements (that’s fine if you’re buying bed and mattress together, but if you buy a mattress there to fit on your bedframe at home it probably won’t fit).
Which one you choose rather depends on your circumstances. If you have children and pets who tend to join you in the night and sleep diagonally or sideways, then buy the biggest bed you can find that will fit in your room (you’ll probably still end up on the outermost 10cm though). If you don’t have the space to fit in a Super-King bed without having to hang part of it out of the window, or if you have wardrobes that you might need to open every now and again, then you may have to opt for a King-Sized bed instead.
If you have room for a King-Sized bed in your spare room then great; your guests will be very comfortable, and it’s even better if you can fit a Super-King in your main bedroom as the largest bed in the house is normally reserved for the master bedroom. Super-Kings are more expensive, and of course once you’ve bought the bed you’ll have to buy mattresses to fit in the future when your existing mattress is ready to retire.
Sleeping in a Super-King-Size bed might not suit people who like to snuggle up to their partners because there can be quite some space that opens up between you in the night. On the other hand, if you’ve been married for a while and your partner’s every move annoys you because it makes you wake up then a Super-King is great because you both have plenty of personal space.