Immersion Heater Element size

[Marka]

Hi, I need to replace a clapped out immersion heater element in my hot water tank but I'm unsure as to the correct replacement size.
The current element has a length of 16 inchs overall with the actual heater element being 13inches.
any ideas as to the replacement size I should get?
Thanks in advance of any help.

 

[Chalked]

If it fitted to the side, it will be a 12 inch. If it's on the top it will be 27 inch.
If it's a pressurised cylinder, get a G3 registered plumber to get a new one, as they ar diferent, depending on manufacturer.