Descaling a kettle without buying anything
Written down because I look this up roughly twice a year and always half-remember the proportions.
Fill to about a third with equal parts water and plain white vinegar. Bring it to a boil, switch off, leave it an hour. Two hours if the scale is thick enough to feel with a fingernail. Pour it out, rinse three times, then boil a full kettle of clean water and throw that away too.
The third rinse is the one people skip. Skip it and the next cup of tea tastes like a chip shop.
Citric acid works faster if there happens to be some in the cupboard — a tablespoon in a half-full kettle, same boil-and-wait. It leaves less smell. But I never have any when I need it, and vinegar is always there.
Do not do this with a kettle that has an exposed heating element unless the manual says it is fine. Ours did not, and the element now has a dull patch that will not polish out.