Two answers, and they stand on different money
Ask what rent you can afford and you get two kinds of answer. The rule of thumb takes a share of your pay before tax, usually thirty per cent. The budget answer starts from what actually reaches your account and takes off everything already spoken for. Both are in use, they rarely agree, and the difference between them is not small.
This page gives both and labels the base of each, because that is where the confusion lives. Thirty per cent of gross pay can be forty per cent or more of take-home pay, depending entirely on where you are taxed. Someone applying the familiar rule in a high-tax country and someone applying it in a low-tax one are doing two different things with the same sentence.
When both halves are filled in, the answer is the lower of the two. A limit that binds is the one that binds.
The rule of thumb, and where it came from
The thirty per cent figure is a convention with a long history in housing policy, and its usefulness is that it is a single number everyone has heard of. Its weakness is that it ignores everything that makes one household different from another: how many people the income supports, whether there is a car loan, whether the city has transport worth using, whether utilities are included in the rent.
It is also a share of gross pay, which means the same percentage leaves wildly different amounts behind in different tax systems. Treat it as a starting point and a sanity check, not as a budget.
The budget answer, which is yours
The second half is simple arithmetic on real numbers: take-home pay, less the commitments you already have, less what you intend to save. Whatever is left is what can go on rent.
Two warnings about it. The commitments line should include the payments that do not vary, so debt payments, insurance, transport and childcare; leaving out an expensive one produces a confident number that is wrong. And treating savings as something to do with whatever is left over rather than as a line in the budget is how people rent well and save nothing for years. Putting it above the rent line is the point of that field.
If the commitments and savings already use the whole packet, the page says so rather than printing a negative rent. That is a real answer, and it means something has to change before a tenancy is signed.
What rent does not cover, and what the page cannot see
Rent is rarely the whole cost of a home. Utilities, broadband, contents insurance, any service or local charge the tenant pays, and the cost of getting to work all come after it. A cheaper flat an hour further out is not always cheaper.
The page also knows nothing about what a landlord will accept. Many require income of some multiple of the rent, or a guarantor, or several months up front; those tests use gross income and their own multiples, and they are not affordability so much as a filter.