Two different questions, and they disagree constantly
"Can I afford this?" and "will I pass referencing?" feel like the same question and they are not, which is why people get caught out in both directions.
Referencing is an arithmetic test on your gross annual income, applied before anybody has looked at your actual outgoings. The usual form is around 30 times the monthly rent. It does not know about your commute, your council tax band, your student loan repayments or your childcare.
Affordability is what is left once tax, national insurance, council tax, bills, travel and food have gone. That is the number you live on.
So two things happen regularly. People pass referencing on a flat they cannot live in, because the gross figure cleared a multiple. And people who could comfortably pay fail on the multiple, usually because their income is structured oddly or they are relying on savings. This tool runs both and says which one is the problem.
The multiple is a convention, not a law
There is no statutory affordability ratio for renting in the UK. Nobody legislated 30x. Letting agents and referencing firms have converged on something around it, they differ between themselves, and some use annual rent times a factor rather than monthly rent times a multiple, which produces different answers again.
It is worth asking what figure the agent is using rather than assuming, because the difference between 30x and 36x on the same income is several hundred pounds of monthly rent. The multiple is an input here for exactly that reason.
The same goes for the "spend no more than 30% of your income on rent" guideline. It has no authority behind it at all, and it is usually quoted without saying whether it means gross or net, which are wildly different tests. The tool shows both.
The move-in cash is what stops people
Monthly affordability gets all the attention and the thing that most often prevents a move is the lump sum needed on day one.
In England the deposit is capped by the Tenant Fees Act at five weeks' rent where the annual rent is under £50,000, and six weeks above that. Add the first month up front and you are looking at well over two months' rent in a single payment.
Worth knowing: anything above the cap is not a deposit, it is an unlawful payment, and it is recoverable. If you are asked for two months' deposit in England, that is not a negotiation, it is a breach.
That cap is England only. Wales, Scotland and Northern Ireland have their own regimes, and rather than quoting England's figure at you as though it applied everywhere, this tool declines to show a cap outside England and says so.
If you are sharing, find out whether the tenancy is joint
Whether the tenancy is joint matters more than anything else here, and it is rarely explained properly.
On a joint tenancy, every tenant is liable for the whole rent, not their share of it. If one person leaves, stops paying, or simply cannot, the others owe the difference and the landlord is entitled to pursue any of you for all of it. Splitting the rent between you is a private arrangement; the contract does not know about it.
Individual tenancies, where each person contracts separately for a room, work differently and are much more common in purpose-built shared housing than in an ordinary flat share.
What a guarantor is signing up to
Usually far more than either party expects. A guarantor is typically liable for the whole rent rather than one tenant's share, and frequently for the whole length of the tenancy including renewals rather than the first year only.
They are also normally tested on a higher multiple than the tenant, because they are expected to be able to cover the rent while still paying their own housing costs. That is why a parent on a decent income can still fail as a guarantor for a flat their child could nearly afford alone.
It is worth both of you reading the deed rather than treating it as a formality, particularly the part about how long it lasts.
Common questions
How much do I need to earn to rent a flat?
Referencing usually wants gross annual income of around 30 times the monthly rent, so a £1,200 a month flat wants roughly £36,000. But that is a market convention rather than a rule, agents and referencing firms set their own, and it is a completely separate question from whether you can live there once tax, council tax, bills and travel come out.
Is the 30x rule a legal requirement?
No. There is no statutory affordability ratio for renting in the UK. It is a habit that referencing firms and letting agents have converged on, and it varies between them. Ask what multiple the agent is using before assuming, because a point either way moves the rent it allows by a surprising amount.
Can I pass referencing and still not afford somewhere?
Easily, and it happens constantly. Referencing is arithmetic applied to a gross figure before anybody looks at what goes out of your account. It does not know about your commute, your council tax band, your student loan or your childcare. This tool runs both tests separately and tells you which one is failing.
How much is the deposit?
In England it is capped by the Tenant Fees Act at five weeks' rent where the annual rent is under £50,000, and six weeks above that. Anything more than the cap is not a deposit, it is an unlawful payment, and it is recoverable. That cap is England only: Wales, Scotland and Northern Ireland run their own regimes.
How much cash do I need to move in?
The deposit plus the first month up front, which comes to well over two months' rent in one go. This is the number that stops people moving, and no monthly affordability test asks whether you have it. Plan for it separately from the rent.
What does a guarantor actually agree to?
Usually far more than they expect. A guarantor is typically liable for the whole rent rather than one tenant's share, and often for the whole length of the tenancy rather than a single year. They are also usually tested on a higher income multiple than the tenant, because they are expected to cover the rent while still paying their own housing costs.