A mortgage calculator is usually a tool rather than a regulated service, so it may not need FCA authorisation. However, if the website offers to arrange a mortgage or give personalised advice, check the firm’s name and reference number on the FCA Financial Services Register at register.fca.org.uk, and confirm it has permission for mortgage advice or arranging. Treat calculator results as estimates, because lenders assess your income, outgoings, credit history, deposit and affordability under their own criteria. If you are unsure, speak to an FCA-authorised mortgage adviser and verify their details on the Register.