Loan-to-value, usually shortened to LTV, compares what you borrow with what the home is worth. Divide the loan by the value and multiply by 100: a £255,000 mortgage on a £300,000 flat is 85% LTV. The deposit is the other side of the same number, so a 15% deposit and an 85% LTV describe one purchase.
It is the figure most mortgage rules hang on. In Ireland the Central Bank requires a deposit of at least 10% from people buying a home to live in and 30% from buy-to-let buyers, which is a 90% or 70% ceiling, with a share of each lender's loans allowed above it. New Zealand works with speed limits instead: the Reserve Bank lets banks make up to 25% of new owner-occupier lending above 80%, which New Zealanders call the LVR, and 10% of investor lending above 70%. In Sweden the belåningsgrad sets how fast you must repay: above 70% of the home's value you amortise at least 2% of the loan a year, according to Finansinspektionen. In the United States a conventional loan with less than 20% down normally carries private mortgage insurance, as the CFPB explains.
LTV is not the same as the loan-to-income ratio. One measures the loan against the security, the other against what you earn, and a lender checks both. LTV also moves after you buy: each repayment lowers it, and so does a rise in the home's value, while a fall in prices pushes it up.
The mortgage calculator works out the LTV from your price and deposit, uses it to decide whether mortgage insurance applies and which premium band you fall in, and shows the result in the derivation under that line.