Loan amount (Principal)
The amount you are borrowing from the lender. Also called the principal. Interest is charged on whatever balance is outstanding each month.
Loan basics
Interest rate
The annual percentage rate (APR) charged on the outstanding balance. The calculator divides this by 12 to compute the monthly interest. If your actual mortgage is a fixed-rate deal, you can use the remortgage feature to switch to a different rate later.
Term
How long you have to pay the loan off, in years. A longer term reduces your monthly payment but increases total interest paid over time.
Start date
The month the first mortgage payment is due. The calculator uses this to label the schedule and project your payoff date.
Repayment type
Repayment (Principal + Interest): each monthly payment pays off some interest and some of the principal, so the balance shrinks every month until it hits zero at the end of the term.
Interest only: each payment only covers the interest. The principal stays the same until the end of the term, when it must be paid in a lump sum. Usually only used for buy-to-let or bridging loans.
Monthly payment
The fixed amount due each month on a repayment mortgage. Calculated so that the loan is paid off exactly at the end of the term (assuming the rate does not change).
Overpayments
Recurring overpayment
An extra amount paid every month on top of your regular payment. Because it goes straight to principal, it reduces the balance faster and saves interest over the life of the loan. You can also choose when the recurring overpayment starts — useful if you want to wait until after a fixed-rate period ends.
One-off overpayment
A lump sum paid at a specific month (for example, a bonus or inheritance). Each one-off payment is applied entirely to the principal at that month, immediately reducing the interest charged from that point onwards.
Interest saved & time saved
The calculator compares two scenarios: your loan with overpayments and the same loan without. The difference is the total interest you avoid and the number of months you shave off the term.
Advanced features
Remortgage schedule
At a future month, switch to a new interest rate and/or term — exactly like signing a new fixed-rate deal when your current one expires. You can optionally add arrangement fees, which are rolled into the balance. After a remortgage the monthly payment is recalculated for the remaining balance and term.
Payment holiday
A stretch of months where you pay nothing. Interest still accrues during that time and is added (capitalized) to the balance, so the loan becomes slightly more expensive afterwards. Many UK lenders allow short holidays for borrowers in financial difficulty.
Offset savings
Money in a linked savings account reduces the portion of the mortgage that accrues interest. If you have £10,000 offset against a £200,000 loan, you only pay interest on £190,000. The offset does not pay off the loan — it just cuts the interest bill while the savings sit there.
Loan-to-value (LTV)
The mortgage amount expressed as a percentage of the property's value. Lenders offer better rates at lower LTV bands (e.g. 60%, 75%, 85%). Enter your property value to see your starting LTV.
Real-terms total cost
Future payments are worth less than today's money because of inflation. Entering an assumed inflation rate discounts each payment back to today's value (net present value), giving the real cost of the loan in 2020-something pounds.
Early repayment charge (ERC)
Most fixed-rate deals let you overpay up to a certain percentage of the balance each year (typically 10%) without penalty. Overpay more than that and the lender charges an ERC — usually 1–5% of the excess. The calculator estimates total ERCs across the life of your overpayment plan.
Charts & outputs
Balance chart
Shows the outstanding balance each month for the baseline scenario (no overpayments) against your actual plan. The gap between the two lines is money you no longer owe thanks to overpaying.
Principal vs Interest
A stacked monthly bar showing how much of each payment goes to principal versus interest. Early on, most of your payment is interest; over time the principal portion grows. This is known as the amortisation profile.
Cumulative interest
Running total of all the interest you have paid so far. Useful for seeing how overpayments bend the curve.
Total paid breakdown
The stacked bar under the chart shows how the grand total splits across regular payments, recurring overpayments, and one-off payments — so you can see exactly where your money is going.
Amortisation schedule
The month-by-month (or year-by-year) table showing every payment: how much interest, how much principal, any overpayment applied, and the remaining balance. Export the schedule as CSV to work with it in a spreadsheet.
How the maths works
Standard monthly payment
The calculator uses the standard amortisation formula:
M = P × r / (1 − (1 + r)^(−n))
Where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments.
Monthly interest
Interest for the month = outstanding balance × monthly rate. With an offset account, the balance used for interest is reduced by the offset savings (but the actual balance is unchanged).
Assumptions & limitations
This is an estimation tool. It assumes the interest rate is constant between remortgages, interest is compounded monthly, and it does not include fees, taxes, insurance, or changes to your circumstances. Always confirm numbers with your lender before making financial decisions.