Mortgage comparison & overpayment calculator

Compare multiple UK mortgage products side-by-side and model overpayments. See the true cost over your fix, the principal you actually repay, and — if you overpay — how many months come off the term and how much lifetime interest you save. The "total cost" figure lenders quote hides both effects.

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How to read the comparison

  • Monthly payment — what you actually pay each month during the fixed period.
  • Upfront cost — arrangement / product fees you pay on completion, minus any cashback. Zero if you opted to add the fee to the loan.
  • Starting loan — your purchase price minus deposit. If you added the fee to the loan, this is higher (and you pay interest on the fee for the whole term).
  • Paid over horizon — sum of monthly payments inside the horizon (usually the fixed period).
  • Interest in horizon — of that amount, how much was interest.
  • Principal repaid — the rest. Higher is better. This is the equity you've bought.
  • Balance at horizon — what you'll owe when the fix ends. This is what gets remortgaged onto the next product.
  • True cost over horizon — upfront fees + payments made. Equal to the "total cost" lenders quote.

The hidden-principal trap

If product A costs £73 less than B on the lender's "total cost" line but also repays £312 more of your loan, A's real advantage is closer to £385, not £73. The lender's framing treats interest paid and principal paid identically — they aren't. Principal is equity in your house; interest is rent paid to the bank.

Modelling overpayments

Each product card has an "Overpayments" toggle. Two inputs to play with:

  • Monthly overpayment — extra principal you add to every payment, indefinitely. £100/month on a £290k 25-year mortgage at 5% saves around £23,000 in interest and shortens the term by roughly 2 years. £200/month saves ~£46k and ~4 years. The effect is non-linear because compounding works in your favour.
  • Lump sums — one-off payments at specific months. A £10,000 lump sum at the start of year 3 saves ~£20k over the life of a £290k mortgage at 5%. The earlier the lump, the more interest you avoid.

The annual cap field reflects what your lender allows without an early-repayment charge — typically 10% of the outstanding balance per year on a fix. Set it to 0 to remove the cap (e.g. if you're on a tracker with no ERC).

Comparing two products with the same overpayment schedule shows whether the lower-rate-with-fee product saves you more once you factor in the extra principal that overpayments accelerate. Spoiler: it usually does, because lower rates compound faster on the principal you've already paid off.

The maths

Monthly payment uses the standard amortising formula:

M = P × i × (1+i)n / ((1+i)n − 1)

where P is the starting loan (including any added fee), i is the monthly interest rate (annual ÷ 12) and n is the total number of payments. Remaining balance after k months follows:

B(k) = P × (1+i)k − M × ((1+i)k − 1) / i

This is the same formula every UK lender uses. Real-world differences are sub-£50 from day-count conventions (365 vs 360) and first-payment proration — they don't change the comparison.

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General-information tool, not a mortgage offer or financial advice. Your actual rate depends on lender, LTV, credit profile, product fees and stress-test outcomes. Speak to a regulated mortgage adviser before committing.