Mortgage Overpayment Calculator

Who this is for

What overpaying does to your mortgage. Enter the balance, your rate and the monthly payment from your own statement, then try a regular extra amount, a one-off lump sum or both on each line. The sheet works out how many months each scenario takes to clear the balance and what it saves in interest against carrying on as you are.

Homeowners with some spare money each month or a lump sum to hand, deciding whether to put it against the mortgage, and anyone checking a lender's overpayment figures against their own.

What this helps you track

• How many months the mortgage has left if nothing changes
• The interest still to pay as things stand
• What a regular overpayment, a lump sum or both do to the months left
• The interest each scenario saves against the baseline
• Several amounts side by side, so you can see where the saving levels off

How to use it

1. Write the lender and copy the balance, your interest rate and the monthly payment from your latest statement.
2. Check the baseline months in the summary against the years left on your term; if they are far apart, recheck the figures.
3. Give each scenario a row: a name, an extra monthly amount, a one-off lump sum paid now, or both.
4. New payment, months to clear, interest and the savings are worked out in the browser and the workbook.
5. Compare the rows, then check any overpayment limits and charges in your own mortgage terms before paying anything extra.

Formats

• Printable PDF (A4 and US Letter)
• Fillable PDF with form fields
• Fill in online, saved in your browser
• Excel / Google Sheets workbook

The printable is laid out inside a box that fits both A4 and US Letter, so the two files are identical and neither one is scaled. The fillable PDF has real form fields. The workbook uses plain formulas only — SUM, IF, COUNTIF and the like — so it opens in Excel, LibreOffice and Google Sheets without anything breaking, and every calculated cell is grey so you can see which figures it worked out.

How to print

Download the A4 or US Letter file to match the paper in your printer, open it in a PDF reader and print at 100% — not "fit to page", which shrinks the writing space. Both files leave a 14 mm margin, which clears the unprintable edge on home printers. To print the online version with your entries in it, use the Print this sheet button, which prints only the sheet and none of the page around it. More on printing.

Questions

How accurate is it?

It assumes one rate for the rest of the term, interest worked monthly at your yearly rate divided by twelve, and a lump sum paid today. Lenders calculate in their own way, so treat the results as a close estimate and ask yours for exact figures.

Why are the months shown with a decimal?

The last payment is usually smaller than the others. A result of 212.4 means 212 full payments and a smaller one in month 213. The interest figures allow for that.

What if my rate changes when a deal ends?

The sheet uses one rate throughout. Run it with the rate you expect after the deal, or fill a second sheet from the balance and rate at the point the deal ends.

Can I overpay as much as I like?

That depends on your mortgage. Many deals limit overpayments or charge for going over a limit. Check your own offer or ask the lender; this sheet assumes no charge.

Why does a row show a dash?

The payment does not cover a month's interest on the balance, so the mortgage never clears on those figures. Usually one of the figures at the top has been mistyped.