£400,000 at 4.92% over 25 years · £100 a month extra

£100 a month clears it 1 year 11 months early

You'd pay £2,420 a month instead of £2,320, and save £26,207 in interest.

Interest £295,926 → £269,719Cleared in 23 years 1 monthPayment £2,320

Every £1 off the balance saves you 4.92% a year, guaranteed and tax-free.

Interest falls from £295,926 to £269,719. Green is what overpaying saves.

Or invest it instead

Investing must beat

4.95%

a year, after charges and tax

Overpay, then invest

£60,904

clear it 1 year 11 months early, then invest the whole payment

Invest instead

£111,557

keep the mortgage, invest the £100 from the start

Both end after 25 years with the mortgage gone, if returns average 9.3% a year — a guess, unlike the figure above it. Change it in the form below. Where 9.3% comes from and what it hides.

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What this assumes

Saving £26,207 in interest is certain. The investing figure is not.
Assumes 4.92% for the whole 25 years. Almost everyone is on a two- or five-year fix.
Interest charged monthly on the outstanding balance. Some lenders charge daily, which makes overpaying slightly better than shown.
Nothing here is advice. An emergency fund, your pension and any dearer debt all come first, and none of them are in this calculator.