Capital and interest.
Every monthly payment is split between interest on what is outstanding and a slice of the balance itself. The slice starts small and grows every month, which is why the elevation on the right lays its first courses slowly and its last ones fast.
£1,379 a month on this case at 4.19%, over 25 years.
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Completion. Five courses.
The £28,450 deposit is the only part of the house that is yours on the day you get the keys. Everything above it belongs to the lender until it is bought back, course by course.
Month 60. Ten courses.
Five years of £1,379 a month has bought five more. £223,774 is still outstanding, which is why the rate on the next five years matters roughly as much as the rate on the first five.
Month 120. Seventeen courses.
The curve has stopped crawling. Each payment now carries more principal than interest for the first time; the crossover on this case happens in month 110.
Month 180. Twenty-five courses.
Half the house. It took fifteen of the twenty-five years to reach it, and the remaining ten will do the other half. That asymmetry is the annuity, and it is the same shape on every repayment mortgage ever written.
Month 240. Thirty-five courses.
£75,336 left. From here the balance falls faster every month, because the interest share of a fixed payment falls with the balance it is charged on.
Month 300. Forty-eight courses.
Redemption. £162,673 of interest paid over the term, and the deeds are yours.