how to calculate home loan interest step by step

What interest really means

Home loans are usually amortized, so each payment covers both interest and principal. Interest is simply the cost of borrowing, charged on the outstanding balance and reset each period.

The calculation in plain words

Inputs you need

• Principal (P): the amount borrowed.
• Annual rate (APR): quoted percentage.
• Term (n): total number of payments.
• Frequency: monthly in most mortgages.

Steps

1. Convert APR to a periodic rate: r = APR/12 for monthly.
2. Find the payment: Payment = P × r × (1+r)^n ÷ [(1+r)^n − 1].
3. Compute interest for the next payment: Outstanding balance × r.
4. Principal paid equals Payment minus Interest; reduce the balance and repeat.

Example: P = 300,000, APR 6%, 30 years (n = 360). r = 0.06/12 = 0.005. Payment ≈ 1,798.65. First month interest is 300,000 × 0.005 = 1,500; principal ≈ 298.65.

Practical tips

Check whether your lender uses daily compounding, and try small extra payments; even one extra monthly amount per year can cut years off your loan and reduce total interest dramatically.