Loan Payment Calculator — monthly installment, total interest, year-by-year amortization
Enter the loan amount, annual rate and term to get the equal-payment monthly installment, total interest, an equal-principal comparison, and a year-by-year breakdown of how much you are actually paying off. Runs entirely in your browser.
Equal installment, year by year — early years are almost all interest:
| Year | Paid in year | of which interest | of which principal | Balance left |
|---|
Where the monthly payment comes from
What the monthly number is made of
An equal-installment loan flattens principal and interest into one horizontal line: you pay the same amount every month, but the mix inside that amount shifts continuously. Interest each month is simply outstanding balance × monthly rate, which is why the first few years are dominated by it.
The monthly rate is the annual rate divided by 12 (a 6.5% APR becomes 0.5417% per month), matching how most loan agreements quote it:
payment = principal × r × (1+r)^n ÷ ((1+r)^n − 1)
At a 0% rate that denominator is zero, so the calculator falls back to a straight split of the total. That fallback is also worth remembering when a “0% interest” installment plan quietly charges you somewhere else.
Equal installment vs equal principal
The second row of cards puts both side by side, and the decision reduces to two numbers:
- How much higher the first payment is. Equal principal starts noticeably larger. If your budget is already tight, that gap is the approval threshold.
- How much interest you save. That is the “interest saved” card. The saving is real, but it is bought with heavier payments in the early years.
The trap: stretching the term often raises the installment only slightly while doubling the total interest. Which is why the table below matters more than the monthly figure alone.
Reading the year-by-year table
Each row is one year of payments split into interest and principal. On long loans the “of which interest” column stays bigger than the principal column for the first several years — you made 12 payments, and the bank is barely closer to being repaid.
This is the whole reason “is early repayment worth it” is even a question: interest already paid is not refunded. Paying off early saves the interest in the remaining years, so judge the decision on the tail of the table, not on what you have already paid in.
When this estimate will be wrong
- Variable rates. If your rate resets annually, this is a snapshot. Re-run it after each reset.
- Split funding. A loan blended from two products with different rates must be calculated twice and added.
- Upfront costs. Down payment, insurance, stamp duty and origination fees are not in here.
- Day-count conventions. Lenders may charge by actual days, round the first partial month differently, or use a 360-day year — expect a few units of drift.
Questions
Why is my bill a little different from this? Real statements often use daily interest and their own rounding. The gap is normally under 1%.
Months or years for the term? Months, always. 20 years is 240.
Can I enter a 0% rate? Yes. Total interest becomes zero and both methods agree, so the “interest saved” card reads zero.