Home, car, renovation, or business — loans are part of life. But many people sign knowing only "the monthly payment", not how much interest they'll really pay. Let's make loan math fully transparent.
The Core Factors in Loan Interest
Total interest depends on three variables: loan amount, loan term, and annual interest rate. Your repayment method (equal installment vs equal principal) also directly affects total interest.
The Equal Installment Monthly Payment Formula
Monthly payment = Principal × [Monthly rate × (1+Monthly rate)^Periods] ÷ [(1+Monthly rate)^Periods − 1]
Where monthly rate = annual rate ÷ 12, and periods = loan years × 12. Equal installment keeps every payment the same; early payments lean heavy on interest, later ones on principal.
Equal Installment vs Equal Principal
| Method | Character | Total interest | Best for |
|---|---|---|---|
| Equal installment | Fixed monthly payment | Higher | Stable-salaried workers |
| Equal principal | Declining monthly payment | Lower | Those with higher early income |
Worked Example
A $200,000 loan at 5% annual rate over 5 years (60 periods):
With the loan calculator: equal installment monthly payment ≈ $3,774, total interest ≈ $26,400; equal principal starts ≈ $4,167 and declines, total interest ≈ $25,400.
Four Ways to Save on Interest
- Shorten the term: same principal, 5 years vs 10 years can more than halve total interest
- Prepay when you can: partial prepayments shrink the principal base
- Pick the right method: if you can handle early pressure, equal principal saves more long term
- Shop the rate: rates differ meaningfully across lenders and timing — compare scenarios with the compound interest calculator
Enter your loan details in the loan calculator to see principal and interest on every single payment — and decide with clarity.