Hong Life CalculatorsUnderstanding the loan interest-to-principal ratio calculator — concept and principle
The monthly payment is what most borrowers look at, but the total interest over the life of a loan is often a surprise. This calculator shows the interest on an equal-payment (amortizing) loan as a percentage of the amount borrowed, the total you repay, and how much of all your payments goes to interest. It is useful when choosing a loan term or deciding whether extra payments are worth it.
How it is calculated
With loan amount P, monthly rate i = annual rate ÷ 12 and number of payments n = years × 12:
- Monthly payment a = P × i ÷ (1 − (1 + i)^(−n))
- Total repaid = a × n
- Total interest I = a × n − P
- Interest as % of principal = I ÷ P × 100
- Interest share of all payments = I ÷ (a × n) × 100
The principle
Each month, interest is charged on the balance still owed, and the rest of the payment reduces that balance. Early in a long loan the balance is large, so most of each payment is interest and the principal falls slowly. Because the balance stays high for longer, interest keeps accumulating. Doubling the term does not simply double the interest; it more than doubles it, since the balance is paid down later. A higher rate has the same effect. The ratio sums this up in one figure: how much each unit borrowed costs in interest by the end.
Worked example
- Loan $200,000, 6% a year, 30 years: i = 0.005, n = 360.
- Payment a = 200,000 × 0.005 ÷ (1 − 1.005^(−360)) ≈ $1,199.10.
- Total repaid = 1,199.10 × 360 ≈ $431,676.38.
- Total interest = 431,676.38 − 200,000 = $231,676.38.
- Ratio = 231,676.38 ÷ 200,000 ≈ 115.8%: every 1 borrowed costs about 1.16 in interest.
- Interest share = 231,676.38 ÷ 431,676.38 ≈ 53.7% of everything you pay.
How to read the result
Above 100% means you pay more in interest than you borrowed. For the same $200,000 at 6%, a 15-year term brings the ratio down to roughly 52%, at the price of a higher monthly payment. Use the ratio to compare terms and rates side by side, but weigh it against what the lower monthly payment lets you do, such as keeping an emergency fund or investing.
Common mistakes
- Assuming interest is simply rate × years × principal; that is simple interest on the full balance and overstates the cost.
- Comparing loans only by monthly payment.
- Ignoring fees and insurance, which raise the real cost.
Frequently asked questions
How much interest is paid on a 30-year loan at 6%?
About 116% of the amount borrowed, or $231,676 on $200,000.
How can I reduce the ratio?
Choose a shorter term, find a lower rate, or make extra principal payments early in the loan.
Why is the interest share lower than the ratio?
The share divides interest by everything repaid, including the principal, so it can never reach 100%.
Sources: Standard time value of money mathematics (amortizing loan payment) · U.S. Consumer Financial Protection Bureau, consumer resources on loan costs and amortization