Loan Repayment Calculator: Repayment, Total Interest, Balance and the Effect of Extra Payments

Repayments on a fixed-rate loan, and what extra payments save.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the amount, the rate and the length of the loan.

Choose how often you pay, and an extra amount if you like.

The calculator gives the repayment, the interest and the savings.

Formulas used

  • A = P\,\frac{r}{1 - (1 + r)^{-n}}
  • B_k = P(1 + r)^k - A\,\frac{(1 + r)^k - 1}{r}
  • I_k = r\,B_{k-1}

Worked example

With the default numbers in the calculator above:

  1. Repayment each period: 2997.75 dollars
  2. Total paid over the life of the loan: 1079190.95 dollars
  3. Total interest over the life of the loan: 579190.95 dollars
  4. Balance owing after the payment number you chose: 465271.78 dollars
  5. Interest in that payment: 2329.70 dollars
  6. Principal repaid in that payment: 668.05 dollars

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Extra amount paid with each payment 500 dollars.

  1. Repayment each period: 2997.75 dollars
  2. Total paid over the life of the loan: 1079190.95 dollars
  3. Total interest over the life of the loan: 579190.95 dollars
  4. Balance owing after the payment number you chose: 465271.78 dollars
  5. Interest in that payment: 2329.70 dollars
  6. Principal repaid in that payment: 668.05 dollars
  7. Payments needed with the extra amount: 252 payments (21 years, 9 years sooner)
  8. Interest saved by the extra amount: 199501.27 dollars

Common questions

How is the repayment worked out?

It is the level payment that pays off the loan, interest and all, by the end of the term. Interest is charged on the balance each period.

Why is so much of the early repayment interest?

The balance is highest at the start, so the interest charged is highest then.

What do extra payments do?

They reduce the balance sooner, which cuts the interest charged from then on and shortens the loan.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any engineering economics or finance text (loan amortisation).

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