Amortization Table by Month Calculator

This calculator estimates the monthly payment and total interest for a fixed-rate loan, using the amount borrowed, annual APR, and term in years. It does not generate a separate row for every month; use the formula and example below to understand how a month-by-month amortization table is built.

Formula

The calculator uses the standard fixed-payment loan formula. First, the monthly interest rate is the annual APR divided by 1,200, and the number of monthly payments is the term in years multiplied by 12.

Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]

Total interest: I = M × n − P

• P is the starting principal, in dollars.
• APR is the annual interest rate, entered as a percentage; r = APR ÷ 1,200 is the monthly rate as a decimal.
• y is the loan term in years; n = 12y is the number of monthly payments.
• M is the monthly payment, in dollars, rounded to the nearest cent.
• I is the estimated total interest over the full term, in dollars. The calculation uses the rounded monthly payment.

Worked Example

Using the calculator’s defaults—$20,000 principal, 8% APR, and a five-year term—there are 5 × 12 = 60 monthly payments. The monthly rate is 8 ÷ 1,200 = 0.0066667.

M = 20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 per month, rounded to cents. Total interest is $405.53 × 60 − $20,000 = $4,331.80. These are the calculator’s estimated payment and total interest; actual lender figures can differ because of payment conventions, fees, or rounding.

How to Build an Amortization Table by Month

An amortization table separates each payment into interest and principal, then tracks the remaining balance. For every month, calculate interest on the balance at the start of that month, subtract that interest from the payment to find the principal repaid, and subtract the principal from the starting balance.

1. Start with the original loan balance and the monthly payment.
2. Calculate monthly interest: starting balance × monthly rate.
3. Calculate principal paid: monthly payment − monthly interest.
4. Calculate ending balance: starting balance − principal paid.
5. Carry the ending balance forward as the next month’s starting balance, and repeat.

For the example, the first month’s interest is $20,000 × 0.0066667, or about $133.33. Of the $405.53 payment, about $272.20 reduces principal, leaving approximately $19,727.80. In month two, interest is calculated on that lower balance, so a slightly larger share of the same payment goes toward principal.

How to Read the Schedule

A typical table has columns for payment number or date, beginning balance, payment, interest, principal, and ending balance. Early in a level-payment loan, the balance is relatively high, so interest takes a larger share of the payment. As principal falls, the interest portion generally shrinks and the principal portion grows. The payment itself remains level when the rate and scheduled payment terms stay fixed.

The calculator provides summary figures rather than all 60 rows. A spreadsheet can generate the complete schedule by applying the monthly steps above. Keep the unrounded balance in calculations when possible and display currency to cents; rounding every intermediate value can create a small final-balance discrepancy. Lenders may apply their own rounding and payment-date conventions.

When This Estimate Helps—and Its Limits

A monthly amortization schedule can help explain how a fixed-rate installment loan is repaid and how the balance changes over time. It can also show why the amount of interest in a payment is not constant. The calculation is useful for examining a loan’s arithmetic when you already have the principal, APR, and term, but it is not a quote or an approval estimate.

This formula assumes equal monthly payments and a constant rate. It does not model a variable rate, skipped or late payments, a payment made on an unusual date, an interest-only period, a balloon payment, or extra principal payments. It also does not include fees, taxes, insurance, PMI, or escrow. Such items may affect the amount paid or the lender’s disclosed figures, but are not part of this calculator’s inputs.

Common Questions and Mistakes

• APR entered in the wrong format: Enter 8 for 8%, not 0.08. The formula divides the entered percentage by 1,200 to obtain a monthly decimal rate.
• Confusing payment with principal reduction: The entire payment does not reduce the balance; monthly interest is paid first in this standard schedule.
• Assuming the last row will be exactly zero: Cent rounding and lender-specific calculations can leave a tiny residual in a hand-built table. A real final payment may be adjusted.
• Assuming extra payments are included: They are not. Extra payments change the balance path and may change the payoff date or interest total.

Assumptions

• The interest rate is fixed for the full loan term.
• Payments are equal, made monthly, and applied at the end of each period.
• Interest compounds monthly at APR divided by 12.
• Fees, taxes, insurance, PMI, and escrow are not included.

Frequently Asked Questions

How do I calculate the interest for one month?
Multiply the balance at the beginning of the month by the monthly rate, which is APR divided by 1,200 when APR is entered as a percentage. The result is that month’s estimated interest charge.

Why does the principal portion increase each month?
With a fixed payment and fixed rate, the balance declines over time. Interest is calculated on that declining balance, so less of each later payment goes to interest and more goes to principal.

Does this calculator show every month’s balance?
No. It estimates the scheduled monthly payment and total interest. To create every row, repeat the monthly interest, principal, and ending-balance calculations for each payment period.

Does an amortization table include extra payments?
Not in this calculator. To model extra principal, add the extra amount to the principal paid in the relevant month, reduce the balance accordingly, and recalculate later rows. Confirm how a lender applies additional payments.

Why might my lender’s schedule differ?
Differences can result from rounding, payment timing, fees, a rate that changes, or terms not represented by this fixed-rate formula. Check the loan documents or ask the lender for its payment schedule.

This result is an estimate, and the lender’s disclosure gives the exact figures.

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2