How to read a loan amortization schedule and check the numbers yourself

An amortization schedule is the payment-by-payment breakdown of a loan: what you pay, how much of it is interest, how much reduces the balance, and what you still owe afterwards. Once you know how a single row is built, every other row follows from it with the same two multiplications, and you can spot whether a lender’s version is doing something it should not. This guide works through a 20,000 loan at 6% over five years, shows where the numbers come from, and quantifies what an extra payment or a shorter term actually saves.

The fixed-payment formula

A standard amortizing loan charges interest on the outstanding balance each period and asks for the same payment every time. The payment that repays the whole loan in exactly n periods is:

M = P × r ÷ (1 − (1 + r)^−n)

P is the amount borrowed, r is the interest rate per period, and n is the number of payments. For monthly payments, r is the annual rate divided by 12 and n is the term in years times 12. If the rate is zero, the formula collapses to P ÷ n.

Take 20,000 at 6% for 5 years, paid monthly. r = 0.06 ÷ 12 = 0.005 and n = 60. Working it through:

1. (1 + r)^n = 1.005^60 = 1.348850, so (1 + r)^−n = 1 ÷ 1.348850 = 0.741372.
2. 1 − 0.741372 = 0.258628.
3. P × r = 20,000 × 0.005 = 100.
4. M = 100 ÷ 0.258628 = 386.656, which rounds to 386.66.

The payment is 386.66 a month. The loan amortization schedule rounds the payment to the cent at this point, and so does almost every lender.

How each payment splits

Every row applies the same two steps. Interest for the period is the previous balance multiplied by r. Principal is whatever is left of the payment after interest. The new balance is the old balance minus that principal.

Row 1: interest = 20,000 × 0.005 = 100.00; principal = 386.66 − 100.00 = 286.66; balance = 20,000 − 286.66 = 19,713.34.

Row 2: interest = 19,713.34 × 0.005 = 98.57; principal = 386.66 − 98.57 = 288.09; balance = 19,425.25.

Because interest is charged on what you still owe, and you owe the most at the start, the first payments carry the largest interest share. As the balance falls, the interest portion shrinks and the principal portion grows by the same amount, so the split slides steadily from mostly interest to almost entirely principal. On a five-year loan the crossover is early; on a 30-year mortgage the first payment can be 85% interest and the halfway point in the split arrives years in.

Payment	Payment amount	Interest	Principal	Balance after
1	386.66	100.00	286.66	19,713.34
2	386.66	98.57	288.09	19,425.25
3	386.66	97.13	289.53	19,135.72
…
59	386.66	3.84	382.82	384.49
60	386.41	1.92	384.49	0.00

Notice the last row. The final payment is 386.41 rather than 386.66 because each period’s interest was rounded to the cent along the way, and the small leftovers accumulate. The common convention, and the one the tool uses, is to let the final payment absorb the difference so the balance closes at exactly zero. Some lenders put the adjustment on the first payment instead; either way, expect one row to differ by a few cents.

Total interest and the cost of the loan

Add the interest column and you get the true cost of borrowing. Over 60 payments this loan collects 3,199.35 in interest, so the total paid is 23,199.35 on a 20,000 principal. A quick sanity check: 60 × 386.66 = 23,199.60, which is 25 cents more than the actual total because of that shortened final payment.

Total interest is the number to compare between offers, not the monthly payment. A lower payment over a longer term almost always costs more in total. The same 20,000 at 6% over seven years drops the payment to 292.17 but raises total interest to 4,542.37, a difference of 1,343 for the slower repayment.

What an extra payment or a shorter term saves

Anything paid above the required amount goes straight to principal, which means it stops accruing interest for every remaining period. The saving is larger than it looks because it compounds in your favour, the same effect the compound interest calculator shows for savings, but working against the lender.

Three ways to shorten the same loan, each computed with the schedule’s rounding:

Option	Monthly payment	Number of payments	Total interest	Interest saved
Baseline, 5 years	386.66	60	3,199.35	0
Extra 100 every month	486.66	47	2,444.36	754.99
One-off 1,000 with the first payment	386.66	57	2,865.64	333.71
Sign for 4 years instead	469.70	48	2,545.66	653.69

The recurring extra payment wins because it removes the most balance earliest. A single lump sum at the start is also efficient; the same 1,000 paid in year four would save far less, since by then most of the interest has already been charged. Before overpaying, confirm the loan has no prepayment penalty and that the lender applies extra amounts to principal rather than holding them against future payments.

What APR includes

The rate used in the schedule is the nominal annual rate, sometimes called the note rate. APR, the annual percentage rate, folds compulsory fees into the calculation so that two loans with different fee structures can be compared on one number. The method is to take the amount you actually receive after fees, keep the contractual payment, and solve for the rate that connects them.

On this loan, an origination fee of 500 deducted at closing means you receive 19,500 but still pay 386.66 for 60 months. The rate that reproduces that payment on 19,500 is about 7.06%, so the APR is 7.06% even though the schedule is built at 6%. The wider the gap between the two, the more of the loan’s cost is hidden in fees. Rules on exactly which fees count vary by country, so read the lender’s disclosure and ask about anything not listed.

Sanity-checking a lender’s schedule

You do not need to recompute every row. Pick a few checks and run them on the document you were given.

• Recalculate the first row by hand: balance × monthly rate should equal the interest shown, and payment minus interest should equal the principal. If it does not, ask whether interest is charged on actual days rather than equal months.
• Confirm the payment against the formula using the stated rate and term. A payment that is too high may include insurance, escrow or a fee rolled into the loan.
• Check that the principal column adds up to the amount borrowed and the balance reaches zero on the last row.
• Look for a first payment that is much larger than the rest; it often means interest was charged from a disbursement date weeks before the first due date.
• Compare total interest with the tool’s figure for the same inputs. A difference of a few cents is rounding; a difference of tens of dollars needs an explanation.
• Ask how extra payments are applied and whether there is any charge for early repayment before you plan around overpaying.