Amortization, Visualized: Where Your Monthly Payment Actually Goes

The setup

You take out a $25,000 personal loan at 9.99% APR for 60 months. The lender's calculator tells you your monthly payment will be $531.18. You agree, you sign, the cash arrives.

A month later you make the first payment. $208.10 went to interest. $323.08 went to principal. Your remaining balance is now $24,676.92.

A month after that you make the second payment. $205.41 went to interest. $325.77 went to principal. Your remaining balance is now $24,351.15.

The total payment is constant. The split is shifting. Why?

The mechanism

Each month, the lender charges you interest on the remaining balance, not the original balance.

Month 1's interest: $25,000 × (9.99% / 12) = $208.13. Month 1's principal: $531.18 - $208.13 = $323.05. Month 1's ending balance: $25,000 - $323.05 = $24,676.95.

Month 2's interest: $24,676.95 × (9.99% / 12) = $205.43. Month 2's principal: $531.18 - $205.43 = $325.75. Month 2's ending balance: $24,676.95 - $325.75 = $24,351.20.

(My quick numbers above are off by $0.05 from a precise calculator's because of rounding. The mechanism is exact.)

The remaining balance shrinks every month, so the interest portion of each constant payment shrinks every month, so the principal portion grows every month. This is amortization.

The full table — selected months

For a $25,000 loan at 9.99% APR, 60 months:

Month	Payment	Interest	Principal	Balance
1	$531.18	$208.13	$323.05	$24,676.95
2	$531.18	$205.43	$325.75	$24,351.20
6	$531.18	$194.42	$336.76	$23,032.54
12	$531.18	$179.21	$351.97	$21,180.20
24	$531.18	$144.96	$386.22	$17,019.82
36	$531.18	$107.27	$423.91	$12,453.52
48	$531.18	$66.04	$465.14	$7,388.69
60	$531.18	$4.40	$526.78	$0.00

Look at the interest column. Month 1's interest is 47x larger than month 60's interest. That's the entire point.

Total interest paid

Add up the interest column for all 60 months: about $6,871.

Total payments: $531.18 × 60 = $31,871. Total interest: $31,871 - $25,000 = $6,871.

You paid $6,871 in interest to borrow $25,000 over 5 years. That's the loan's true cost (assuming no fees).

Why prepayment in early months saves more

A $1,000 prepayment in month 6 reduces your remaining balance by $1,000. From that month on, the lender calculates interest on a $1,000-smaller number. Over the remaining 54 months, you save approximately:

$1,000 × (9.99% / 12) × ~54 months × 0.5 (averaging) ≈ $225 in interest

A $1,000 prepayment in month 50 reduces your remaining balance by $1,000 — but you only have 10 months of borrowing left. You save approximately:

$1,000 × (9.99% / 12) × ~10 months × 0.5 ≈ $42 in interest

The same $1,000 prepayment is worth ~5x more in month 6 than in month 50. Time-of-prepayment is asymmetric in your favor early.

This is why "prepay aggressively in the first half of the loan" is rarely bad advice on simple-interest installment loans.

Why refinancing earlier is also asymmetric

Same logic applies to refinancing. If you can drop your APR from 12% to 9% in month 12 (with 48 months left), you save the 3-percentage-point delta on the entire $24k+ remaining balance over 48 months. If you refinance the same loan in month 50 (with 10 months left), you save 3 percentage points on a balance that's mostly already been paid down.

Refinance breakeven calculations should always start by asking how much principal remains. A loan with 12 months left rarely justifies the refinance hassle even at a meaningfully lower rate.

Quick takeaways

1. Your monthly payment is constant, but the interest/principal split shifts every month.
2. Early-loan interest is huge; late-loan interest is small.
3. Prepayment in the first half of the loan is dramatically more efficient than prepayment in the second half.
4. Refinancing in the first third of the loan is dramatically more impactful than refinancing in the last third.
5. The published "total interest paid" on a loan is exact — it's the sum of every interest charge in the amortization table — and it's usually 25–35% of the principal on a 5-year personal loan in the 10–14% APR range.

Want to see your own loan?

Most spreadsheet apps have an IPMT and PPMT function that compute the interest and principal portion of a given month's payment. Build a sheet, plug in your APR, term, and principal — you'll see the table for your specific loan. Worth doing once, even if you never look at it again.