Where Your Payment Actually Goes: Amortization Math Without the Mystery
Amortization is one rule applied 360 times: monthly interest equals the remaining balance times the monthly rate; everything past that is principal. Run the rule on one loan — $400,000, 30 years, 6.5% — and every statement mystery resolves into arithmetic.
First-Payment Anatomy: The Exact Split and Why
From first principles: the monthly rate is 6.5% ÷ 12 = 0.5417%, so month 1's interest is $400,000 × 0.5417% = $2,166.67 — exact to the penny. The standard payment computes to $2,528.27; subtract, and principal is $361.61. Month 1 sends 85.7% of your payment to interest.
Why so lopsided? The charge scales with the balance, which starts at its lifetime maximum; the payment is engineered so exactly that amount, paid every month, zeroes the balance at payment 360 — forcing the earliest payments to be almost all interest. Nothing was done wrong. Progress is glacial: a full year of payments ($30,339) retires only $4,471 of principal — 1.1% of the loan.
The Crossover Point: When Principal Finally Beats Interest
Because the interest slice shrinks as the balance falls, some payment eventually flips the split. Here that takes a while: principal first exceeds interest at payment 233 — 19.4 years in — balance $232,985: $1,266.27 principal vs $1,262.00 interest. By then 84% of all lifetime interest ($429,446 of $510,178) is paid; only 44% of the loan retired. Dollars to date at each checkpoint:
| Checkpoint | Interest · principal split | Balance | Principal retired |
|---|---|---|---|
| Year 3 | 84% · 16% | $385,669 | $14,331 (3.6%) |
| Year 10 | 80% · 20% | $339,105 | $60,895 (15.2%) |
| Year 15 | 76% · 24% | $290,237 | $109,763 (27.4%) |
| Year 20 (flip at payment 233) | 71% · 29% | $222,661 | $177,339 (44.3%) |
| Year 25 | 64% · 36% | $129,217 | $270,783 (67.7%) |
The final decade contributes only $80,732 of interest — 15.8% of the total — while clearing most of the balance: the 20-year-old mortgage is the efficient one; the fresh refinance drowns in interest. Shorter terms compress the crossover: a 15-year at 6.0% flips at payment 43 — 3.6 years in — because larger mandatory principal ($1,375.43 in month 1 vs $361.61) pulls the interest charge visibly down every month.
Total Interest on Standard Schedules: 30 vs 15 Years
Same $400,000, four standard schedules, computed:
| Loan | Payment | Total interest | Interest as % of loan |
|---|---|---|---|
| 30-yr at 6.5% | $2,528.27 | $510,178 | 128% |
| 20-yr at 6.5% | $2,982.29 | $315,750 | 79% |
| 15-yr at 6.5% | $3,484.43 | $227,197 | 57% |
| 15-yr at 6.0% | $3,375.43 | $207,577 | 52% |
Read the deltas. Halving the term at the same rate costs $956.16 more per month and removes $282,981 of interest — 38% more payment, 55% less interest. The lower rate compounds it: the 15/6.0 loan ends $302,601 ahead of the 30-year baseline, and a half-point alone is worth $46,825 at 30 years. The 20-year column is the sleeper: $2,982.29 ($454 above the 30-year payment, $502 below the 15-year) removes $194,428 of interest; most lenders offer it, almost nobody shops it.
Why Extra Early Crushes More Extra Later
Prepaid principal kills interest for the entire remaining term: a dollar prepaid in month 1 works 359 months; in month 300, 60. Take the same $12,641 of extra principal, two placements. Early: $210.69/month for years 1–5, then stop — $57,345 interest saved, payoff 27 months sooner. Late: the same $12,641 spread over years 6–30 at $42.14/month — $17,990 saved, 11 months sooner. Identical cash; 3.2× the benefit when placed early. Scaling instead — $210.69/month for years 6–30 costs $63,207 and saves $74,481 — still clears less per dollar.
A single $10,000 lump shows the decay: at month 1 it saves $54,998 and ends the loan 25 months early; at month 200, $13,212; at month 359, $14. Same money, same rate, a 3,900-to-1 spread. The corollary most people miss: serial refinancing into fresh 30-year terms resets the crossover to zero and refills the schedule with front-loaded interest — a "lower payment" can coexist with a far larger lifetime bill.
Equity-Building Curves: What a Decade Buys You
Paydown equity starts almost flat and steepens — the mirror image of the interest curve. Computed on the standard 30-year schedule:
| Year | Balance | Principal retired | Cum. interest paid |
|---|---|---|---|
| 3 | $385,669 | $14,331 | $76,687 |
| 5 | $374,444 | $25,556 | $126,140 |
| 10 | $339,105 | $60,895 | $242,497 |
| 15 | $290,237 | $109,763 | $345,326 |
| 20 | $222,661 | $177,339 | $429,446 |
| 25 | $129,217 | $270,783 | $487,698 |
At year 15 the bank has collected $345,326 of interest while 27% of the loan is retired — paid before built. Add $210.69/month from day one and the year-5 balance drops to $359,554, year-10 to $303,624, year-15 to $226,283 — and the loan dies at payment 290 instead of 360. Most of the gap opens in the first decade, where the interest share is highest. Contrast the 15/6.0 curve: $345,896 at year 3, $174,596 at year 10 — the 30-year borrower does not reach that balance until month 274, 22.8 years in. Same money, earlier balance, less interest: the whole game in one line.