Mortgage Amortization Calculator Project payments, generate schedules, model extra payments, and visualize principal vs. ...
Mortgage Amortization Calculator
Project payments, generate schedules, model extra payments, and visualize principal vs. interest — with 2025 U.S. benchmarks.
Monthly Payment Formula**:
$$PMT = P \frac{r(1+r)^n}{(1+r)^n - 1}$$
Where: • $P$ = loan amount • $r$ = monthly interest rate (annual ÷ 12) • $n$ = number of payments (years × 12)
Example** ($300K @ 6.8%, 30 yrs): $$PMT = 300{,}000 \times \frac{0.005667(1.005667)^{360}}{(1.005667)^{360} - 1} = \mathbf{\$1{,}952.33}$$
✅ Key Insight**: Early payments are mostly interest — Year 1: $20,361 interest, $3,066 principal Year 10: $16,242 interest, $7,185 principal Year 30: $131 interest, $1,821 principal
⚠️ Avoid these costly traps:
- Interest-only ARMs** — payments jump 40–70% after 5–10 yrs
- Negative amortization** — balance grows if payment < interest (option ARMs)
- Teaser rates** — 3% intro → 8%+ adjustment (check lifetime caps!)
- Prepayment penalties** — 1–2% if paid off early (banned for most FHA/conventional, but still in private loans)
✅ Pro Tips**:
- Make **extra principal payments in Year 1–5** — max interest reduction
- Switch to **bi-weekly payments** → 13 full payments/year = ~25% faster payoff
- Refinance only if break-even <3 years (closing costs ÷ monthly savings)
| Loan | Payment | Total Interest | P/I Yr 1 |
|---|---|---|---|
| $300K @ 6.8%, 30 yr | $1,952/mo | $402,839 | 87% interest |
| + $200/mo extra | $2,152/mo | $338,822 | 22.3 yrs |
| Bi-weekly ($976 × 26) | $2,115 eq | $353,724 | 24.8 yrs |
| $300K @ 6.1%, 15 yr | $2,532/mo | $155,760 | 69% interest |
📉 Extra Payment Impact**:
- $100/mo extra on $300K @ 6.8% → **4.2 yrs saved**, **$32K interest saved**
- $5,000 lump sum in Year 1 → **6.1 yrs saved**, **$49K interest saved**
➡️ Standard Amortization
“$300K loan @ 6.8% — what’s my payment and total interest?”
➡️ Extra Payments
Model $200/mo extra or $10K lump sum — see months and interest saved.
➡️ Refinance
Compare 7.2% → 5.8%: monthly savings, break-even, total interest change.
➡️ Tax & Insurance
Add $3,600 tax + $1,400 insurance + $900 PMI → full PITI.
Note: Uses exact amortization math. All extra payments go to principal. Bi-weekly = 26 half-payments/year (= 13 full).