An extra $200 a month. What does it actually do to your mortgage?
Most mortgage payoff calculators give you a single headline number ("you'll save $X in interest") and call it done. This one shows the full month-by-month amortization, side by side — original payoff vs accelerated payoff — so you can see exactly when the curves diverge, when the new payoff date lands, and what the interest savings look like compounding over time. Add a one-time lump sum (tax refund, bonus, inheritance) on top, and watch it stack with the monthly extra. The standard amortization formula is shown.
Side-by-side
Original vs accelerated
Monthly
P / I / balance schedule
Lump-sum
+ monthly extra stacked
Show-the-math
Standard amortization formula
1. Your current mortgage
Use today's numbers — current balance, current rate, months remaining. If you're early in a 30-year loan and unsure, your monthly statement shows all three.
$
Today's principal balance on the loan.
%
Your fixed mortgage rate. For ARMs, use the current rate (this tool assumes a fixed schedule).
Months left until original payoff. 30-year new loan = 360. A loan that's 4 years in = 312.
$
Principal & interest only — not taxes or insurance. Leave at 0 to auto-compute from balance + rate + term using the standard amortization formula.
2. Your acceleration plan
Pick one or both. Extra principal goes against the loan immediately, every month. Lump sum is applied in month 1.
$
An additional principal-only payment with every regular monthly payment. Try $100, $200, $500 — the leverage is surprising.
$
Tax refund, bonus, inheritance, sale proceeds applied to principal in month 1.
Headline numbers
MONTHS SAVED
—
—
TOTAL INTEREST SAVED
—
—
NEW PAYOFF DATE
—
—
MONTHLY P&I
—
—
Side-by-side comparison
ORIGINAL SCHEDULE
Pay the minimum — ride it out
Months to payoff—
Years to payoff—
Payoff date—
Total interest paid—
Total paid (P + I)—
ACCELERATED SCHEDULE
With your extra payments
Months to payoff—
Years to payoff—
Payoff date—
Total interest paid—
Total paid (P + I + extras)—
Month-by-month amortization — both schedules
The full schedule — principal, interest, and balance for every payment under both plans. Years are highlighted so you can scroll through anniversaries. Use the toggles to switch view.
Show the math — the standard amortization formula
Monthly payment (P&I) for a fully-amortizing fixed-rate loan:
M = L × [ r × (1 + r)n ] / [ (1 + r)n − 1 ]
where: M = monthly P&I payment L = loan balance (principal) r = monthly interest rate = annual APR ÷ 12 n = number of months remaining
Each month's interest:interestm = balancem−1 × r Each month's principal applied:principalm = M − interestm New balance:balancem = balancem−1 − principalm − extram
Total interest saved = sum of all interest paid on original schedule − sum of all interest paid on accelerated schedule.
Why the small extra payment has outsized impact: in the early years of a mortgage, almost every dollar of your regular payment goes to interest, not principal. The first $1,000 of extra principal you throw at a 30-year loan in year one prevents that $1,000 from accruing interest for the next 29 years. The amortization curve is heavily back-loaded toward principal — extra payments flip the curve forward.
RELATED GUIDES & TOOLS · FROM THE BARATELLI INSTITUTE
Where to go next
Before you commit the extra $200/mo — emergency fund first → Emergency Fund Builder — 3-to-6 month buffer comes before mortgage prepayment in the standard sequence.
Buying or refinancing — the closing costs you're about to pay → Closing Costs Calculator — itemized lender, title, transfer, escrow, prepaid interest with low/high ranges.
Underwriting the loan in the first place → Mortgage Underwriting — Conventional / FHA / VA / Jumbo. Front-end + back-end DTI, LTV, MIP/PMI.
The full family financial picture (Money Reality series) → All Baratelli Institute Money Reality tools at baratelliinstitute.com — first apartment, paycheck walkdown, 401(k) match, college NPV, lease vs buy, and more.
Disclosure. This calculator handles fixed-rate, fully-amortizing mortgages. It does not handle: adjustable-rate mortgages after the rate-change date, interest-only periods, balloon payments, biweekly payment plans (which differ from extra monthly principal), recasting, PMI removal triggers, escrow shortages/refunds, or prepayment penalties (rare on residential loans post-2014, but possible on commercial and some non-QM products). Property tax, homeowners insurance, HOA fees, and PMI are not included — those are separately escrowed amounts that don't affect the P&I amortization. Confirm with your lender that extra payments are applied to principal (the standard, but verify) and check your loan documents for any prepayment penalty before sending large lump sums. Educational only. Not financial advice.
WANT THE METHODOLOGY BEHIND THIS TOOL?
Read more in the Money Reality.
The tool gives you the answer. The guide gives you the argument — the case law, the worked examples, the negotiation playbook, the cross-check tables, the exception cases.
The methodology behind this calculator is in Home ownership economics · payoff acceleration of the reference guide.