Calculator inputs
How to use this calculator
- Enter your loan principal, rate, term, and payment frequency.
- Add monthly income, non-loan expenses, savings goal, and a buffer.
- Choose your extra payment and optional lump sum timing.
- Turn on affordability capping to stay within a safe budget.
- Review payoff dates, interest saved, and warning checks.
- Download the CSV for the full schedule or PDF summary.
Formulas used
Available per period = Monthly disposable * 12 / periods_per_year
Affordable extra = max(0, Available per period − Scheduled payment)
Budget-first prepayment sizing
This calculator converts monthly cashflow into a safe extra payment per period. It subtracts non-loan expenses, a savings goal, and an income buffer, then spreads the remainder across your payment frequency. If disposable cash is $1,200 per month and you pay monthly, that is $1,200 available. After a $950 scheduled payment, the maximum affordable extra is $250, before adding a cushion.
Payoff acceleration impact
Extra payments target principal, so they reduce future interest charges. For example, on a $250,000 balance at 6.50% with a 30-year term, a modest extra can cut years from repayment because the balance drops faster early on. The results panel reports the new payoff date, time saved in periods, and interest saved versus a no-prepayment schedule using the same rate and frequency.
Sensitivity to rate and frequency
Higher rates increase the value of principal reduction, so interest saved usually grows as the annual rate rises. Frequency matters too: biweekly or weekly schedules apply principal reduction more often, which can improve outcomes even when the monthly-equivalent cash is unchanged. Use Payment Override if your lender uses a different rule, or if escrow and insurance are included in your bill.
Penalties and caps in real contracts
Some loans limit annual prepayments or charge a penalty. The calculator supports an annual cap as a percent of original principal and models penalties as a percentage plus an optional fixed fee. This enables a net-savings view: interest saved should be weighed against penalty cost. If penalties exceed interest saved, the plan may be smaller extras, a later lump sum, or no prepayment.
Decision rules for choosing extra
Start with the recommended extra, which applies a cushion to keep flexibility. If the monthly buffer left is near zero, reduce extra or raise the cushion percent. Prefer consistent extras over one-time spikes unless you have stable surplus cash. Recalculate after income or expense changes, then export the CSV to audit the schedule and confirm the payoff trajectory for your scenario.
FAQs
What does “affordable extra” mean?
It is the maximum extra amount you can add per payment period after reserving expenses, savings goals, and your buffer. If it is zero, reduce costs, increase income, or pause prepayments.
How does the cushion percent affect my plan?
The cushion reduces the affordable extra to a safer recommended figure. A 20% cushion keeps one‑fifth of the calculated extra uncommitted, which helps absorb irregular bills or income swings.
What if my loan has a prepayment penalty?
Enter the penalty rate and any fixed fee. The calculator adds penalties when extra or lump sums occur, so you can compare interest saved against penalty cost and choose a plan that still nets positive.
Why would the calculator show a warning about interest coverage?
If your scheduled payment is near or below periodic interest, the balance may not decline normally. In that case, focus on adjusting the payment amount or term assumptions before planning prepayments.
Can I model biweekly or weekly payments?
Yes. Choose the frequency and the schedule will apply interest and principal each period. Use Payment Override if your lender’s billing method differs, so the projection matches your real statement.
What do the CSV and PDF downloads include?
The CSV contains the full period-by-period schedule with balances and cash applied. The PDF provides a summary plus a short schedule preview, useful for sharing a quick payoff and savings snapshot.
Example data table
| Scenario | Principal | Rate | Term | Income | Expenses | Savings | Payment | Affordable extra | Recommended extra |
|---|---|---|---|---|---|---|---|---|---|
| Sample | $250,000 | 6.50% | 30 yrs | $6,500 | $2,800 | $500 | $1,580.17 | $969.83 | $775.86 |