How a New Zealand extra-repayment calculator works

Build and test the baseline schedule

Enter current principal, contract rate, remaining term, next payment date and regular principal-and-interest amount. Accrue interest over actual calendar gaps, then apply each payment consistently.

Compare the estimated first payments and balance with a statement. Differences can arise from lender day count, cut-off time, fees, rounding and irregular first periods. Document rather than conceal them.

Define what each extra input means

A lump sum and recurring extra reduce principal on stated dates. An offset balance reduces the interest-bearing balance without reducing contract principal. Revolving credit changes with daily income and spending and is outside a fixed repayment schedule.

The plan is compared with the same baseline rate and dates. A lender-quoted break fee can be deducted from gross interest reduction as an estimated cost, but the engine cannot calculate that fee.

Method audit checklist

1. Use current balance and the actual next payment date.
2. Disclose 365- and 366-day interest treatment.
3. Schedule fortnightly payments every fourteen days.
4. Keep offset separate from principal.
5. Exclude revolving-credit cash-flow claims.
6. Show constant-rate assumptions and fees.

Limits every result needs

A generic tool cannot know fixed-loan limits, predict refix rates, reproduce break fees or value accessible cash. It also cannot combine materially different splits exactly with one blended rate.

• Do not promise exact savings or a payoff date.
• Use separate runs for different loan splits.
• Rerun after every rate or contract change.
• Use lender figures for payout and fees.
• Keep a zero-extra comparison visible.

Use a calculator as a scenario, not a promise

Payoff Atlas follows this disclosed daily-balance method and reports its assumptions. It is designed for repeatable scenario comparisons, not lender-specific quotes or revolving-credit management.

The New Zealand model accrues interest across actual calendar days, using 365 days or 366 in a leap year. It supports monthly, fortnightly and weekly schedules and treats a stable offset balance separately from principal.

The estimate holds the entered rate constant, assumes extras reach principal and excludes revolving-credit cash flow. Fixed-loan limits, refix rates, break fees, posting and lender rounding can change the actual result.

Review the plan before money leaves

Compare a zero-extra case, the intended plan and an interrupted plan with identical starting inputs. For floating or split debt, rerun at other rates without presenting them as predictions. Stop the projection at a refix date when the next rate is not known.

Keep the loan agreement, latest statement and written lender answers about fixed-loan limits, break fees and payment allocation. Model each split separately when its rate, fixed expiry, offset treatment or flexibility differs.

• Secure required repayments and essential costs first.
• Keep emergency and planned-expense cash accessible.
• Name the intended loan split for an extra payment.
• Verify the transaction and principal on the next statement.
• Rerun after a refix, rate change, fee or product restructure.