How a Canadian mortgage calculator should work

Start by reproducing the current schedule

Enter current principal, contract rate, remaining amortization, payment frequency and next date. Convert the nominal rate compounded semi-annually into the periodic rate. Estimate the regular payment and compare it with the lender’s principal-and-interest amount.

If they differ materially, inspect frequency, acceleration, rounding, first-period timing and any payment change. Do not add a prepayment until the baseline is credible.

Build baseline and plan with the same engine

The baseline keeps the current payment schedule without optional principal. The plan applies the lump sum and recurring extra under the same rate, dates and rounding. Interest savings equal baseline interest minus plan interest; a stated penalty can then be deducted for a rough net figure.

For an accelerated frequency, the regular amount itself is higher than the standard conversion. It should not also be counted as a separate recurring extra unless the borrower truly plans both.

Audit checklist

1. Confirm nominal semi-annual rate conversion.
2. Check standard and accelerated frequency formulas separately.
3. Use remaining amortization for payment count.
4. Record the current term as a rate horizon.
5. Apply extras directly to principal on stated dates.
6. Show fees, warnings and assumptions beside results.

Limits every result should disclose

A generic engine does not know the lender’s exact rounding, posting date, privilege reset, penalty method or future renewal rate. It should not present a precise dollar as guaranteed or label every difference a saving without accounting for fees.

• Keep APR separate from the contract rate input.
• Warn when the rate is held beyond term end.
• Do not invent an unused prepayment privilege.
• Compare payoff dates only under identical assumptions.
• Use the lender’s statement for binding amounts.

Use a calculator as a scenario, not a promise

Payoff Atlas follows this method and exposes the assumptions with each result. Compare its baseline payment with FCAC examples and your own lender’s figure, then treat any residual difference as a reason to investigate—not hide—model limitations.

The Canadian model converts the quoted nominal annual rate using semi-annual compounding, then derives the payment-period rate. It distinguishes standard from accelerated schedules and keeps the mortgage term separate from the remaining amortization.

The estimate holds the entered rate constant, assumes extras reach principal and cannot discover a lender’s privilege or penalty method. Compare its regular payment with your statement, and rerun at renewal or after a variable-rate change.

Review the plan before money leaves

Compare the proposed prepayment with a zero-extra case and an interrupted plan using identical dates and rate assumptions. Record the current term end even when the schedule spans the remaining amortization, because the contractual rate and privileges can change at renewal.

Keep the agreement, annual statement and lender response that states the available privilege in dollars. If the mortgage has several segments, review each rate, maturity and penalty separately rather than treating one combined balance as a single contract.

• Protect the required payment and essential household costs.
• Keep emergency and known near-term cash accessible.
• Confirm payment-increase and lump-sum privileges separately.
• Reconcile the first prepayment with the lender statement.
• Rerun at renewal, after a variable-rate move or after any contract change.