Why accelerated biweekly works
A regular biweekly payment is just your monthly payment split in two, paid every two weeks — 26 payments a year, which is the same total as 12 monthly payments, just spread out differently. Accelerated biweekly is different: it's half of your monthly payment, paid every two weeks, which adds up to 26 half-payments — the equivalent of 13 monthly payments a year instead of 12. That single extra payment, made automatically because there are 52 weeks in a year rather than a neat 48, is what shaves years off your amortization without you having to think about it again or change your budget in any noticeable way.
Why extra payments save more in interest than they cost in dollars
Every extra dollar you pay toward your mortgage goes straight to principal, since your regular payment already covers that period's interest. Reducing principal earlier means every future interest calculation — for the rest of your amortization — is based on a smaller balance. That compounding effect is why a relatively modest extra monthly payment, sustained over years, tends to save far more in total interest than the extra payments themselves add up to. It's the same mechanism as compound growth working in reverse, in your favour.
Check your prepayment privileges before committing
Most Canadian mortgage contracts cap how much extra you can pay in a given year without triggering a prepayment penalty — commonly somewhere between 10% and 20% of the original principal, though the exact figure and how it's calculated (some lenders reset annually on your mortgage anniversary, others on the calendar year) varies by lender and by contract. This calculator doesn't know your specific limit, so before committing to a large monthly increase or a big lump sum, it's worth a quick call to your lender or a look at your mortgage documents to confirm you're within your allowance — going over it can trigger a real penalty that offsets the interest savings you were trying to capture.
Accelerated biweekly, extra monthly, or both
You don't have to choose just one. Switching to accelerated biweekly captures that automatic "13th payment" effect with essentially no change to your monthly budget, since the biweekly amount is calculated directly off your existing payment. Adding extra on top of that compounds the effect further. A lump sum from a bonus, tax refund, or inheritance works differently again — it's a one-time reduction to principal rather than an ongoing change, but the same compounding logic applies: paid earlier in your amortization, it saves more than the same amount paid later.
Curious what a lump sum from a bonus or tax refund would do on its own, without the ongoing monthly changes? Enter it above alongside or instead of the monthly extra, or see how a full refinance compares if you're also considering a rate change.