Built a calculator to compare offset accounts, fortnightly payments, and lump sums - surprised by the results

I got frustrated trying to figure out which mortgage strategy actually saves more money - offset accounts, switching to fortnightly payments, or throwing in lump sums. Most calculators only do one thing, so I built one that compares them all.

mortgage.hazuu.com

Some things I found interesting while testing:

• Fortnightly payments (half the monthly amount, paid fortnightly) can shave 4-5 years off a 30 year loan. You end up making 26 half-payments per year - that's 13 monthly equivalents instead of 12.
• Money in a 100% offset from day one typically saves more interest than the same amount as a lump sum years later. Earlier interest reduction has more impact due to how amortisation works.
• The interest-only period trap is real. 5 years of I/O on a 30 year loan means your P&I payments jump significantly afterwards because you're now paying off the same principal in 25 years.

Covers NSW and VIC stamp duty + LMI calculations too (with the FHB exemptions).

It's free, no login needed. Let me know if the calculations seem off or if there's something else that would be useful to add.