Fixed vs Variable Mortgage Rates in Japan

The fixed-versus-variable question in Japan has a distinctive flavour, because the standard Japanese variable-rate mortgage carries a notable safety valve: the rate moves, but your monthly payment mostly does not — and the cap on how fast it can move creates a quiet trap of its own. Here is how the three product families actually behave.

Variable rate (変動金利)

The classic Japanese variable-rate housing loan tracks a short-term benchmark, reviewed — with the individual bank's own spread — typically about twice a year. What makes the product distinct is the payment rule: the monthly instalment is normally reviewed only once every five years, and each review may raise it by at most 1.25 times the previous payment. The borrower's monthly exposure therefore moves slowly even when the rate moves fast.

The catch is called unpaid interest (未収利息). When the accrued interest exceeds the payment — possible once rates rise enough — the shortfall is added to the balance instead of the payment. The loan then negatively amortises: you pay every month and owe more. Japanese disclosure rules require lenders to flag when this approaches, but the flag is a sentence in a document most borrowers skim. A variable-rate plan deserves a stress test at rates well above the one you are quoted — the mortgage calculator holds whatever rate you enter, so enter a pessimistic one and see.

Fixed-period selection (固定期間選択型)

The middle family fixes the rate for an initial period — commonly 2, 3, 5, 10 or 20 years — then usually reverts to the variable structure above. It trades a higher rate than pure variable for a decade of payment certainty, and it is a widely chosen middle option in the Japanese market precisely because many borrowers' horizons (births, school fees, retirement) fit inside the fixed window. The classic error is treating the initial rate as the loan's rate: it is only the first act. What happens at reversion — and what the bank's conversion options cost at that moment — belongs in the decision before signing, not after.

Full-term fixed (全期間固定) and Flat 35

The third family fixes the rate from the first payment to the last. Its standard-bearer is Flat 35, the full-term fixed product administered through the Housing Finance Support Agency (JHF): terms up to 35 years, one rate for life, with published pricing that distinguishes loans at or below 90% loan-to-value from loans above 90%. Because a constant-rate annuity model matches the product exactly, the totals from a constant-rate calculator describe Flat 35 honestly — the same totals are merely a snapshot for a variable loan. Private banks also offer their own full-term fixed products; compare them on total repayment over the term you actually expect to hold.

How the choice is usually made — and how it should be

Variable rates in Japan have typically started below fixed rates, so the visible saving recruits borrowers immediately. The decision should instead run on three inputs the advertisement never shows: how much your monthly budget could absorb if the payment rose the full 1.25x at a review; how likely unpaid interest becomes at rates a few points above today's; and how long you actually expect to hold the loan — moving, refinancing or repaying early shortens the horizon that justifies paying up for certainty.

A quieter strategy many Japanese borrowers use is splitting (借り分け): one portion variable, one portion fixed, sometimes across different lenders. The split caps the damage of either regime being wrong. Whatever mix you lean toward, run it through the mortgage calculator at both an optimistic and a pessimistic rate, and check the payment the affordability calculator says your income actually supports. The companion guides on affordability basics and home buying affordability cover the rest of the frame.