Last updated: 2026-06-29

Japanese Mortgage Mix-Loan Comparison

A mix loan (Japanese: ミックスローン) is a way of borrowing a Japanese mortgage in which the total principal is split into a variable-rate portion and an all-period fixed portion. When one borrower buys one property, part of the loan can be taken at a variable rate and the rest at a fixed rate that lasts to payoff, aiming for less exposure to rising rates than an all-variable loan while keeping a lower initial payment than an all-fixed loan. This calculator varies the variable allocation and compares, on a single screen, the monthly payment, the total repayment, and the total repayment after rates rise. It is a decision aid for a borrower who cannot settle on variable or fixed alone.

How a mix loan works

Japanese mortgage rates fall into two broad types: a variable rate that is reviewed during repayment, and an all-period fixed rate that does not change until the loan is cleared. A variable rate usually starts lower but carries the risk that payments rise if rates climb. An all-period fixed rate is higher but fixes the repayment for the whole term, which makes the household budget easier to project.

A mix loan combines these two types within a single borrowing. Increasing the variable share lowers the initial payment; increasing the fixed share raises stability against a rate rise. Rather than committing fully to one type, it takes a position in between, balancing risk against payment.

The repayment formula

Both portions use equal-payment repayment, in which a constant amount is paid each month. For a balance $B$, annual rate $r$, and number of payments $n$ (in months), the monthly payment $P$ is

This is the standard mortgage formula: it sets a level monthly payment that clears both principal and interest in exactly $n$ payments. In a mix loan the formula is applied separately to the variable and fixed portions, and the two are added to give the monthly payment. With a total loan $L$ and a variable share $w_v$, the variable portion is $L \cdot w_v$ and the fixed portion is $L \cdot (1 - w_v)$.

Worked example

Take a total loan of 40 million yen, a 35-year term (420 payments), a variable rate of 0.4%, an all-period fixed rate of 1.5%, and a variable share of 50%. The variable and fixed portions are 20 million yen each.

The monthly payment is about 112,275 yen, and the total repayment over 35 years is about 47.16 million yen. That sits midway between an all-variable loan, whose total is about 42.87 million yen, and an all-fixed loan, at about 51.44 million yen. The roughly 4.29 million yen difference from the all-variable total is the practical premium for holding a fixed portion as a hedge against rising rates.

Choosing the allocation

There is no single correct split between variable and fixed. If rates never rise, a larger variable share gives the smaller total; if rates rise sharply, a larger fixed share limits the increase. The basic trade-off is to favour variable when the lowest initial payment matters most, and fixed when protection against a rate rise matters most. Moving the allocation slider while watching both the monthly payment and the total after a rate rise helps locate a split the household can accept.

Switching the rate-rise scenario from flat to gradual or shock applies the rise to the variable portion only and shows how far the combined total moves above the base case. Because the fixed portion's rate does not change, a smaller variable share produces a smaller increase. This calculator uses a simplified model in which the payment is recalculated each year from the remaining balance and term; it does not model the five-year rule, the 125% payment cap, or unpaid interest. A more detailed variable-rate stress test would additionally model those rules.

Points to keep in mind

A mix loan requires two mortgage contracts, so some costs such as administration fees and stamp duty are charged on both, and the paperwork increases. This calculator does not include the duplicated fees, so the actual total paid will be slightly higher than shown. Not every lender offers a mix loan, and some restrict which repayment terms can be combined. Arrangements that raise the borrowing amount or the mortgage tax deduction across a married couple raise different questions about the number of debtors and how the deduction is used.