This short post was inspired by comments.
I’ve mentioned this along the way, but when you pay the Japan social insurances, these are deductions to your Japan national tax, and to your residence tax. Additionally, since your taxable income is reduced, it creates a further reduction, the next year, of your health insurance premiums.
To take a basic example, let’s talk about someone with a 80,000 yen health insurance premium in their second year in Japan. (The first year premium is very small, since it’s based off 0 yen earnings in Japan.)
This 80,000 yen payment is going to shelter, at minimum, 5% national tax, and then 10% residence tax. So 12,000 yen is a tax reduction, and the insurance is only costing you, net, 68,000 yen for the year.
Additionally, since you paid 80,000 yen and reduced your taxable income by 80,000, the next year’s health insurance premium is based on not including the 80,000. If the ward health insurance is 8%, that means 6,400 less in the next year (the third year).
A number of people work this out as gross, but the real way to do it is as net.