[Dr. Toto's Interest Rate Course] What is an Interest Rate? A Gentle Introduction Through Generational Dialogue
# [Dr. Toto’s Interest Rate Course] What is an Interest Rate? A Gentle Introduction Through Generational Dialogue
Every time we hear in the news that “the Bank of Japan is raising rates,” “mortgage rates are rising,” or “deposit interest rates have been hiked for the first time in 17 years,” we know it sounds important, but surprisingly few people can answer immediately if asked, “So, what exactly is moving with interest rates?” Kenta, a middle school student, also paused during social studies class when he saw “Policy Interest Rate 0.5%” written on the blackboard.
Today, through a generational dialogue between Dr. Toto and two young people, we will translate the true nature of interest rates into everyday language. From the concept of rental fees for money, the difference between deposit and lending rates, the impact of policy rates on household budgets, the situation in Japan as of 2026, to a mechanism for experiencing interest rates with children’s pocket money, we will unravel it all in one go.
[CHAT:boy: Doctor, ‘Policy Interest Rate 0.5%’ came up in social studies class, but does this have anything to do with my pocket money?]
[CHAT:girl: I’m curious too. Even if I deposit money in the bank, the interest is almost 0 yen, but there’s a lot of noise about how much monthly payments will increase due to mortgage rate hikes. I don’t understand why they are different stories when they are both called ‘interest rates’.]
[CHAT:doctor: That’s a good question. Well then, today, let’s line everything up on the same ruler, from a single 100-yen coin to mortgages and the Bank of Japan’s policy rate.]
1. Interest rates are ‘rental fees for money’; the trick is to think in terms of time axes
First, let’s start with the essence. An interest rate is the percentage of a rental fee paid to the lender when borrowing money for a certain period of time. Expressing it as an annual rate (percentage) is a common rule worldwide.
For example, if you lend 10,000 yen to a friend for a year and they return 10,300 yen, that is a 3% annual interest rate. The structure is exactly the same as renting a bicycle for 500 yen a day; the only difference is that the object is ‘money’. We distinguish between the percentage of the rental fee, called the ‘interest rate’, and the rental fee itself, called ‘interest’.
What is important is the ‘time axis’. Interest rates are always discussed in conjunction with a period, and ‘3% per year’ and ‘3% per day’ have completely different meanings. Comparing a mortgage at ‘1.0% per year’, a consumer loan at ‘18% per year’, and the old loan shark ‘toichi’ (10% every 10 days) shows how the cost of borrowing is determined by the period.
[CHAT:boy: Oh, it’s the same as renting a bicycle.]
[CHAT:doctor: That’s right. If you remember that money is buying and selling ‘time’, it will make sense immediately.]
Why does a fee arise for ‘lending’? There are three reasons. The value of time (10,000 yen right now is more valuable than 10,000 yen a year from now), default risk (an insurance premium for the probability that it might not be returned), and inflation adjustment (compensation for the loss of value in a world where prices rise). These three factors accumulate to determine interest rates in the world.
Key points of this section
2. Deposit rates and lending rates: Banks live on ‘interest margins’
Interest rates have two main faces. They are deposit interest rates and lending interest rates.
Deposit interest rates are the percentage of interest that banks pay when we deposit money. As of April 2026, ordinary deposits at mega-banks are around 0.2% per year, and time deposits (1 year) are around 0.3% to 0.5% per year. Even if you deposit 10,000 yen for a year, the interest is only 20 to 50 yen, and it is reduced by another 20% after taxes.
Lending interest rates are the percentage of interest paid when borrowing money from a bank. As of April 2026, variable mortgage rates are 0.5% to 1.0% per year, 10-year fixed rates are 1.5% to 2.0% per year, and unsecured card loans jump to 14% to 18% per year.
[CHAT:girl: Even though it’s the same bank, 0.2% when depositing and 18% when lending—isn’t that a rip-off…?]
[CHAT:doctor: No, no, this is the core of the bank’s business.]
Banks collect money from depositors at low interest rates and lend it to companies and individuals at high interest rates. This difference between lending rates and deposit rates is called the ‘interest margin (spread)’, which is the bank’s main source of profit. If they collect at 0.2% and lend at 1.0%, the margin is 0.8%, which is 8 billion yen in annual revenue on 1 trillion yen in deposits. The reason card loans have such high interest rates is that they include an insurance premium for default risk. While foreign exchange fees and investment trust sales commissions are also sources of income, their main business is still the ‘collect and lend’ spread business.