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Flat vs Reducing Rate Calculator
Two loans can quote the same interest rate yet cost very different amounts. Enter a loan amount, rate and term to see what a flat rate really costs against a reducing-balance loan.
Written by TopicDrill Editorial Team·Updated June 2026
Outstanding balance over time
Flat Reducing
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How this comparison works
The calculator takes one quoted rate and applies it two ways. Under the flat method it multiplies your original loan amount by the rate and the number of years, then spreads that fixed interest evenly across every month. Under the reducing method it amortises the loan like a normal bank repayment, charging interest only on the balance that is still outstanding each month.
The chart traces the outstanding principal for both schemes. The flat line falls in a straight diagonal because principal is returned evenly, while the reducing area curves as early payments are mostly interest and later payments are mostly principal. The headline figure is the extra interest the flat structure quietly adds.
A worked example
Borrow 20,000 dollars at a 9 percent flat rate over 5 years. The flat scheme charges 9 percent of 20,000 every year for 5 years, which is 9,000 dollars of interest. The same 9 percent applied on a reducing balance costs roughly 4,900 dollars, because by year three you owe far less. That 9 percent flat rate behaves like an effective reducing APR near 16 percent, almost double the headline number.
Things to keep in mind
When you compare two loan offers, always confirm which method each uses before judging the rate. A consumer protection primer on loan costs from the CFPB is a useful neutral reference. Once you know the equivalent reducing rate, plug it into our loan calculator to see the full repayment schedule.
Frequently asked questions
What is the difference between flat and reducing interest?
A flat rate charges interest on the full original loan amount for the whole term, no matter how much you have already repaid. A reducing-balance rate charges interest only on what you still owe, so as the balance falls each month the interest portion falls too. For the same quoted rate the flat method always costs more.
Why does a flat rate cost more than the same reducing rate?
Because under a flat loan you keep paying interest on money you have already returned to the lender. Even though half your principal might be repaid by the midpoint, the flat scheme still charges interest as if the whole amount were outstanding. The calculator shows how a flat rate of one number behaves like a much higher reducing APR.
How do I convert a flat rate to an effective APR?
Take the flat monthly payment and find the reducing-balance rate that produces that same payment on the same principal and term. As a rough rule the effective reducing rate is close to the flat rate times two for typical terms, but it depends on the length of the loan. This tool solves for the exact equivalent and shows it.
Which loans usually quote a flat rate?
Flat rates are common on car loans, consumer durable financing, some personal loans and many informal lenders, because the headline number looks smaller and more attractive. Mortgages and most bank term loans use reducing balance. Always ask which method applies before comparing two offers.
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