Flat Rate vs Reducing Balance: Why a 12% Flat Loan Really Costs About 21.6%
A store offers you a phone, a motorbike or a sofa at "12% a year" over 24 months. A bank quotes 12% a year on a personal loan for the same term. Same number, same term — yet on a $10,000 purchase the store plan charges $2,400 in interest and the bank loan about $1,298. The difference is not the rate. It is what the rate is charged on.
With a flat rate, interest is always calculated on the original amount, even after you have repaid most of it. With a reducing balance rate (also called declining balance), each month's interest is charged only on what you still owe. Both are legal, both are common, and they are almost never labeled clearly on a price tag.
Loan Calculator shows the two methods side by side for any loan you type and converts the flat rate into its effective reducing-balance rate, so a store offer and a bank offer can finally be compared on the same scale.
What should you compare?
Two quotes with the same headline rate can hide very different costs. Before signing, line up these four numbers:
- Monthly payment — what leaves your account every month.
- Total interest — the price of the loan over its whole life.
- Effective rate — the reducing-balance rate that produces the same payment. This is the only fair way to put a flat-rate quote next to a bank quote.
- Fees and insurance — processing fees and insurance can add several points on top. They are covered in the true APR guide.
Flat rate vs reducing balance at a glance
All figures below are for $10,000 over 24 months at 12% a year.
| Flat rate | Reducing balance (equal payments) | |
|---|---|---|
| Interest is charged on | The original $10,000, every month | The balance still owed |
| Monthly payment | $516.67 | $470.73 |
| Interest in month 1 | $100.00 | $100.00 |
| Interest in month 24 | $100.00 | $4.66 |
| Total interest | $2,400.00 | about $1,297.65 |
| Effective annual rate | about 21.6% | 12% |
| Where it usually appears | Store installment plans, some consumer and vehicle finance | Bank loans, mortgages, most personal loans |
Notice the first month: both methods charge $100 of interest. The gap opens because the flat plan keeps charging $100 every month, while the reducing loan charges less and less as the balance falls.
1. Flat rate: interest on the original amount
Best for: understanding store and dealer installment quotes, which are often flat even when the word "flat" never appears.
A flat loan splits the principal evenly across the term and adds the same interest every month:
Monthly payment = P ÷ n + P × (annual rate ÷ 12)
= 10,000 ÷ 24 + 10,000 × 1%
= 416.67 + 100.00 = $516.67The total interest is simply P × annual rate × years — here $10,000 × 12% × 2 = $2,400. That simplicity is exactly why flat rates are popular in advertising: the math fits on a sticker. The catch is that by month 23 you owe only about $417, yet you are still paying interest as if you owed $10,000.
2. Reducing balance: interest on what you still owe
Best for: bank loans and mortgages, and as the yardstick for comparing any two offers.
Here the rate applies to the remaining balance. With equal payments (an annuity), the monthly amount stays the same while its interest share shrinks and its principal share grows:
r = annual rate ÷ 12 = 0.01
Monthly payment = P × r ÷ (1 − (1 + r)^−n)
= 10,000 × 0.01 ÷ (1 − 1.01^−24) = $470.73Over 24 months you pay about $1,297.65 in interest. There is also a second reducing-balance style, equal principal, where the payment starts higher ($516.67 here) and falls every month to $420.76; it costs $1,250 in total interest. The differences are explained in equal payment vs equal principal.
3. Effective rate: turning a flat quote into a comparable number
Best for: deciding between a flat-rate offer and a bank loan.
The effective rate answers one question: which reducing-balance rate would give the same monthly payment as this flat plan? For our example, a reducing loan needs an annual rate of about 21.57% to produce a $516.67 payment. The flat "12%" is really about 1.8 times as expensive.
The ratio is fairly stable across common terms, which is why "roughly 1.8×" is a useful rule of thumb:
| Term | Flat total interest | Reducing total interest | Effective rate of 12% flat |
|---|---|---|---|
| 12 months | $1,200 | $661.86 | about 21.5% |
| 24 months | $2,400 | $1,297.65 | about 21.6% |
| 36 months | $3,600 | $1,957.18 | about 21.2% |
| 60 months | $6,000 | $3,346.77 | about 20.3% |
4. How to compare both methods in Loan Calculator
Best for: checking a real offer in under a minute, even without a signal.
- Enter the loan amount, the annual interest rate and the term. If the offer is quoted per month (for example 1% a month), type 12 — the app takes the rate per year.
- Tap Calculate. Your result uses the interest method selected on the form.
- Scroll to the Interest methods compared card. It shows the first payment and total interest for equal payments, equal principal and flat rate, plus a line such as "Flat rate: $1,102.35 more interest than equal payments".
- Read the note under it — "Flat 12%/year ≈ 21.57%/year effective (equal payments)". That is the number to compare with a bank's rate.
- To see the month-by-month difference, switch the interest method to Flat rate and open the amortization schedule.
Every calculation runs on your phone with exact decimal arithmetic, so the figures match a lender's statement to the cent. Nothing you type is uploaded.
Which method should you choose?
You rarely choose the method — the lender does. What you can choose is which offer to take, once both are expressed the same way.
| Situation | What to do |
|---|---|
| A store quotes "0.8–1.5% a month" for an installment plan | Assume it is flat until the contract says otherwise; multiply by 12 and check the effective rate |
| You are choosing between a store plan and a bank loan | Compare the store's effective rate with the bank's rate plus fees |
| Two banks quote the same rate | They are likely both reducing balance — compare fees and insurance instead |
| The contract says "interest on the outstanding balance" | It is reducing balance; the headline rate is already comparable |
| You plan to repay early | Reducing balance rewards early repayment; on a flat plan, ask how interest is refunded |
FAQ
Is a flat rate always more expensive than a reducing rate?
At the same headline number, yes — for any loan longer than one payment, a flat rate charges more total interest. But a flat 7% can be cheaper than a reducing 15%. That is why you convert to the effective rate before comparing.
How do I know if a loan quote is flat?
Look for interest calculated "on the original principal" or "on the loan amount", or a fixed interest figure that is the same every month. If the contract instead mentions the "outstanding" or "remaining" balance, it is reducing. When in doubt, ask the lender for the full repayment schedule.
Why is the effective rate about 1.8 times the flat rate?
On average you owe only about half of the original amount over the life of an amortizing loan, yet a flat plan charges interest on all of it. Paying interest on roughly twice the average balance works out to about 1.7–1.8 times the rate for typical terms.
Can I enter a monthly flat rate like 1% a month?
Yes. Loan Calculator takes the rate per year, so type 12 for 1% a month or 18 for 1.5% a month. The comparison card then shows the equivalent effective annual rate.
Does the app work offline?
Yes. All calculations, including the method comparison and the effective rate, run on your device and work with no internet connection.