Flat rate vs reducing balance interest: what a borrower really pays

A lender who says "12% interest" could mean two very different things. On a flat rate loan, 12% is charged on the full amount for the full term. On a reducing balance loan, 12% is charged on whatever is still owed each month. The label is the same, the cost is not. Because this choice touches every loan product, it is worth understanding exactly how the two methods behave.

This guide compares them on one loan: 1,000 borrowed for 12 months, repaid in 12 equal monthly installments.

How flat rate interest is calculated

With a flat rate, interest is calculated once, on the original principal, for the entire term.

• Interest = 1,000 × 12% × 1 year = 120
• Total repayable = 1,120
• Monthly installment = 1,120 ÷ 12 = 93.33

Every month the borrower pays 93.33, made up of 83.33 of principal and 10.00 of interest. The interest never changes, even though the amount owed shrinks each month. In month 12 the borrower owes only 83.33 but still pays 10.00 in interest on it, a 12% charge for that month on a balance that is a twelfth of the original.

How reducing balance interest is calculated

With reducing balance interest, each month's interest is the monthly rate times the balance still owed.

• Monthly rate = 12% ÷ 12 = 1%
• Month 1 interest = 1,000 × 1% = 10.00
• Month 6 interest is about 5.98, on a balance of about 597.79
• Month 12 interest is about 0.88, on a balance of about 87.97

With equal installments the payment is 88.85 a month, giving total interest of 66.19. The borrower pays less each month and far less in total, because they are only charged for the money they still have.

Side by side on the same loan

In the first month both loans charge the same 10.00. By month 12 they are very far apart. Flat rate keeps charging interest on principal the borrower has already repaid.

Why flat rate loans cost more than they look

A borrower on the flat rate loan effectively has the use of the full 1,000 only in the first month. From then on they have less, and a smaller and smaller amount, yet they pay the same interest. To compare properly you need the rate that would produce the same payments on a reducing balance basis, the effective rate.

For this loan, 12% flat is equivalent to about 21.5% a year on a reducing balance basis. The table shows how the gap grows with the quoted rate, for 12-month loans with equal monthly installments:

A useful rule of thumb: a flat rate is roughly 1.7 to 1.8 times as expensive as the same number quoted on a reducing balance basis. Our detailed guide to the effective interest rate on flat rate loans shows how to calculate the exact figure.

Which is better for the borrower? For the lender?

For the borrower, reducing balance is almost always cheaper for the same headline rate, and it rewards early repayment: pay off principal sooner and you stop paying interest on it. On a flat rate loan an early payoff often saves little unless the agreement gives a rebate.

For the lender, flat rate is simple to explain and produces a higher yield for the same label. That is exactly why regulators in many countries require lenders to disclose the effective rate or total cost of credit, so that borrowers can compare a flat rate offer with a reducing balance one. If you lend to borrowers who compare offers, expect them to compare effective costs, and price accordingly.

There is also a practical point: flat rate is easy to compute by hand, which is why it grew up in small lending. Once you have software that does the arithmetic, that advantage disappears.

How to compare two loan offers fairly

1. Ask for the total repayable, in currency, not only the rate.
2. Ask how often payments are made and for how long.
3. Ask about fees and how they are taken: deducted at the start, or added to the loan. See deductible vs capitalized fees.
4. Compare the total repaid, or convert both to an effective annual rate.

If one lender quotes 15% flat and another 24% reducing balance for the same 12 months, the effective rates are roughly 26.6% and 24%, so the "higher" 24% offer is actually cheaper.

Choosing a method for your loan products

• Use reducing balance for larger or longer loans, for formal lending, and wherever borrowers or regulators expect it.
• Use flat rate only where your market expects it, and always disclose the total cost.
• If you offer both, keep them as separate loan products with clear names, so officers cannot mix them up.

Flat rate and reducing balance in LoanTabs

LoanTabs supports both. On each loan product you choose the interest method: flat, declining balance, reducing balance with equal installments or equal principal, interest-only, or compound. When you create a loan, the live schedule preview shows every installment and the total, so you can compare the two methods on the same loan before you decide. See loan origination for how products and previews work, and the guide to how to calculate loan interest for every method side by side.

FAQ

Is flat rate or reducing balance better?

Reducing balance is cheaper for the borrower at the same headline rate. Flat rate is simpler to quote and costs the borrower more, so it should always be accompanied by the total repayable.

How do I convert a flat rate to a reducing balance rate?

Work out the installment from the flat calculation, then find the periodic rate at which that installment repays the principal over the same number of periods. Most spreadsheets do this with the RATE function. For a 12-month loan the effective annual rate is roughly 1.75 times the flat rate.

Does flat rate interest change if the borrower pays early?

Not by itself. Because interest was calculated on the original principal, paying early only helps if the loan agreement gives a rebate on unearned interest. Reducing balance loans naturally cost less if repaid early.

Can I offer both methods?

Yes. Create a separate loan product for each so the method is fixed per product and clearly named.