Effective interest rate on flat rate loans: how to calculate the true cost

A flat rate is easy to quote and easy to misread. "12% flat" sounds like 12%, but the borrower is repaying principal all the time while still being charged interest on the original amount, so the real cost is much higher. The effective interest rate puts flat rate loans on the same footing as reducing balance loans, so borrowers and lenders can compare them honestly.

Why the quoted flat rate understates the cost

On a flat rate loan, interest is calculated once on the original principal and spread across the installments. Take 1,000 at 12% flat for 12 months:

• Interest = 1,000 × 12% × 1 = 120
• Total repayable = 1,120
• Installment = 1,120 ÷ 12 = 93.33

After the first installment the borrower owes less than 1,000, yet the next month's interest is calculated as if they still owed all of it. By the last month they owe about 93 but are charged interest as if they owed 1,000. The average amount the borrower actually holds is roughly half the loan, so the real rate is roughly double the quoted one.

A quick approximation

For loans repaid in equal installments, a widely used rule of thumb is:

Effective annual rate ≈ 2 × n × flat rate ÷ (n + 1)

where n is the number of installments and the flat rate is the annual figure.

For 12 installments at 12% flat: 2 × 12 × 12% ÷ 13 = 22.15%. It slightly overstates the exact answer, but it is close enough for a quick check, and it makes the point clearly: about 22% is the cost, not 12%.

The exact method

The exact effective rate is the internal rate of return of the loan cash flows: the periodic rate at which the installments repay the principal over the number of periods.

1. Calculate the total repayable and the installment.
2. Find the rate r that satisfies: Principal = Installment × (1 − (1 + r)^−n) ÷ r.
3. Multiply r by the number of periods in a year to get the annualized (nominal) rate, or compound it to get the effective annual rate.

There is no algebraic shortcut for step 2, so use a spreadsheet's RATE function (in Excel or Google Sheets: =RATE(12, -93.33, 1000)) or a calculator.

For 12% flat over 12 months, RATE returns about 1.79% a month, which is:

• 21.5% a year as a nominal annualized rate (1.79% × 12), and
• 23.7% as an effective annual rate once monthly compounding is included.

Most regulators and comparison tools use the nominal annualized figure (often called the annual percentage rate, APR), but the definition varies by country, so check how yours is defined.

Worked examples

12% flat, 12 months, monthly installments

• Installment: 93.33
• Exact rate: about 1.79% a month, 21.5% APR
• Rule of thumb: 22.15%

24% flat, 6 months, monthly installments, on 500

• Interest = 500 × 24% × 0.5 = 60; total repayable = 560; installment = 93.33
• Exact rate: about 3.34% a month, 40.0% APR

10% flat in total, 10 weekly installments, on 1,000

Here the 10% is charged for the whole 10-week term, not per year.

• Interest = 100; total repayable = 1,100; installment = 110
• Exact rate: about 1.77% a week
• Annualized: about 1.77% × 52 = 92% a year (over 149% if weekly compounding is included)

The last example shows why short-term, high-frequency loans can be very expensive even when the flat charge looks small. A "10%" charge over ten weeks is not a 10% loan.

Effective rate for different flat rates

For monthly equal installments, the effective annualized rate for common flat rates is approximately:

Adding fees to the effective rate

Fees raise the effective rate further, because the borrower pays them on top of interest. The correct way to include a fee is to treat it as a cash flow: if a fee is deducted at disbursement, the borrower receives less than the principal but repays the full installments, so run the rate calculation with the smaller amount received. For example, a 5% fee deducted from a 1,000 reducing balance loan (12% a year, 12 months) means the borrower receives 950 and repays 12 installments of 88.85, which is an annualized rate of about 21.9%, against 12% without the fee. Our guide to deductible and capitalized loan fees explains the difference.

Should you show borrowers the effective rate?

In many countries you must. Consumer-credit and microfinance rules often require lenders to disclose the total cost of credit, or an APR, before the borrower signs. Even where it is not required, showing the total repayable in currency is good practice: it is the number the borrower actually cares about, and it prevents disputes later. If you lend on flat rate terms, publish both the flat rate and the total to repay.

Flat rate loans in LoanTabs

LoanTabs supports flat rate loans alongside declining and reducing balance, interest-only and compound methods, set per loan product. When you create a loan, the live schedule preview shows every installment and the total to be repaid, so you can quote a borrower the real cost. You can compare methods on the same loan by creating one product for each. See how to calculate loan interest for every method, and flat rate vs reducing balance interest for a side-by-side.

FAQ

What is the effective interest rate?

The effective interest rate is the true annual cost of a loan once the timing of payments (and often fees) is taken into account. For a flat rate loan it is the reducing balance rate that gives the same payments.

How do I calculate the effective rate of a flat rate loan in Excel?

Calculate the installment, then use =RATE(number_of_installments, -installment, principal) and multiply by the number of periods per year.

Is a 10% flat rate the same as a 10% reducing balance rate?

No. A 10% flat rate is equivalent to roughly 18% on a reducing balance basis for a 12-month loan.

Why do regulators require APR disclosure?

Because flat rate and reducing balance offers are otherwise hard to compare. A single, consistent measure lets borrowers see which loan is cheaper.