Interest Rate Converter โ Flat to Reducing & Nominal to Effective
Convert flat interest rates to reducing balance rates, nominal rates to Effective Annual Rates (EAR), and compare loan or deposit offers on a like-for-like basis. RBI mandates EIR disclosure on all loans โ verify dealer and bank claims here.
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How to Use This Calculator
Flat to Reducing tab
Enter the flat interest rate quoted by the dealer or lender (e.g. 8%), the loan tenure in years (e.g. 5 years), and a reference loan amount (default โน1,00,000). The calculator finds the equivalent reducing balance rate using bisection โ the rate at which the standard EMI formula produces the same EMI as the flat rate method. You will see the reducing rate, a side-by-side EMI comparison, and the total cost of the loan under both methods.
Nominal to Effective tab
Enter the nominal (stated) interest rate (e.g. 12%) and select the compounding frequency (annually, semi-annually, quarterly, monthly, or daily). The calculator computes the Effective Annual Rate (EAR) using the standard formula: EAR = (1 + r/n)^n - 1. A comparison table shows the EAR for all compounding frequencies side by side, so you can see exactly how compounding frequency impacts the effective cost or yield.
Rate Comparison tab
Enter up to 3 offers โ each with a nominal rate and compounding frequency. Toggle between Loan (lowest EAR is best) and Deposit (highest EAR is best). The calculator converts all offers to EAR and ranks them, highlighting the best deal. This is useful when comparing a home loan from SBI (monthly compounding) against HDFC (quarterly compounding), or comparing FD rates across banks.
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The Formula
This calculator uses three core formulas to convert and compare interest rates:
EMI = (P + P × r × t) / (t × 12)
Where P = principal, r = flat rate as decimal, t = tenure in years.
Interest is charged on the full principal for the entire tenure.
Reducing Balance EMI:
EMI = P × rm × (1 + rm)n / ((1 + rm)n − 1)
Where rm = monthly reducing rate (annual rate / 12 / 100), n = total months.
Interest is charged only on the outstanding (reducing) balance.
Flat to Reducing Conversion:
Find the reducing rate reff such that Reducing EMI(P, reff, n) = Flat EMI(P, rflat, t).
Solved numerically using the bisection method (200 iterations, precision < 0.001).
Effective Annual Rate (EAR):
EAR = (1 + r/n)n − 1
Where r = nominal annual rate as decimal, n = compounding periods per year.
Accounts for intra-year compounding to give the true annual cost or yield.
The flat-to-reducing conversion always yields a higher rate because the flat method overcharges interest by applying it to the full principal, not the declining balance. The ratio is typically 1.7x to 1.9x for 3-7 year tenures.
Example
Priya — Buying a car in Pune, dealer quotes 8% flat rate on โน5,00,000 for 5 years
Priya (32) is buying a car worth โน8,00,000. She is paying โน3,00,000 down and financing โน5,00,000. The dealer quotes "8% interest" โ but is this the real cost? She uses this calculator to find out.
Step 1: Flat to Reducing Conversion
Step 2: Results
Step 3: What Priya discovers
The dealer's "8% interest" is actually ~14.5% APR on a reducing balance basis. The same EMI of โน11,667 that sounds cheap at "8%" is actually a 14.5% loan. Priya checks her bank's car loan rate โ SBI offers 9.15% reducing rate, which gives an EMI of โน10,389 on the same โน5L for 5 years. She saves โน1,278 per month (โน76,680 over 5 years) by choosing the bank loan over the dealer's "cheaper" offer.
Step 4: Comparing bank offers using EAR
Despite having the highest nominal rate (9.50%), ICICI is the cheapest because annual compounding results in the lowest effective cost. This is why comparing nominal rates directly is misleading โ always use EAR.