Flat vs Reducing Interest Calculator

Flat: total interest = P × rate × years, EMI = (P + interest) ÷ n. Reducing: standard EMI on the falling balance. The equivalent reducing rate is solved numerically.

Why a flat rate is not what it seems

A flat rate charges interest on the full original loan amount for the entire term — even though you are steadily paying that amount down. A reducing (or reducing-balance) rate charges interest only on what you still owe, which falls every month. So the same number means two very different things: a flat 10% is far more expensive than a reducing 10%, because under the flat structure you keep paying interest on money you have already repaid. The headline looks identical; the cost is not.

This calculator puts the two side by side. Enter your rate and say whether it is flat or reducing, and it shows both EMIs, both interest totals, the extra the flat structure costs, and — the key number — the reducing rate that the flat quote actually equals.

Worked example — ₹5,00,000 over 5 years at a flat 10%: Flat interest is 5,00,000 × 10% × 5 = ₹2,50,000, for an EMI of about ₹12,500. That same EMI corresponds to a reducing rate of roughly 17.3% — so a “flat 10%” is really a reducing ~17%, and costs about ₹1,10,000 more than a genuine reducing 10%.

The rule of thumb — and the exact figure

There is a well-known rule of thumb that a flat rate is roughly 1.8 times the equivalent reducing rate — so a flat 10% is about a reducing 18%. It is a handy sanity check, but the true multiple depends on the tenure: the longer the loan, the larger the gap, because there is more repaid principal for the flat structure to keep charging on. Rather than rely on the rule, this calculator computes the exact equivalent rate for your loan, so you can compare a flat quote against a reducing one on equal footing.

This matters because flat rates are still widely advertised — on car loans, two-wheeler loans, consumer durables and some personal loans — precisely because the number looks low. Knowing the reducing equivalent is the difference between comparing like with like and being quietly overcharged.

Reading the comparison

The side-by-side schedule tells the story year by year. Under the flat loan the interest charged stays the same every year, regardless of how much you have repaid — that flat line of unchanging interest is the overcharge made visible. Under the reducing loan the interest falls each year as the balance shrinks. The total-cost chart shows the same thing in one view: the flat bar is longer, and the gap between the bars is the extra you pay for the privilege of a low-looking headline.

If both loans carry fees, add them under the advanced options for a fair total-cost comparison, or use “compare both” to test a specific flat offer against a specific reducing one.