Guide · 6 min read
How EMI Is Actually Calculated
Every loan EMI you’ve ever paid — home loan, car loan, personal loan — comes from the same formula. Understanding it isn’t just academic: it explains why your early payments barely dent the principal, why prepaying a new loan saves so much more than prepaying an old one, and why two loans with similar EMIs can cost wildly different amounts overall.
The formula
Indian banks and NBFCs calculate EMI using the reducing-balance method:
EMI = P × r × (1+r)ⁿ / ((1+r)ⁿ − 1)
Where P is the principal (loan amount), ris the monthly interest rate (annual rate ÷ 12 ÷ 100), and nis the number of monthly installments. The output is a fixed monthly payment that stays the same for the entire tenure, as long as the rate doesn’t change.
Why “fixed EMI” doesn’t mean “fixed split”
The EMI amount is constant, but what it’s made of changes every single month. Each installment is split into two parts:
- Interest — calculated on whatever principal is still outstanding at the start of that month.
- Principal— whatever’s left of the EMI after interest is deducted.
In month one of a 20-year home loan, the outstanding balance is the full loan amount — so the interest portion of that first EMI is close to its maximum. As the balance shrinks month by month, the interest portion shrinks with it, and the principal portion grows to fill the gap. By the last few years of the loan, almost the entire EMI is going toward principal.
This is why a 20-year loan can end up costing more in interest than the loan amount itself, even at a moderate rate — you’re paying substantial interest on a large balance for a long time before it meaningfully shrinks.
What this means for prepayment
Because interest is calculated on the outstanding balance, paying down principal early has an outsized effect: every rupee you prepay stops accruing interest for every remaining month of the loan. Prepaying ₹1,00,000 in year 2 of a 20-year loan saves far more total interest than prepaying the same ₹1,00,000 in year 18 — because in year 2, that money would otherwise have sat on the books accruing interest for 18 more years; in year 18, only 2 more years.
This is also why tenure matters more than most people expect. Stretching a loan from 15 to 20 years might only lower the EMI by 10–15%, but it can add years of extra interest, because the balance stays higher, for longer, than it would on the shorter tenure.
Where to see this in practice
The EMI Calculator shows the full month-by-month split for any loan, along with a year-by-year chart of principal versus interest. The EMI Interest Calculatorputs the tenure trade-off front and center, comparing total interest at different tenures side by side. And if you’re specifically weighing a prepayment, the Prepayment Strategy Calculatorshows exactly how much a lump sum saves, and whether it’s better used to shorten your tenure or lower your EMI.