How EMI Is Calculated: The Complete Formula, Math & Amortization Guide
Equated Monthly Installments (EMIs) are structured so that you pay a constant monthly sum, but the internal split between principal and interest changes dynamically every month.
Key Financial Takeaways
- Indian banks use the reducing balance method where interest is computed solely on the outstanding principal balance.
- In the early years of a 20-year loan, up to 70-80% of your monthly EMI goes directly towards paying interest.
- Even a 0.50% reduction in interest rate or making 1 extra EMI payment per year can save lakhs in interest outgo.
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The Mathematical Formula for EMI
The standard formula used by Indian banks and financial institutions is: EMI = P × r × (1+r)^n / ((1+r)^n - 1)
Where P is the Principal Loan Amount, r is the Periodic Monthly Interest Rate (Annual Rate / 12 / 100), and n is the Total Number of Monthly Installments (Tenure in Years × 12).
For example, if you take a loan of ₹30 Lakh at an annual interest rate of 8.5% for 20 years (240 months), r = 0.085/12 = 0.007083. The resulting EMI comes out to exactly ₹26,035 per month.
Understanding the Amortization Split Over Time
In month 1 of a ₹30 Lakh loan, interest = ₹30,00,000 × (8.5%/12) = ₹21,250. This means from your ₹26,035 EMI, only ₹4,785 goes toward paying down principal balance.
By Year 15, because principal balance has decreased substantially, the interest component drops to under ₹8,000, and over ₹18,000 goes toward paying off the loan principal.
| Loan Phase | Monthly EMI | Interest Portion | Principal Portion | Remaining Balance |
|---|---|---|---|---|
| Month 1 | ₹26,035 | ₹21,250 (81.6%) | ₹4,785 (18.4%) | ₹29,95,215 |
| Year 5 (Month 60) | ₹26,035 | ₹18,742 (72.0%) | ₹7,293 (28.0%) | ₹26,38,400 |
| Year 10 (Month 120) | ₹26,035 | ₹14,923 (57.3%) | ₹11,112 (42.7%) | ₹20,95,100 |
| Year 15 (Month 180) | ₹26,035 | ₹9,105 (35.0%) | ₹16,930 (65.0%) | ₹12,68,900 |
| Year 20 (Month 240) | ₹26,035 | ₹183 (0.7%) | ₹25,852 (99.3%) | ₹0 |
Frequently Asked Questions
Why do banks charge more interest initially?
Because interest is calculated directly on the remaining outstanding principal, which is highest at the start of the loan.
How does partial prepayment reduce interest?
When you make a lump sum prepayment, 100% of the payment reduces the outstanding principal balance immediately, permanently dropping all future monthly interest calculations.
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