Prepay vs Invest Calculator

When you find yourself with surplus cash at the end of the month, or after receiving an annual bonus, you essentially have two choices: reduce your liabilities (debt) or increase your assets (investments). The purely mathematical answer lies in comparing the effective rates of return of both options.

If your home loan interest rate is 8.5%, every extra rupee you pay towards the principal gives you a guaranteed, risk-free, and tax-free return of exactly 8.5%. Because prepayments go entirely toward the principal (not the interest), they drastically reduce your loan tenure, saving you a massive amount of compound interest over the years.

The Concept of "Effective Interest Rate"

However, 8.5% isn't usually your true cost of borrowing. If you are a salaried professional in the 30% tax bracket, the Income Tax Act (Section 24b) allows you to claim a deduction of up to ₹2,00,000 on home loan interest paid. This tax shield essentially means the government is subsidizing a portion of your interest.

When you factor in this 30% tax saving, an 8.5% home loan actually has an effective interest rate of roughly 5.95%.

Now compare this to investing. If you invest that same surplus cash into an equity mutual fund SIP that historically generates 12-15% annually, your money is growing at double the speed of your effective debt cost. Over a 15 or 20-year horizon, the exponential compounding effect of equity investing creates far more wealth than the interest saved by prepaying a subsidized home loan.

While the calculator above provides the absolute mathematical winner based on your inputs, personal finance is highly psychological. The right decision heavily depends on your comfort with debt, job security, and life stage.

• When to strictly PREPAY: If the sheer thought of a ₹50 Lakh debt causes you anxiety or affects your sleep, mathematically optimal returns don't matter—pay it off. Prepayment is also superior if you are nearing retirement (and want to eliminate mandatory EMIs), if you work in an industry with low job security, or if your loan is a high-interest unsecured personal loan (>12%) where investing cannot reliably beat the debt cost.
• When to strictly INVEST: If you are in your 20s or 30s with a long runway for compounding, investing is the clear winner. Equity markets are volatile in the short term but reliably outpace debt costs over 10+ year periods. Furthermore, if you are currently maximizing your Section 24(b) tax deduction, prepaying the loan actually destroys your tax shield, making investing the financially superior choice.