How to Use:
How to Use the Interest Crusher Simulator
Model the exact impact of extra EMI payments on any loan. See how many months disappear from your tenure and how many lakhs you avoid paying in interest — before you commit a single rupee.
What You'll Learn
- How to model extra payment scenarios on a home loan, personal loan, or vehicle loan
- Why even ₹2,000–₹3,000 extra per month has a dramatically outsized impact on total interest
- How to use the simulator before a windfall to decide exactly which loan to prepay
What It Does
The Interest Crusher Simulator is one of the most motivating tools in Fin OS — because it makes the invisible visible. When you're paying a ₹40 lakh home loan, the interest you're paying over 20 years is an abstract number. This simulator makes it concrete, then shows you exactly how much of it you can eliminate with small, consistent extra payments.
Who This Guide Is For
You have at least one active loan — home loan, personal loan, or vehicle loan — and want to understand the impact of overpaying before committing to it.
Open the Interest Crusher
Navigate to Decision → Interest Crusher. You'll see a list of all loans you've entered in Fin OS. Select the loan you want to simulate. If you haven't entered your loans yet, go to Decision → Debt Journey → Add Debt first. The simulator pulls all loan parameters (principal, rate, tenure) directly from your debt entries.
The simulator works best on loans with remaining tenures of 3+ years. Short-tenure loans already have most of the interest paid — the impact of extra payments is smaller.
Review Your Current Loan Profile
The simulator opens with your loan's baseline metrics: outstanding principal, monthly EMI, annual interest rate, remaining tenure in months, and — the most important number — your projected total interest payable at current EMI. For a ₹40L home loan at 8.5% with 18 years remaining, this number is typically ₹38–42 lakhs in interest alone. Read it carefully. This is what you're working to reduce.
If the remaining tenure or outstanding principal looks wrong, go back and update your debt entry. Accuracy in your debt profile directly determines the accuracy of the simulation.
Set Your Extra Monthly Payment
Use the slider or input field to set an extra monthly payment amount. Start conservatively — try ₹2,000, then ₹5,000, then ₹10,000 to see the step-change in impact. As you move the slider, three numbers update in real time: your new projected debt-free date, months saved off your tenure, and total interest saved in rupees.
The relationship between extra payment and interest saved is non-linear. The earlier you start paying extra, the higher the impact — each rupee you save from interest today was compounding against you for years.
Read the Side-by-Side Comparison
Below the slider, the simulator shows a direct comparison table: Current path (your debt-free date, total interest paid) vs. Optimized path (new debt-free date, total interest paid, interest saved). For most users running this simulation on a home loan for the first time, the 'Interest Saved' figure is large enough to be genuinely surprising — sometimes ₹4–12 lakhs on a typical ₹40–60L loan.
The comparison also shows your effective cost reduction: if you're paying ₹3,000 extra per month for 10 years (₹3.6L total extra payments) to save ₹9L in interest, your net gain is ₹5.4L. This is the number that makes the trade-off obvious.
Simulate a Lump-Sum Prepayment
Switch from 'Monthly Extra' to 'One-Time Prepayment' mode using the toggle at the top of the simulator. Enter a lump-sum amount — your year-end bonus, a tax refund, or any windfall. The simulator recalculates your new outstanding principal, revised tenure, and total interest saved. Use this before any significant lump-sum decision to see where it has the highest impact across your debt stack.
If you have multiple loans, run the lump-sum simulation on each one. The loan where the prepayment saves the most total interest is always the one with the highest interest rate and the most remaining tenure — typically your personal loan before your home loan.
Pro Tip
Run the simulator on the day you receive your annual salary hike. Calculate what ₹2,000–₹3,000 of the increase (if redirected to loan prepayment instead of lifestyle) would save over your loan's remaining tenure. This single exercise has converted more users to consistent prepayers than any other feature in the app.
RBI regulations prohibit prepayment penalties on floating-rate home loans from banks. Fixed-rate loans and loans from NBFCs may still charge penalties. Check your loan agreement or call your lender before making a large prepayment. The Interest Crusher has an optional 'Prepayment Penalty' field where you can enter the percentage — it will factor it into the net savings calculation.
The break-even is your loan interest rate. If your home loan rate is 8.5%, prepayment is risk-free 8.5% return. To beat that through investment, you'd need expected post-tax investment returns above 8.5% — achievable with equity over long periods, but not guaranteed. Financial planners typically recommend: clear high-interest debt first (above 12%), then split surplus between home loan prepayment and equity investment.
This depends on your lender's policy. Many Indian banks allow you to choose between tenure reduction and EMI reduction on prepayment. Tenure reduction always saves more total interest. EMI reduction improves monthly cash flow. The simulator currently models tenure reduction — if you choose EMI reduction, your actual savings will be somewhat lower than shown.
Ready to try it?
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