Home Loan Part Prepayment Calculator and EMI Savings

A home loan part prepayment calculator estimates how an extra lump-sum payment changes your remaining loan balance, interest cost, and payoff time. Enter your current balance, annual interest rate, remaining term, regular monthly payment, and planned prepayment to see an estimate of the revised payoff period and interest saved.

Formula

This estimate assumes the current monthly payment is the level payment needed to repay the outstanding balance over the remaining term. It models the extra payment as being applied immediately, before the next scheduled monthly payment, then keeps the regular payment unchanged. The outputs are the new estimated payoff time and the interest avoided compared with making no extra payment.

• Monthly rate: r = APR / 1200, where APR is the annual rate entered as a percentage.
• Number of scheduled payments: n = remaining years × 12.
• Regular monthly payment: M = round[B × r / (1 − (1 + r)⁻ⁿ), 2]. B is the current balance in dollars.
• Balance after the immediate prepayment: B′ = B − E, where E is the extra payment in dollars.
• Estimated new payoff months: k = ceil[−ln(1 − r × B′ / M) / ln(1 + r)].
• Interest avoided: (M × n − B) − (M × k − B′). This compares the scheduled-plan interest with estimated interest after prepayment, using the rounded regular payment.

The payoff-month expression requires the regular payment to exceed monthly interest on the reduced balance. If the prepayment is at least the whole balance, the ordinary monthly-payoff calculation no longer describes the result. This calculator’s formula is intended for a positive balance after prepayment and a payment that can amortize it.

Worked Example

Using the exact defaults above: balance B = $200,000, APR = 8%, remaining term = 20 years, and extra payment E = $10,000.

1. Convert the rate and term: r = 8 / 1200 = 0.0066667 per month; n = 20 × 12 = 240 months.
2. Calculate the monthly payment: M = round[$200,000 × 0.0066667 / (1 − (1.0066667)⁻²⁴⁰), 2] = $1,673.29.
3. Apply the prepayment: B′ = $200,000 − $10,000 = $190,000.
4. Find the approximate new payoff period: k = ceil[−ln(1 − 0.0066667 × $190,000 / $1,673.29) / ln(1.0066667)] = 217 months (rounded up to a whole monthly payment).
5. Compare modeled interest: without prepayment, $1,673.29 × 240 − $200,000 = $201,589.60. After prepayment, $1,673.29 × 217 − $190,000 = $173,903.93. Estimated interest avoided = $27,685.67.

The rounded payoff month represents a final payment that may be smaller than the regular payment. Actual payment dates, lender rounding, and the way the prepayment is credited can change the result.

Assumptions

• The rate stays fixed throughout the estimate.
• Monthly payments are made at the end of each monthly period; the extra payment is applied immediately before the next scheduled payment.
• Interest is modelled monthly at APR divided by 12.
• The regular payment is calculated from the current balance and remaining term, rounded to cents, and held constant after prepayment.
• Fees, taxes, insurance, mortgage insurance, escrow, rate changes, and lender-specific prepayment treatment are not included.

How a Part Prepayment Changes a Loan

A part prepayment is an additional payment toward principal rather than a replacement for the scheduled installment. Reducing principal generally reduces the balance on which later interest is calculated. If the regular installment remains unchanged, more of each later payment can go toward principal, which may shorten the repayment period.

Some servicing arrangements instead lower the required monthly installment while retaining the original end date. This calculator does not model that option: it holds the payment constant. Ask the servicer how it will apply an extra payment and whether you can select a shorter term, a lower installment, or another treatment. Do not assume an amount sent above the installment is automatically treated as principal.

How to Use and Interpret the Estimate

• Use the current principal balance, not the original amount borrowed. Check a recent loan statement or request a payoff/balance figure from the servicer; a payoff quote may include accrued interest or charges that this model excludes.
• Enter the remaining term. Use the time left to repay, rather than the original loan term. The calculator derives a modeled regular installment from that balance, rate, and term.
• Enter the planned lump sum. The extra payment must be less than the balance for this formula to produce a positive reduced balance.
• Read payoff months as an estimate. The whole-month result rounds up, and the final installment may differ from the regular installment.
• Compare interest only on the stated assumptions. The interest difference is not a guaranteed saving or a complete measure of a loan’s costs.

For a reliable comparison, check that the derived payment is close to the actual required installment. If it is not, the balance, remaining term, rate, payment structure, or payment history may differ from the simplified model. Enter the actual scheduled payment instead only if using a calculator designed to accept it; this page’s estimate derives the payment from its inputs.

Limits and Common Errors

Home loans may have variable rates, offset or redraw features, different payment frequencies, daily interest calculations, or servicing rules that affect when an extra amount reduces principal. This calculator uses a fixed rate and monthly periods, so its results may diverge when those features apply. It also does not establish whether a prepayment charge applies; obtain any relevant charge and payment instructions directly from the servicer.

• Do not enter an APR such as 8% as 0.08; enter 8 because the input is already a percentage.
• Do not confuse the loan’s original term with the remaining term, or the original principal with the current balance.
• Do not treat the estimated interest difference as the amount of the lump-sum payment; the payment reduces principal, while the interest difference is a separate modeled total.
• Do not interpret cents or a precise month count as a promise. Actual accrual dates and rounding can change both.

Frequently Asked Questions

Does a part prepayment reduce EMI or loan tenure?
This calculator keeps the modeled monthly payment unchanged and estimates a shorter payoff time. A servicer may instead recalculate the installment or offer a choice; confirm its process.

How much interest can I save by prepaying my home loan?
The estimate depends on the current balance, rate, remaining term, prepayment amount, payment timing, and how the lender applies it. Enter those details to calculate a modeled difference; the actual result can vary.

Can I enter the original loan amount?
Use the outstanding principal balance at the time of the planned prepayment. The original amount is appropriate only if no principal has yet been repaid.

Does this calculator include prepayment fees?
No. It models principal and interest only. Check your loan documents or ask the servicer whether any fee or other condition applies.

Why does my lender’s result differ?
The lender may use a different balance date, daily interest accrual, payment schedule, rounding method, rate, or prepayment-crediting rule. Compare the inputs and assumptions with the lender’s calculation.

This result is an estimate for educational purposes, and the lender’s disclosure gives the exact figures.

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in bal | Outstanding loan balance | $ | 200000 | 0.01 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in rem | Remaining term | years | 20 | 0.0833 | 50
in extra | Part prepayment | $ | 10000 | 0 | 100000000