Calculator Inputs
Layout auto-adjusts: 3 columns large, 2 medium, 1 mobile.Enter your loan details and optional extra payments. Add lump sums using the table; leave unused rows blank.
Example Data Table
Use these sample inputs to test the calculator quickly.
| Field | Example | Why it matters |
|---|---|---|
| Loan Amount | 250000 | Sets the starting balance for interest and principal. |
| Annual Rate | 6.25% | Drives the monthly interest portion each period. |
| Term | 30 years | Defines the baseline payment and payoff horizon. |
| Extra Monthly Principal | 150 | Accelerates principal reduction every payment. |
| Lump Sums | 5000 on 2027-06-01 | Can shorten term and reduce total interest. |
Formula Used
- Monthly rate: r = (APR / 100) / 12
- Number of payments: n = years × 12 + months
-
Scheduled payment (principal + interest):
PMT = P × [ r(1+r)^n ] / [ (1+r)^n − 1 ]If r = 0, then PMT = P / n. -
Each period:
Interest = Balance × r
Principal = Payment − Interest
Ending Balance = Balance − Principal − Extra − LumpSum - Payoff logic: Extra and lump sums are capped to avoid negative balance.
How to Use This Calculator
- Enter loan amount, interest rate, and the term.
- Set the first payment date to match your loan.
- Add an optional extra monthly principal payment.
- Add lump sums with dates and amounts.
- Choose when lump sums are applied for modeling.
- Click Calculate Amortization to view results.
- Use CSV or PDF export to save your schedule.
Core payment math in practice
The calculator starts with a fixed principal-and-interest payment based on loan amount, annual rate, and term. Using the example inputs (250,000 at 6.25% for 30 years), the scheduled payment is 1,539.29 per month. The first month’s interest is about 1,302.08, so only 237.21 reduces principal initially. Over 360 payments, the baseline produces about 304,145.48 of interest and total payments near 554,145.48. The chart highlights how the balance declines slowly early, then faster as principal share rises.
How lump sums change the timeline
Lump sums reduce principal immediately, which reduces future interest because interest is computed on the remaining balance. With an extra 150 monthly plus a 5,000 lump applied in an early payment month, a sample run paid off in 273 payments. That is 87 months sooner than standard, moving payoff to roughly year 23 of the loan. A single early lump also lowers later interest charges on every subsequent payment.
Interest savings and cash-flow tradeoffs
In the same sample run, total interest dropped to about 214,506.57, implying interest savings near 89,638.91. Savings come from faster principal reduction, but they require higher out-of-pocket cash through recurring extras and one-time lumps. Use the “standard” totals to judge whether the acceleration fits your budget. Total paid will rise when you add extras, yet the reduced interest can outweigh the added cash.
Timing assumptions you can test
The “Apply lump sums” option lets you model timing. Applying a lump before interest accrues is a planning case that can slightly increase savings versus applying after the scheduled payment. If your lender posts the extra on the payment date, the “after” option may match statements more closely.
Reporting, review, and validation
Use the schedule table to audit each period: beginning balance, interest, principal, extras, and ending balance should reconcile. Export CSV for spreadsheet checks and PDF for shareable summaries. Results assume a fixed rate and monthly payments; fees, escrow, and penalties vary by lender. Minor rounding differences are normal here.
FAQs
1) What counts as a lump sum here?
Any one-time principal payment with a date. The amount is applied in the period that includes that date, reducing balance directly and lowering future interest.
2) Why is my interest higher at the start?
With fixed payments, interest is calculated on the full opening balance. Early payments therefore contain more interest and less principal. As the balance falls, the interest portion shrinks and principal grows.
3) Should I choose “before” or “after” for timing?
Choose “after” to resemble many lender statements where extras post on the payment date. Choose “before” to model funds applied ahead of the period’s interest, which can slightly increase savings.
4) Can extra payments ever increase total cost?
Extra payments raise out-of-pocket cash, so total paid can be higher even when interest falls. Use the interest saved and the earlier payoff date to judge whether the tradeoff is worthwhile for your goals.
5) Why does the last payment look different?
When the remaining balance is small, the calculator caps the final payment, extra, and lump sum so the ending balance reaches zero. This avoids negative balances and keeps totals consistent.
6) Do CSV and PDF exports include my inputs?
Yes. The exports include a summary block with key inputs and results, followed by the full schedule rows. Run a calculation first, then use the download buttons to save your report.