Loan Amortization Calculator
Principal, rate, and term → monthly payment, total interest, full amortization schedule, and principal vs interest pie chart.
This free online loan amortization calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Supports both metric (SI) and imperial units with built-in unit selection dropdowns on every input field, so you can work in whatever units your problem provides. Designed for engineering students and professionals working through coursework, design projects, or quick reference calculations.
Loan Amortization Calculator
Monthly Payment
$1,580.17
Total Interest
$318,861.22
Total Paid
$568,861.22
Principal vs Interest
Amortization Schedule
| Month | Payment | Principal | Interest | Balance |
|---|---|---|---|---|
| 1 | $1580.17 | $226.00 | $1354.17 | $249774.00 |
| 2 | $1580.17 | $227.23 | $1352.94 | $249546.77 |
| 3 | $1580.17 | $228.46 | $1351.71 | $249318.31 |
| 4 | $1580.17 | $229.70 | $1350.47 | $249088.61 |
| 5 | $1580.17 | $230.94 | $1349.23 | $248857.67 |
| 6 | $1580.17 | $232.19 | $1347.98 | $248625.48 |
| 7 | $1580.17 | $233.45 | $1346.72 | $248392.04 |
| 8 | $1580.17 | $234.71 | $1345.46 | $248157.32 |
| 9 | $1580.17 | $235.98 | $1344.19 | $247921.34 |
| 10 | $1580.17 | $237.26 | $1342.91 | $247684.07 |
| 11 | $1580.17 | $238.55 | $1341.62 | $247445.53 |
| 12 | $1580.17 | $239.84 | $1340.33 | $247205.69 |
| 13 | $1580.17 | $241.14 | $1339.03 | $246964.55 |
| 14 | $1580.17 | $242.45 | $1337.72 | $246722.10 |
| 15 | $1580.17 | $243.76 | $1336.41 | $246478.34 |
| 16 | $1580.17 | $245.08 | $1335.09 | $246233.26 |
| 17 | $1580.17 | $246.41 | $1333.76 | $245986.86 |
| 18 | $1580.17 | $247.74 | $1332.43 | $245739.12 |
| 19 | $1580.17 | $249.08 | $1331.09 | $245490.03 |
| 20 | $1580.17 | $250.43 | $1329.74 | $245239.60 |
| 21 | $1580.17 | $251.79 | $1328.38 | $244987.81 |
| 22 | $1580.17 | $253.15 | $1327.02 | $244734.66 |
| 23 | $1580.17 | $254.52 | $1325.65 | $244480.13 |
| 24 | $1580.17 | $255.90 | $1324.27 | $244224.23 |
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Loan Amortization Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Loan Amortization Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Loan Amortization Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Loan Amortization Calculator when solving homework or exam problems that require quick numerical verification of your hand calculations — instant feedback helps identify arithmetic errors before they propagate.
- •Use it during the early design phase to rapidly iterate on parameters and narrow down feasible configurations before committing time to detailed finite element simulations or full design packages.
- •Use it when reviewing a colleague's calculation or checking a vendor's data sheet for plausibility — a quick sanity check can prevent costly downstream errors.
- •Use it to generate reference data for a technical report or presentation without manual computation, ensuring consistent, reproducible numbers throughout the document.
- •Use it in the field when a quick estimate is needed and a full engineering software package is not available.
About This Calculator
The Loan Amortization Calculator is a precision engineering calculation tool designed for students, engineers, and technical professionals. Principal, rate, and term → monthly payment, total interest, full amortization schedule, and principal vs interest pie chart. All calculations are performed using established engineering formulas from the relevant scientific literature and standards. Inputs support both metric (SI) and imperial unit systems, with unit conversion handled automatically — simply select your preferred unit from the dropdown next to each field. Results are computed instantly in the browser without sending data to a server, ensuring both speed and privacy. This calculator is intended as a supplementary tool for learning and design exploration; always verify results against authoritative references for safety-critical applications.
The Theory Behind It
Loan amortization is the process of paying off a loan through regular installments that include both principal and interest. For a fixed-rate amortizing loan, the monthly payment is computed by the amortization formula: PMT = P × i × (1+i)^n / ((1+i)^n − 1), where P is the loan principal, i is the periodic (monthly) interest rate, and n is the total number of payments. The payment is constant, but the split between interest and principal changes each period. In early payments, most of the amount goes to interest because the outstanding balance is large. As the balance shrinks, the interest portion decreases and the principal portion grows. The total interest paid over the life of the loan is n·PMT − P, which can be surprisingly large: on a 30-year mortgage at 6%, total interest exceeds the original principal. Shorter loans (15 years vs 30 years) dramatically reduce total interest but increase monthly payments. Extra principal payments have a cascading effect — each dollar of extra principal reduces all future interest. Amortization schedules tabulate each payment's interest portion, principal portion, and remaining balance over the loan's life. The calculator generates full amortization schedules and summarizes total interest, final payment date, and effect of extra payments.
Real-World Applications
- •Mortgage calculation: compute monthly payment and full amortization schedule for a fixed-rate home loan.
- •Car loan analysis: determine monthly payment, total interest, and payoff schedule for auto financing.
- •Student loan planning: compute standard and extended payment amounts for federal and private student loans.
- •Refinance comparison: compare two loan options (old vs new) by computing total interest, break-even point, and monthly payment difference.
- •Extra principal payment analysis: see how adding $100-500/month to the principal affects total interest and payoff time.
Frequently Asked Questions
What's the amortization formula?
PMT = P·i·(1+i)^n / ((1+i)^n − 1), where P is loan amount, i is periodic interest rate (annual/12 for monthly), and n is number of payments. For $200,000 at 6%/yr for 30 years: i = 0.005, n = 360, PMT = $1,199/month. Total paid = 360 × $1199 = $431,676; total interest = $231,676 (115% of the principal).
Why are early payments mostly interest?
Each month's interest = current balance × monthly rate. When the balance is near the original principal, most of the payment covers interest. As you pay down principal, the interest portion decreases. For a 30-year 6% loan, the first payment is about 83% interest and 17% principal. By year 15, it's 50/50. By year 30, almost all principal.
How much interest do I pay on a mortgage?
For a 30-year fixed at 6%: total interest is about 1.15× the principal. $300,000 mortgage = $345,000 interest. At 4%: about 0.72×, or $216,000. At 8%: about 1.64×, or $492,000. Higher rates and longer terms massively increase total interest. Shorter terms (15-year) reduce total interest by 60-70%.
How much can I save with extra payments?
Depends on loan amount, rate, and extra amount. For a $300,000 mortgage at 6%/yr 30-year: paying an extra $100/month cuts about 4.5 years off the term and saves ~$55,000 in interest. An extra $300/month cuts about 9 years and saves ~$110,000. The earlier you start extra payments, the bigger the impact because each principal reduction avoids years of compounded interest.
When should I refinance?
When you can save more in monthly payments than you'll pay in closing costs over the time you plan to stay in the home. Rule of thumb: refinance if you can reduce rate by 1% AND stay in the home for at least 3 years. Compute total savings (lower payment × months) and compare to closing costs (typically 2-5% of loan). If savings exceed costs within your planned stay, refinancing wins.
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