Mortgage Calculator

#	Payment	Interest	Principal	Balance	Copy
{{ row.idx }}	{{ row.payment }}	{{ row.interest }}	{{ row.principal }}	{{ row.balance }}
No schedule yet.

Mortgage payments are structured installments that retire a home loan and cover interest on the remaining balance. Understanding how principal and interest share each payment helps you plan and compare choices with a clear amortization schedule.

Enter the purchase price, down payment, annual rate, and years, then choose monthly or biweekly payments so you can preview the periodic amount and how quickly the balance falls. Try extra payments or a one time lump sum to see how the term shortens and interest drops.

Property tax, insurance, and homeowners association fees can be included for a fuller view of cash flow. Private mortgage insurance can be modeled when the down payment is under twenty percent.

For a price of 350,000 with ten percent down and a four percent rate over 30 years the first payment for principal and interest is about 1,504. About 1,050 goes to interest and about 454 reduces the balance.

Estimates assume a fixed rate and rounding to cents, so totals can differ by a few cents between views. Use consistent units and realistic figures for fair comparisons. Biweekly schedules use 26 periods per year.

Technical Details:

The calculator measures periodic payment sizing and balance reduction for a fixed rate mortgage. It focuses on principal and interest for each period and then adds recurring costs for an all‑in periodic figure used for budgeting.

Computation starts with the standard level payment formula on the net loan amount after down payment. The periodic rate equals the stated annual percentage rate divided by the number of payment periods per year. The schedule records interest as the current balance times the periodic rate, with the remainder of the payment applied to principal. Extra payments and a specified lump sum are applied to principal to shorten the term.

Results are most useful for comparing the pace of repayment and lifetime interest across monthly versus biweekly schedules. Values near the start of a loan are interest‑heavy by design; they shift toward principal as the balance falls.

P = L \cdot \frac{r}{1 - {(1 + r)}^{- n}}

Symbols and units used in the calculations
Symbol	Meaning	Unit/Datatype	Source
L	Loan amount after down payment	Currency	Input‑derived
r	Periodic interest rate	1/period	APR ÷ periods/year
n	Total number of payment periods	Count	Term × periods/year
P	Base payment for principal and interest	Currency	Computed
I₁	First period interest	Currency	Computed
Pr₁	First period principal	Currency	Computed
f	Payment periods per year	12 or 26	Input

Worked example

Inputs: price 350,000; down payment 10%; APR 4; term 30 years; frequency monthly.

\begin{array}{lll} L & = & 315000 \\ r & = & \frac{0.04}{12} \approx 0.0033333333 \\ n & = & 360 \end{array}

P \approx 1503.8

I_{1} = L \cdot r = 1050.00

\Pr_{1} = P - I_{1} \approx 453.8

About thirty percent of the first payment reduces principal; this share grows over time.

Units, precision, and rounding

Currency values are rounded to two decimals per period.
Displayed numbers follow your locale for separators; CSV uses a dot as the decimal point.
Property tax, insurance, PMI, and HOA are converted to a per‑period amount for display.

Validation and bounds

Input validation and bounds derived from the interface
Field	Type	Min	Max	Step/Pattern	Error Text
Loan amount	number	0	—	—	—
Down payment	number	0	—	—	—
Down payment type	enum	—	—	percent \| dollar	—
Interest rate (%/yr)	number	0	—	0.01	—
Loan tenure (years)	number	1	—	—	—
Payment frequency	enum	—	—	monthly \| biweekly	—
Extra payment (per period)	number	0	—	—	—
Lump sum amount	number	0	—	—	—
Lump sum at period	number	1	—	—	—
Property tax	number	0	—	0.01	—
Property tax type	enum	—	—	%/yr \| amount/yr	—
Insurance (per year)	number	0	—	—	—
HOA fee (per month)	number	0	—	—	—
PMI rate (%/yr)	number	0	—	0.1	Shown only when down payment is percent and under 20.

I/O formats

Input and output formats
Input	Accepted Families	Output	Encoding/Precision	Rounding
Numbers for price, down payment, rate, years, extras	Integers and decimals	Amortization table; payment summaries; charts; CSV	Two decimal places for currency	Rounded per period to cents
Enums for frequency and types	monthly, biweekly, percent, dollar, amount	CSV header: Period,Payment,Interest,Principal,Balance	Comma separated, dot as decimal point	As displayed

Performance and determinism

Schedule generation is linear in the number of periods.
A guard stops iteration at total periods plus 120 to avoid runaway loops.
Identical inputs produce identical schedules and totals.

Security, privacy, and compliance

Processing is browser‑based; no data is transmitted to a server.
No sensitive account numbers are required; avoid entering unrelated personal data.
Outputs are educational and not financial advice.

Assumptions and limitations

Fixed nominal rate across the entire term.
Interest compounds at the chosen payment frequency.
Property tax as percent uses purchase price and does not change over time.
PMI is modeled as a flat annual percent of the original loan and does not auto‑cancel.
Extra payments and the lump sum reduce principal immediately in the matched period.
Heads‑up Zero percent interest is not supported by the closed‑form payment formula.
Heads‑up Down payment can exceed price; this yields a non‑positive loan and no schedule.
Biweekly uses 26 periods per year; taxes, insurance, PMI, and HOA are allocated per period for display only.
Per‑period rounding can cause a few cents of difference when summing totals.
Locale formatting affects separators in the UI; CSV always uses a dot for decimals.
Lump sum period counts payment periods, not months or calendar dates.

Edge cases and error sources

Very small rates near zero can cause unstable results.
Extremely short terms produce very large payments and rounding sensitivity.
Lump sum greater than the remaining balance is capped to the payoff amount.
Non‑integer or empty inputs can coerce to invalid numbers until corrected.
Floating point rounding may accumulate minor discrepancies over long schedules.
Very large loan values magnify rounding drift between charts and table.
Setting down payment above 100 percent yields a negative computed loan.
CSV opened in a locale that expects commas as decimals may misinterpret values.
Switching frequency changes the period count; ensure lump sum is still aligned.
Rapid input edits can briefly show partial or stale results while recalculating.

Step‑by‑Step Guide:

Mortgage amortization and cash flow preview with principal and interest split.

Enter Loan amount or price with down payment.
Set Interest rate (% per year) and Term (years).
Choose Monthly or Biweekly payments.
Optional extras: add tax, insurance, HOA, and PMI if applicable.
Model prepayment with an extra amount per period or a lump sum at a period.
Review the schedule, totals, and charts; download the CSV if needed.

Example: 350,000 price, 10% down, 4% APR, 30 years, monthly, extra 100 per period.

You now have the periodic payment, payoff horizon, and interest saved by prepayment.

FAQ:

Is my data stored?

No. Calculations run in your browser and nothing is sent to a server. CSV files are generated locally.

Avoid entering unrelated personal information.

How accurate are the results?

Amounts are rounded to cents each period. Minor differences can appear when summing long schedules or switching views, which is normal for rounded arithmetic.

Which units and formats are supported?

Enter plain numbers. Rates are percent per year. Taxes can be a percent per year or a yearly amount. HOA is per month. CSV uses a dot for decimals.

How do I export the amortization schedule?

Use the Download CSV action in the schedule view. The file lists period, payment, interest, principal, and remaining balance.

How do I compare monthly and biweekly?

Switch the payment frequency. Monthly uses 12 periods per year; biweekly uses 26. Compare payoff length and total interest in the schedule and charts.

When does PMI apply here?

If the down payment is below 20 percent and a PMI rate is set, a per‑period PMI amount is added for budgeting. It does not auto‑cancel in this model.

What if the interest rate is zero?

The closed‑form payment formula does not support a zero rate. Use a very small positive value to approximate interest‑free repayment.

Troubleshooting:

Payment shows as NaN — ensure a positive interest rate and valid numbers.
Lump sum not applied — check that its period matches the schedule index.
PMI not visible — set down payment to percent under 20 and enter a PMI rate.
CSV uses the wrong separator — your spreadsheet may expect commas as decimals; set the import locale.
Charts look stale — switch tabs to refresh; resizing the window also reflows the view.
Totals seem off by cents — rounding is applied every period; this is expected.

If valid inputs still produce a blank schedule, reset the fields to defaults or reload the page; zero rate or a non‑positive loan amount prevents calculation.

Advanced Tips:

Tip Compare monthly versus biweekly using the same inputs to isolate frequency effects.
Tip Try an extra payment roughly equal to one twelfth of the monthly payment to simulate one extra payment each year.
Tip Place a lump sum early in the schedule for the largest interest reduction.
Tip Include tax, insurance, PMI, and HOA to plan cash needs beyond principal and interest.
Tip Use the CSV to extend analysis with scenarios or charts in a spreadsheet.
Tip Keep units consistent across runs so differences reflect inputs and not formatting.

Glossary:

Amortization: Gradual repayment of a loan through equal periodic payments.
Principal: The outstanding loan balance excluding interest and fees.
Interest: The cost of borrowing, computed on the remaining balance.
APR: Annual percentage rate used here as a nominal rate.
PMI: Private mortgage insurance added when equity is low.
HOA: Homeowners association fee collected each month.