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Categories: Calculator , Finance
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A mortgage converts a large real-estate purchase into a series of smaller payments spread across a fixed term. Each payment blends interest—charged on the outstanding balance—and principal that steadily reduces that balance. Over time the interest share falls while the principal share rises, producing the familiar amortisation curve.

This tool lets you explore that curve interactively. Enter the property price, choose a down-payment method, set interest, term, and payment frequency, then optionally add extra or lump-sum repayments plus taxes, insurance, fees, or private-mortgage-insurance. A reactive engine recalculates the amortisation schedule and redrafts three charting-layer visualisations in real time.

Use it when comparing loan offers, estimating affordability, or testing how additional payments shorten repayment length and lower total interest. *Figures are illustrative; always confirm terms with your lender.*

Technical Details:

An amortising loan applies compound interest to a declining principal and repays both through regular, equal-sized instalments. The key variables are loan amount L, nominal annual interest rate ra, payments per year m, and total periods n = m × years. A periodic rate r = ra/m underpins every subsequent calculation.

FormulaVariableMeaning
P= L⁢r 1− 1+r−n P Base principal-and-interest payment per period
StageTypical Interest %Principal %
Early≈ 70 – 90≈ 10 – 30
Mid-term≈ 40 – 60≈ 40 – 60
Late≈ 10 – 30≈ 70 – 90

A falling interest share means later payments accelerate principal reduction; extra payments early in the term therefore achieve the greatest interest savings.

  • Price – contracted purchase price.
  • Down-payment – entered as percentage or currency amount.
  • Term – loan length in years.
  • Interest % / yr – nominal annual rate.
  • Frequency – monthly (12) or bi-weekly (26) payments.
  • Extra/Lump-sum – optional additional reductions.
  • Tax %, Insurance, HOA, PMI – ancillary housing costs.

Example (US$ 280 000 loan, 4 % APR, 30 years, monthly):

r=0.04/12=0.003333 n=30×12=360 P=280000⁢0.003333 1−1.003333−360 ≈1336.76
  • Constant interest rate throughout the term.
  • No negative amortisation or interest-only phases.
  • Lender compounds interest at the chosen frequency.
  • Taxes and insurance are estimated, not exact.
  • Zero or near-zero rates flatten amortisation; rounding errors grow.
  • Balloon payments after the schedule ends are unsupported.
  • Early payoff triggers unused periods; tool stops when balance ≤ 0.
  • Extremely high extras may produce negative balance; tool caps to zero.

Formulas mirror standard annuity theory documented in academic mortgage mathematics and consumer-finance guidelines from central-bank publications.

No personal data leaves your browser; calculations remain local and exempt from GDPR transfer rules.

Step-by-Step Guide:

Follow the steps below to generate an amortisation schedule and explore repayment scenarios.

  1. Enter the Loan amount or leave default.
  2. Specify a Down-payment and choose percentage % or currency ¤.
  3. Add the Interest rate and Term years.
  4. Open Advanced to adjust payment Frequency, extras, taxes, insurance, fees, or PMI.
  5. Select a results tab to view the schedule, payment breakdown, balance pie, or cumulative totals.
  6. Press Download CSV to export the full amortisation table.

FAQ:

Is my data stored?

All inputs are processed client-side and vanish when you close the page; nothing reaches external servers.

Can I model bi-weekly payments?

Yes—select “Bi-weekly” under Payment frequency and the tool recalculates using 26 periods per year.

What if I pay extra each month?

Enter an Extra payment; the schedule shows the shortened term and reduced interest automatically.

How is PMI handled?

If the down-payment is below 20 %, you may enter a yearly PMI rate; its cost is added until the balance drops below 80 % LTV.

Why does the balance end slightly negative?

Rounding to two decimals can overshoot by a few cents; the tool clips the final value to zero for clarity.

Glossary:

Amortisation
Systematic repayment of principal plus interest.
Principal
Outstanding loan balance before interest.
Interest
Charge for borrowing, expressed as a percentage of principal.
PMI
Insurance protecting lenders when down-payment < 20 %.
LTV
Loan-to-value ratio: loan ÷ property price.