Complete guide
Mortgage Amortization Calculator: formula, worked example and interpretation
Build a complete mortgage amortization schedule with optional extra principal. Enter the known values and the result updates instantly with an auditable breakdown.
What the Mortgage Amortization Calculator answers
Build a complete mortgage amortization schedule with optional extra principal.
Use the breakdown to compare the result with the source values before making a decision.
Formula and variables
The calculation uses interest=opening balance×rate/12; principal=payment−interest+extra. Every variable comes directly from a labeled field.
interest=opening balance×rate/12; principal=payment−interest+extraWorked example
A $300,000 30-year mortgage at 6% is recalculated with $200 extra principal each month.
Changing any field recalculates the result immediately, so nearby scenarios can be compared without reloading the page.
How to use the result
Treat the result as a planning value and compare it with the detailed rows shown beside it.
Keep units and time periods consistent. Use the share or PDF action to preserve the exact assumptions.
Assumptions and scope
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
The calculator exposes its assumptions instead of filling missing facts with hidden estimates.
Accuracy checks
Confirm the units, dates, rates and source figures before relying on the output.
Round only the displayed answer; Calculator24 keeps full JavaScript precision for intermediate arithmetic and exported values.
Input and output checklist
| Item | What to enter or review |
|---|---|
| Inputs | Mortgage principal ($), Annual rate (%), Term (years), Extra monthly principal ($) |
| Equation | interest=opening balance×rate/12; principal=payment−interest+extra |
| Example | A $300,000 30-year mortgage at 6% is recalculated with $200 extra principal each month. |
| Limit | Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule. |
Frequently asked questions
What does this Mortgage Amortization Calculator calculate?
Build a complete mortgage amortization schedule with optional extra principal.
Which formula does it use?
interest=opening balance×rate/12; principal=payment−interest+extra
Does the result update while I type?
Yes. Valid changes recalculate after a short delay and the previous result is not substituted for invalid input.
Which assumptions should I check?
Check units, rates, dates and every source value shown in the form.
Is the result exact?
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
Can I save or share the calculation?
Yes. Copy, Share, WhatsApp, PDF/Print, CSV, comparison and parameterized-link actions are available.
What to keep in mind
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
Results display up to 8 decimal places; exported numbers preserve calculation precision. This is a calculation summary, not an official certificate.