Mortgage Amortization Calculator: method, checked example and practical limits
Build a complete mortgage amortization schedule with optional extra principal. The schedule demonstrates how early payments contain more interest and later payments retire more principal under a fixed rate.
Inputs used by this calculator
Mortgage Amortization Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Mortgage principal ($) | 300,000 | 0.01 to 1000000000000 | Supplies the mortgage principal ($) quantity used by the published equation or result breakdown. |
| Annual rate (%) | 6 | 0 to 100 | Supplies the annual rate (%) quantity used by the published equation or result breakdown. |
| Term (years) | 30 | 1 to 50 | Supplies the term (years) quantity used by the published equation or result breakdown. |
| Extra monthly principal ($) | 200 | 0 to 1000000000 | Supplies the extra monthly principal ($) quantity used by the published equation or result breakdown. |
Inputs and measurement conventions
The quantities used on this page are Mortgage principal ($), Annual rate (%), Term (years), Extra monthly principal ($). Start with values for the same situation and read the units attached to each field. The field reference below shows supported choices and numerical limits.
Lender daily accrual and payment rounding can create small differences. A variable-rate contract is not represented by a constant schedule.
Method and equation
Calculate the fixed installment, split each payment between opening-balance interest and principal, and repeat across the term.
Mortgage Amortization Calculator uses the following stated relationship: interest=opening balance×rate/12; principal=payment−interest+extra. Its assumptions determine which question the answer can resolve.
interest=opening balance×rate/12; principal=payment−interest+extraWorked calculation with the starting values
Mortgage principal ($): 300000; Annual rate (%): 6; Term (years): 30; Extra monthly principal ($): 200. These starting values give 1,998.651575 planned monthly principal and interest. The calculation breakdown is Required payment: 1,798.651575; Payoff months: 279; Interest: 256,341.1328; Interest saved: 91,173.43433.
A $300,000 30-year mortgage at 6% is recalculated with $200 extra principal each month.
Reading the answer
The schedule demonstrates how early payments contain more interest and later payments retire more principal under a fixed rate.
Required payment: 1,798.651575; Payoff months: 279; Interest: 256,341.1328; Interest saved: 91,173.43433. These intermediate quantities make the stated Mortgage Amortization Calculator model inspectable; they should be read together with the main answer rather than treated as independent guarantees.
Common errors and model boundaries
Lender daily accrual and payment rounding can create small differences. A variable-rate contract is not represented by a constant schedule.
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
Independent verification and next decision
As the outstanding balance falls, the interest charge falls, leaving more of the unchanged installment for principal.
For the Mortgage Amortization Calculator method, compare interest=opening balance×rate/12; principal=payment−interest+extra with the starting inputs and the calculation table. The linked CFPB: mortgages and CFPB: Loan Estimate explainer references identify the relevant definitions, conventions or domain limitations. Retain intermediate precision when comparing a manually worked answer.
Mortgage Amortization Calculator worked-example ledger
| Quantity | Example value | Role |
|---|---|---|
| Mortgage principal ($) | 300000 | Entered assumption |
| Annual rate (%) | 6 | Entered assumption |
| Term (years) | 30 | Entered assumption |
| Extra monthly principal ($) | 200 | Entered assumption |
| Main answer | 1,998.651575 planned monthly principal and interest | Calculated from the starting inputs |
| Required payment | 1,798.651575 | Intermediate or contextual output |
| Payoff months | 279 | Intermediate or contextual output |
| Interest | 256,341.1328 | Intermediate or contextual output |
| Interest saved | 91,173.43433 | Intermediate or contextual output |
| Months saved | 81 | Intermediate or contextual output |
Frequently asked questions
Why does the principal portion grow?
As the outstanding balance falls, the interest charge falls, leaving more of the unchanged installment for principal.
Which relationship does the Mortgage Amortization Calculator use?
Calculate the fixed installment, split each payment between opening-balance interest and principal, and repeat across the term. The displayed relationship is interest=opening balance×rate/12; principal=payment−interest+extra.
What do the starting values produce in the Mortgage Amortization Calculator?
Mortgage principal ($): 300000; Annual rate (%): 6; Term (years): 30; Extra monthly principal ($): 200. These starting values give 1,998.651575 planned monthly principal and interest. The calculation breakdown is Required payment: 1,798.651575; Payoff months: 279; Interest: 256,341.1328; Interest saved: 91,173.43433.
How should I interpret this Mortgage Amortization Calculator answer?
The schedule demonstrates how early payments contain more interest and later payments retire more principal under a fixed rate.
Which input mistake most affects the Mortgage Amortization Calculator?
Lender daily accrual and payment rounding can create small differences. A variable-rate contract is not represented by a constant schedule.
What is outside this Mortgage Amortization Calculator model?
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
What to keep in mind
Escrow, adjustable rates, fees, payment timing and prepayment restrictions are outside this principal-and-interest schedule.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.