Blackdog Economy Lab
Home›Loan Interest Calculator

Loan Interest Calculator

How much will you pay
each month and overall?

Estimate monthly payments and total interest across repayment methods and an optional interest-only period.

Loan assumptions

Reset

Only interest is paid during this period; principal repayment starts afterward.

Assumes a fixed rate and monthly payments. Origination costs, prepayment penalties, and other fees are excluded.

Completing a calculation publishes selected loan inputs and results in the recent-calculations list for up to 30 days. Privacy policy

COMMON LOAN SCENARIOS

Compare estimated payments and total interest
before you calculate.

These examples assume a fixed rate, monthly calculations, no interest-only period, and no additional fees. A lender's actual result can differ because of daily accrual, payment dates, rounding, product terms, and changing rates.

By loan amount

4% annual rate · 30 years · level payments

Loan amount
$50,000
Estimated monthly payment
$239
Estimated total interest
$35,934
Loan amount
$100,000
Estimated monthly payment
$477
Estimated total interest
$71,868
Loan amount
$200,000
Estimated monthly payment
$955
Estimated total interest
$143,739
Loan amount
$300,000
Estimated monthly payment
$1,432
Estimated total interest
$215,607

By rate and term

$100,000 · level payments · no interest-only period

Scenario
3% · 10 years
Estimated monthly payment
$966
Estimated total interest
$15,873
Scenario
3% · 20 years
Estimated monthly payment
$555
Estimated total interest
$33,103
Scenario
3% · 30 years
Estimated monthly payment
$422
Estimated total interest
$51,778
Scenario
4% · 10 years
Estimated monthly payment
$1,012
Estimated total interest
$21,494
Scenario
4% · 20 years
Estimated monthly payment
$606
Estimated total interest
$45,435
Scenario
4% · 30 years
Estimated monthly payment
$477
Estimated total interest
$71,868
Scenario
5% · 10 years
Estimated monthly payment
$1,061
Estimated total interest
$27,278
Scenario
5% · 20 years
Estimated monthly payment
$660
Estimated total interest
$58,389
Scenario
5% · 30 years
Estimated monthly payment
$537
Estimated total interest
$93,257

By repayment method

$100,000 · 4% annual rate · 30 years · no interest-only period

Repayment method
Level payment
Estimated monthly payment
$477
Estimated total interest
$71,868
Repayment method
Equal principal
First monthly payment
$611
Estimated total interest
$60,166
Repayment method
Bullet
Monthly interest
$333
Estimated total interest
$119,999

Calculation method

Repayment structure changes
both cash flow and total interest.

01

Level payment

Principal and interest combine into a nearly level monthly payment. Interest dominates early payments, while principal grows over time.

02

Equal principal

The same principal amount is repaid each month and interest is charged on the remaining balance. Payments start higher and decline over time.

03

Bullet loans and interest-only periods

A bullet loan pays interest until principal is due at maturity. An interest-only period likewise delays principal reduction and generally increases total interest.

04

Borrowing $100,000 at 4% for 30 years

This example assumes level monthly payments, no interest-only period, and monthly interest calculation.

Loan principal
$100,000
Estimated monthly payment
$477
Estimated total interest
About $71,868

A longer term lowers the monthly payment but keeps principal outstanding longer, which can substantially increase total interest.

05

Frequently asked questions

Why does the lender quote a different payment?

Lenders may use exact payment dates, daily accrual, product-specific rounding, changing rates, and other contract terms.

Are origination and prepayment costs included?

No. Origination fees, insurance, guarantees, taxes, and prepayment penalties must be reviewed separately.

Which repayment method is best?

Faster principal reduction may lower total interest, but affordability and monthly cash flow should be considered alongside the total cost.

Please check the entered values.