Mortgage Simulator

This is a Python program that simulates a mortgage loan and calculates daily or monthly balances, payments, and interest.

Limitations

It currently only works for mortgages where the interest is calculated daily. In fact that is why I wrote this program because other mortgage calculators assumed interest was calculated monthly.

Usage

To run the program, use the following command:

python mortgage_simulator.py START_DATE START_BALANCE INTEREST [--monthly] [--monthly-payment MONTHLY_PAYMENT] [--end-date END_DATE] [--payment-day PAYMENT_DAY]

• START_DATE should be the start date of the loan in the format YYYY-MM-DD.
• START_BALANCE is the starting balance of the loan.
• INTEREST is the annual interest rate as a decimal.
• The --monthly flag is optional. If present, the program will output monthly rows instead of daily rows.
• MONTHLY_PAYMENT is an optional argument that specifies the monthly payment amount.
• END_DATE is an optional argument that specifies the end date of the loan in the format YYYY-MM-DD.
• PAYMENT_DAY is an optional argument that specifies the day of the month on which payments are made. By default, it's set to the 16th day of the month.

Either MONTHLY_PAYMENT or END_DATE must be supplied.

The program will output a CSV file to stdout, with columns for date, balance, interest, and payment amount.

For example, to simulate a loan from January 1, 2023 with a starting balance of $200,000, an annual interest rate of 5%, a monthly payment of $1500, and output daily rows, you would run:

python mortgage_simulator.py 2023-01-01 200000 0.05 --monthly-payment 1500

To output monthly rows instead, you would add the --monthly flag:

python mortgage_simulator.py 2023-01-01 200000 0.05 --monthly --monthly-payment 1500

Output is written to the file mortgage.csv.

Acknowledgments

This program was made with help from ChatGPT, an AI developed by OpenAI. You can view the conversation that led to the creation of this program here.

License

MIT