Mortgage Calculator (inc. balance remaining after a number of payments)

mortgage_calculator.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import sys

mortgage_value = float(sys.argv[1])
mortgage_rate = float(sys.argv[2]) / 100
mortgage_term = float(sys.argv[3])
mortgage_fixed_term_m = int(sys.argv[4]) if len(sys.argv) > 4 else mortgage_term * 12

monthly_interest_rate = mortgage_rate / 12
payments = mortgage_term * 12
total_repayment = payments * mortgage_value * (monthly_interest_rate * pow(1 + monthly_interest_rate, payments)) / (pow(1 + monthly_interest_rate, payments) - 1)
total_repayment_m = total_repayment / payments
total_interest = total_repayment - mortgage_value
average_interest = total_interest / payments
average_repayment = total_repayment_m - average_interest

def print_line_groups(line_groups):
    for line_group in line_groups:
        print('')
        for line in line_group:
            print('{:>40s}:{:>4s}{:>12.2f}{:>2s}'.format(line[0], line[1], line[2], line[3]))


print_line_groups([[
    ('Total interest', '£', total_interest, ''),
    ('Average interest / m', '£', average_interest, ''),
],[
    ('Repayment / m', '£', average_repayment, ''),
    ('Total cost', '£', total_repayment, ''),
    ('Total cost / m', '£', total_repayment_m, ''),
]])

calculation_payments = min(mortgage_fixed_term_m, mortgage_term * 12)

repayment_interest_amount_paid = 0
eom_amount = mortgage_value
for i in range(calculation_payments):
    interest = eom_amount * monthly_interest_rate
    eom_amount -= (total_repayment_m - interest)
    repayment_interest_amount_paid += interest

if calculation_payments < mortgage_term * 12:
    print('\nAfter {} payments, there is £{:10.2f} left to repay'.format(mortgage_fixed_term_m, eom_amount))

interest_only_amount_paid = monthly_interest_rate * mortgage_value * calculation_payments

print_line_groups([[
    ('Repayment mortgage interest', '£', repayment_interest_amount_paid, ''),
    ('Interest-only mortgage interest', '£', interest_only_amount_paid, ''),
], [
    ('Saving with repayment mortgage', '£', interest_only_amount_paid - repayment_interest_amount_paid, ''),
    ('', ' ', 100 * (interest_only_amount_paid - repayment_interest_amount_paid) / interest_only_amount_paid, '%'),
]])

If you are taking out a £400,000 mortgage at a rate of 1.41% over a 5 year period (for the first 63 payments usually) you'd execute the following.

❯❯❯ python mortgage-calculator.py 400000 1.41 30 63

                          Total interest:   £    90778.07  
                    Average interest / m:   £      252.16  

                           Repayment / m:   £     1111.11  
                              Total cost:   £   490778.07  
                          Total cost / m:   £     1363.27  

After 63 payments, there is £ 341624.13 left to repay

             Repayment mortgage interest:   £    27510.29  
         Interest-only mortgage interest:   £    29610.00  

          Saving with repayment mortgage:   £     2099.71  
                                        :            7.09 %