A simple state-based mortgage calculator
This is a very simple mortgage payment calculator, designed to help answer questions like:
The intended target is the American mortgage, but it may well be used for other types of loans also.
There is really not much here other than the “innovation” that complicated mortgage payment schedules can be simulated relatively cleanly by representing the mortgage as a stateful object, whose state (e.g., how much to pay) can be modified from period to period.
An illustrative example will show the supported features.
In this example, we simulate a mortgage that is:
The details below should be self-explanatory:
from mortgage import Mortgage
from decimal import Decimal # avoid floating point conversion
# initial loan: $300K at 4% over 5 years (interest charged monthly)
principal = 300_000
interest_rate = Decimal('0.04') / 12 # monthly rate = 4% / 12
periods = 60 # 5 years = 60 months
m = Mortgage(principal, interest_rate, periods)
# pay loan as usual for one year ...
m.go(12)
# ... then refinance to a better rate
principal = 245_000
interest_rate = Decimal('0.03') / 12
periods = 60
m.refinance(principal, interest_rate, periods, other_costs=2_000)
# go for two months
m.go(2)
# make a large one-time payment and go again
m.pay_extra_one_time(120_000)
m.go(2)
# pay extra each month and go until close
m.pay_extra_each_time(3_000)
m.go()
# see details of how much paid each month
print(m.format())
The output is:
period | principal | principal paid | interest paid | other costs | total paid
-------+-----------+----------------+---------------+-------------+-----------
0 | 300000.00 | 0 | 0 | 0 | 0
1 | 295475.04 | 4524.96 | 1000.00 | 0 | 5524.96
2 | 290935.00 | 4540.04 | 984.92 | 0 | 11049.92
3 | 286379.82 | 4555.18 | 969.78 | 0 | 16574.88
4 | 281809.46 | 4570.36 | 954.60 | 0 | 22099.84
5 | 277223.86 | 4585.60 | 939.36 | 0 | 27624.80
6 | 272622.98 | 4600.88 | 924.08 | 0 | 33149.76
7 | 268006.76 | 4616.22 | 908.74 | 0 | 38674.72
8 | 263375.16 | 4631.60 | 893.36 | 0 | 44199.68
9 | 258728.12 | 4647.04 | 877.92 | 0 | 49724.64
10 | 254065.59 | 4662.53 | 862.43 | 0 | 55249.60
11 | 249387.52 | 4678.07 | 846.89 | 0 | 60774.56
12 | 244693.85 | 4693.67 | 831.29 | 0 | 66299.52
12 | 245000.00 | 0 | 0 | 2000 | 68299.52
13 | 241210.17 | 3789.83 | 612.50 | 0 | 72701.85
14 | 237410.87 | 3799.30 | 603.03 | 0 | 77104.18
14 | 117410.87 | 120000 | 0 | 0 | 197104.18
15 | 113302.07 | 4108.80 | 293.53 | 0 | 201506.51
16 | 109183.00 | 4119.07 | 283.26 | 0 | 205908.84
17 | 102053.63 | 7129.37 | 272.96 | 0 | 213311.17
18 | 94906.43 | 7147.20 | 255.13 | 0 | 220713.50
19 | 87741.37 | 7165.06 | 237.27 | 0 | 228115.83
20 | 80558.39 | 7182.98 | 219.35 | 0 | 235518.16
21 | 73357.46 | 7200.93 | 201.40 | 0 | 242920.49
22 | 66138.52 | 7218.94 | 183.39 | 0 | 250322.82
23 | 58901.54 | 7236.98 | 165.35 | 0 | 257725.15
24 | 51646.46 | 7255.08 | 147.25 | 0 | 265127.48
25 | 44373.25 | 7273.21 | 129.12 | 0 | 272529.81
26 | 37081.85 | 7291.40 | 110.93 | 0 | 279932.14
27 | 29772.22 | 7309.63 | 92.70 | 0 | 287334.47
28 | 22444.32 | 7327.90 | 74.43 | 0 | 294736.80
29 | 15098.10 | 7346.22 | 56.11 | 0 | 302139.13
30 | 7733.52 | 7364.58 | 37.75 | 0 | 309541.46
31 | 350.52 | 7383.00 | 19.33 | 0 | 316943.79
32 | 0.00 | 350.52 | 0.88 | 0 | 317295.19
which reports that the loan is paid off after 32 periods from the origination of the initial mortgage, with a total payment of $317,295.19 (vs. the original $300,000 loan). For each period, the data show: