Compound interest is the eighth wonder of the world

- He who understands it… earns it!
- He who doesn't … pays it!

– *Albert Einstein*

### Terminology

A bit of terminology first:

\[ \color{blue} P - principal / price / present\ value \\ Px - principal + interest\ after\ 'x'\ years \\ \color{red} r - annual\ interest\ rate \\ \color{green} t - time\ in\ years \]

### Simple interest rate

Let * P* be the principal that is borrowed, after first year the balance that needs to be paid back is the principal

*plus the interest:*

**P**\[ \\ P1 = P + r*P \]

after two years we have the principal plus the interest for 2 years: \[ \\ P2 = P + r*P + r*P \\ \]

then after 3 years … \[ \\ P3 = P + r*P + r*P + r*P \\ \]

we can see that * r*P* term repeats for each year, generalize for

*years: \[ \\ Pt = P + t*r*P \\ \]*

**t**
and finally extract * P* and remember the formula for later use:
\[
\color{blue}
S = P*(1 + r*t)
\]

Let's see a simple example, borrow $100 at 12% annual rate for 20 years:

```
p, r, t = 100, 0.12, 20
s = p * (1 + t*r)
print('$%.2f' % s)
```

$340.00

Looks right, $100 at 12% interest rate makes $12/year for 20 years is $240 plus $100.

### Compounding interest rate

#### Yearly compounding

When compounding, we use previous year's balance * Px* (intermediate principal) instead of initial principal

*.*

**P**after 1 year: \[ P1 = P + r*P \\ P1 = P*(1 + r) \]

2nd year: \[ P2 = P1 + r*P1 \\ P2 = P*(1 + r) + r*P*(1 + r) \\ P2 = P(1 + r)(1 + r) \\ P2 = P(1 + r)^2 \]

3rd year \[ P3 = P2 + r*P2 \\ P3 = P*(1 + r)^3 \]

and so on, you've got the idea, by the same technique after * t* years we end up with:
\[
\color{blue}
C = P*(1 + r)^t
\]

Same example as above with annual compound:

```
c = p * (1 + r)**t
print('$%.2f' % c)
```

$964.63

Wow, look at that, * $964* vs

*, the final balance when compounding is a lot bigger, which makes sense, after first year we receive interest on interest and balance adds up quickly.*

**$340**#### Monthly compounding

Let's introduce a new variable * n* which is the number of periods in a year:

- monthly interest rate:
**r/n** - total compounded periods:
**t*n**

The new compounding formula is:

\[ \color{blue} C = P*(1 + \frac{r}{n})^{t*n} \]

When compounding monthly we have 12 months in a year and **n = 12**

```
n = 12
c = p * (1 + r/n)**(t*n)
print('$%.2f' % c)
```

$1089.26

#### Daily compounding

365 days in a year and **n=365**

```
n = 365
c = p * (1 + r/n)**(t*n)
print('$%.2f' % c)
```

$1101.88

Again, * $1101 vs. $1089 vs $964*, the more often the compounding, the bigger the final balance.

#### Continuously compounding

What about hourly compounding or even per minute or per millisecond? we can increase * n* and get the results but I want to find a formula that fits them all starting with:
\[
\color{blue}
C = P*(1 + \frac{r}{n})^{t*n}
\]

Since we compound more and more often then * n* gets bigger and bigger and approaches infinity and we can rewrite the formula as a

*limit*: \[ C = \lim_{n \to +\infty} P*(1 + \frac{r}{n})^{t*n} \]

Define * x* variable as:
\[
x = \frac{n}{r}
\\
n = x * r
\]

then * x* is direct proportional with

*, as*

**n***approaches infinity, then*

**n***approaches infinity as well,*

**x***limit*formula holds and we can substitute

*with*

**n***: \[ C = \lim_{x \to +\infty} P*(1 + \frac{r}{x*r})^{t*x*r} \] extract constant*

**x***outside the limit, cancel out*

**P***and commute the exponents: \[ C = P * (\lim_{x \to +\infty} (1 + \frac{1}{x})^x)^{r*t} \]*

**r**What does the above formula looks like? Calculus anyone? Looks like the Euler's number \[ e = \lim_{x \to +\infty} (1 + \frac{1}{x})^x \]

that gets substituted and we end up with the magic * continuous compounding formula*:
\[
\color{red}
C = P * e^{r*t}
\]

```
from math import e
c = p * e**(r*t)
print('$%.2f' % c)
```

$1102.32

*Takeaway*: Before moving further it is important to understand the difference between * APY* (annual percentage yield) and

*(annual percentage rate), the former include compounding interest, the latter does not. In other words*

**APR***is simple interest and*

**APR***is compounding interest.*

**APY**### Discounted interest-rate

What about the other way around, we are given the final (compounding value) and need to calculate the present value?

Simple, re-arranging the compounding formula above we have:

\[ P = \frac{C}{e^{r*t}} \] \[ \color{red} P = C * e^{-r*t} \]

where * P* is called the present value or discounted value and is very useful in financial valuation.

### A few tricks

#### Double your investment aka 100% profit

OK, now, let's assume that I want to invest $1000 at 6% annual interest rate and I want to know how many years do I need to double my investment?

##### Analytical

Let's do a bit of high school math using the annual compounding formula above.

\[ \textcolor{blue} {C = P*(1 + r)^t} \\ \frac{C}{P} = (1 + r)^t \\ t = \log_{1+r} \frac{C}{P} \\ t = \frac{\ln {\frac{C}{P}}}{\ln 1+r} \]

After putting in the numbers we end up with:

\[ t = \frac{\ln {\frac{2000}{1000}}}{\ln 1+0.06} \\ \textcolor{green} {t = 11.9\ years} \]

Double check in Python:

```
from math import log
t = log(2)/ log(1.06)
print('%.1f years' % t)
```

11.9 years

Unfortunately most of the people cannot do logarithms in their heads but rest assured there is a lot easier solution.

##### Rule of 72

As Investopedia nicely explains it: Rule of 72 is a formula to estimate the number of years required to double the invested money at a given annual interest rate.
Just divide * 72* by the annual interest rate

*and there you have it:*

**6***years.*

**~12**```
t = 72 / 6
print('%.1f years' % t)
```

12.0 years

Why 72?

Detailed explanations here but ultimately it is as simple * ln 2*,

\[ \ln 2 = 0.693 \]

while * 2* comes from doubling ($2000 / $1000), if you need to find the tripling time just use

*or*

**3***for 50% rise.*

**1.5**### References

- https://www.investopedia.com/terms/c/compoundinterest.asp
- https://www.investopedia.com/terms/f/fixedinterestrate.asp
- https://en.wikipedia.org/wiki/E_(mathematical_constant)
- https://www.investopedia.com/personal-finance/apr-apy-bank-hopes-cant-tell-difference/
- https://www.investopedia.com/terms/r/ruleof72.asp
- https://en.wikipedia.org/wiki/Rule_of_72
- https://www.khanacademy.org/economics-finance-domain/core-finance/interest-tutorial
- https://tex.stackexchange.com/questions/21598/how-to-color-math-symbols
- https://texblog.org/2015/05/20/using-colors-in-a-latex-document/