Annuity Formula - How it Works

One formula I wanted to have a look at (just in time for both the accounting exam tomorrow and the finance exam on Friday) the annuity formula. While the math looks rather convoluted, I wanted to strip it down to it's parts to understand how (and why) it works.

First let's look at a few things. Assume that you've already explained how a perpetuity formula works (without growth), you know that the value of a perpetuity is:

(Assume it goes forever beyond period 7).

PV = CF / r

Where:

• PV is the present value
• CF is the cash flow per period
• r is the rate per period

The next question I would propose is this, what is the value of the perpetuity in period n at time 0? Well, it would be:

Well it would be the same as the PV's value at time n, discounted back to 0. Since the cash flows at time n would look the same as now, the PV at time n should be the same as the PV now.

PV @ n = PV / (1+r)^n

= CF / [r x (1+r)^n]

Now the last question, what is th value (both of the cash flows and the PV) of the perpetuity now minus the perpetuity at time n? Well, if you draw a diagram, the answer is an annuity from 0 to n. And the math shows the same:
(Note this graph is merely the first graph minus the second graph in the same way the math is the first PV minus the second.)

PV - PV @ n = PV - PV / (1+r)^n
= PV (1 - 1/(1+r)^n)
= CF (1 - 1/(1+r)^n) / r

This is the annuity formula for a cash flow CF, to period n at discount rate r, which is much easier than doing a DCF for each of the cash flows (imagine doing a DCF for 30 even cash flows mechanically).

This is a slight variation on the question that Kent Womack presented to us at our review session in the ROM and also highlights how the formula for annuities is constructed.