In this article, I will review some option pricing approximations that can be useful to verify the results given by the pricer you just implemented or to answer some interview questions. Here are the points I am going to tackle :

*Basic At-The-Money option approximation**Examples**Other approximations**Negative volatility ??*

*Basic ATM approximation*

The price for a simple european vanilla Call is given by the Black & Scholes formula:

with

With an ATM option, we can write (ATM forward). Then becomes:

Doing the same for leads to:

We can therefore write the call price as follows:

As is close to 0, we can use the Taylor series for evaluated in 0:

We then have:

Let’s find and we are done:

where is the normal density function.

We have the final result which is the same for ATM calls and puts (can be verified with Call-Put parity):

*Examples*

First, let’s compare the result with a simple application on an ATM 1year Call spot 50 (assume interest rate is 0):

The result is pretty good since we only have a 0.43% error.

*Question*: The 1Y ATM put on XX is worth 4, what is the price for the same option with 2Y maturity?

With our approximation this is straightforward :

*Other approximations for ATM options*

- Chooser option: .
- Asian option: where is the time to start of the average period.
- Floating Strike Lookback Call:
- Floating Strike Lookback Put:

*Pricing with negative volatility*

Don’t worry, I won’t tell you that negative volatility exists. However, as depicted in the short and funny cartoon on Espen Haug’s website, one can find the value of a call by inputting a *negative volatility* in its put pricer and multiplying the result by -1.

*Example*:

This last and pretty amazing approximation can be very useful to test a pricer quickly!

*Thank you!*

*References: *

Paul Wilmott: Introduces Quantitative Finance

QuantStackExchange Forum

Espen Haug: The Options Genius