# Estimate simple option price without a calculator

I have been to two different interviews for jobs related to option trading, and both time I have been asked a question, which is pretty basic, and still I could not answer it.

If you have an European call option, with :

• Expiration : 1 year
• Volatility : 10%
• Strike : 100
• Spot : 100 (at the money)
• Risk free rates : 0

What is its price? I could not use the BS formula, because I have no calculator. I think I should probably have used a binomial tree, but I didn't find out how.

There is a good quick well-known approximation for at-the-money options: $$\textrm{Call,Put} = 0.4 S \sigma \sqrt{T}.$$ See further discussion at What are some useful approximations to the Black-Scholes formula?.