If you've no time for Black and Scholes and need a quick estimate for an at-the-money call or put option, here is a simple formula.
Price = (0.4 * Volatility * Square Root(Time Ratio)) * Base Price
Time ratio is the time in years that option has until expiration. So, for a 6 month option take the square root of 0.50 (half a year).
For example: calculate the price of an ATM option (call and put) that has 3 months until expiration. The underlying volatility is 23% and the current stock price is $45.
Answer: = 0.4 * 0.23 * SQRT(.25) * 45
Option Theoretical (approx) = 2.07
Remember, this only works for ATM options, where ATM would be assumed to be the forward price of the underlying given the expiration date of the option; not the actual spot price.