The value of your call option has two distinct measurements, the intrinsic and the time value. The intrinsic value of your option can be determined by subtracting the strike price from the market value. The strike price is the predetermined price agreed upon in your stock option agreement that you are willing to pay for a single share.
The time value is the part of the equation that goes down over time until it completely disappears at expiration of the option, also known as the expiry. There is a time value because of the potential for your option to increase in value before expiry. Options can quickly go up and down in value, so the time value is only based on the odds of your option going up in value. Time value is worth about 5% per year of expiry remaining taken from the intrinsic value.
If you have an option to buy shares at $10 each and the market value happens to be $20, you could assume that the intrinsic value is $10 per share. The time value would be worth about $2.50 for a 5 year expiry period, $2 for a 4 year time period, $1.50 for a 3 year time period and so on. For an option with a 5 year expiry, you could assume the option was worth about $12.50 per share in this case.
You can determine the exact time value using the Theta value of the particular option. The higher the specific Theta value is, the faster your option will decrease in value due to time decay. Options with the longest expiry time periods have the most potential for profit, so they are justly valued much higher.
Your breakeven point for a stock option can be determined by taking market value and subtracting your strike price as well as the non-refundable premiums paid to the seller, also known as the writer of the option.