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Positive Theta, Negative Theta

Theta is a measure of the rate of an option contract’s value decline due to passage of time, in the theoretical pricing model. If all other variables held constant, then the contract decays as the contract’s life decreases.

Thetas are measured in money incremental (for the sake of demonstration, in U.S. dollars); therefore the option’s contract owner pays the time value which part of it is the theta.

Example
The option contract ATM you see above its theta is -0.03 (most trading platforms will show the sensitivities in decimal prices because of the options pricing a premium of $1.86 in real money is $186). This means that today theta will cost -$3.00, if all other variables held constant options price tomorrow will be $183 (1.83).

The theta portion of total options price is 3/183 = 1.63%.
The “today and tomorrow” phrase is used deliberately because the theta graph is not linear and every day and every strike it changes.

OTM call options premium $41 at strike 40. Theta will be about -$2.00.
The theta portion of total options price is 2/41 = 4.78%.

FOTM call options premium $1 at strike 50. Theta will be about -$0.10.
The theta portion of total options price is 0.1/1= 10%.

ITM call options premium $529 at strike 30. Theta will be about -$1.70.
The theta portion of total options price is 1.7/529 = 0.32%.

DITM call options premium $1,500 at strike 20. Theta will be about -$0.21.
The theta portion of total options price is 0.21/1500 = 0.014%.

For theta sensitivity to strikes concluded in dollars, the ATM options contract is the most expensive to hold; as a percentage, the OTM is the most expensive.