formula for the liquid volume inside a bottle at any given time

__Question : __

What is the formula for working out the volume inside a bottle ( eg a 750 ml bottle ), and converting that into mils (millilitres), bearing in mind the different dimensions bottles can have and varying volumes or capacity. What I want to measure is the liquid inside the bottle therefore a 750 ml bottle for example would have 750 mls of liquid inside the bottle but the overall volume of the bottle would need to be greater than 750 ml eg a bottle of wine……therefore how can I meaure the amount of liquid inside the bottle at any one time without having to pour the liquid out into a measuring container.

__Question : __

What is the formula for working out the volume inside a bottle ( eg a 750 ml bottle ), and converting that into mils (millilitres), bearing in mind the different dimensions bottles can have and varying volumes or capacity. What I want to measure is the liquid inside the bottle therefore a 750 ml bottle for example would have 750 mls of liquid inside the bottle but the overall volume of the bottle would need to be greater than 750 ml eg a bottle of wine……therefore how can I meaure the amount of liquid inside the bottle at any one time without having to pour the liquid out into a measuring container.

ps i couldn't paste my diagram