Pushing ISO doubles every time you increase it 1 stop. So if the box speed is 400, then +1 stop would be 800, +2 stops would be 1600, +3 would be 3200. You can also pull your film - it works the same – you’re just cutting the speed in half for each stop you need to pull. So if the box speed is 400, -1 stop would be 200, -2 stops would be 100, and -3 would be 50.
Pushing and pulling will change the look of the film. Pushing is good for lowlight situations - it will add contrast and often make the grain more noticeable. Pulling will take away contrast giving you better detail in your shadows and will make grain less noticeable. Pulling film works great for black and white especially contrasty scenes with a lot of shadow detail.
You will also need let us know how much you push or pull your film so when we develop your film so we can change the developing time to compensate. Write down your push/pull on the order form which is a $2 fee and be sure to write on the film canister itself the push/pull.