There are many different types of cap available for plastic and glass jars, and it's a challenge to interpret cap sizes without instruction. If a bottle and a cap do not have the same size/style designation, they will not work together even if they look similar in size.
Below is an overview of how to pick the correct cap for your bottle.
Caps are identified by two numbers with a dash in between, generally written as either 24-400 or 24/400.
When measuring your cap size, the first number refers to the diameter of the bottle neck (not the cap) from the outside thread on one side to the outside thread on the other side, in millimeters.
The second number represents the GPI or SPI* thread design and height of the cap and container.
To determine the thread style or "thread finish" of a bottle or jar, observe the height, and count how many times the threads pass one another. Then compare it to this chart to find the second number, or thread style.