How do I know the size ballast I need for a fluorescent fixture?

You need to know how many bulbs there are and how long they are. However, make sure you check the starter before you change the ballast. In fact, you may want to replace the whole fixture.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.