If my provisional patent is about to expire, will my deadline to file a nonprovisional in the US be extended another 18 months if I file a PCT?

Yes, what you can do is file a PCT application by the one year deadline. Then you can either wait until the national phase to enter the US OR you can file what is known as a bypass continuation application in the US without waiting until the national phase application. By only filing a PCT application by the one year deadline, you will receive a International Search Report and a Written Opinion within 6-8 months of the PCT filing. That will let you know how "patentable" your application is and only having to file (and pay for) a single patent application by the one year deadline. If you plan to use the provisional application as the PCT application, make sure that you comply with all of the PCT application requirements. Filing only a PCT application by the one year deadline is a good strategy for many clients, especially those of limited funds.

When you file a provisional application, you have one year to file a US non-provisional and/or a PCT application if you intend to seek patents in foreign countries. If your funds are limited, you can file a PCT application and then come back into the US as a national stage application within 30 months, however, the PCT application is much more expensive than filing a US application. Filing a PCT application will probably cost between $3K-$4K, whereas a US non-provisional will cost under $800 for a small entity, $400 for a micro-entity. If the provisional application was in fact a good provisional application, in other words, up to US patent application standards, you can use the provisional as your PCT application with no real changes.