Charlie,
It's not so simple. You could be talking about light air sails or heavy offshore sails; no way for us to know. As a guess I would say nothing less than 7 oz material for a standard sail.
You confuse also the question when you say "canvas". Sails are no longer made of canvas. The sacrificial cover is most often Sunbrella,which though an acrylic; is often still called canvas.
Order a half dozen different sail cloth samples of different weights and compare to your sail.
Dennis