Canning 101: Headspace This little tool is a lifesaver for canning. It is inexpensive and makes measuring headspace quick and easy. Having the correct headspace is important to successful canning. If you have too little headspace, your food can bubble up and get under the lid. This can prevent a safe seal and waste all your hard work. Leaving too much headspace creates a different problem. The vacuum seal that holds the lid on is created during the canning process. The heat forces the air in the