Find how many canning jars a batch of preserves needs.
Jar count depends on finished volume, not the weight of raw produce you started with. Jams and chutneys reduce substantially during cooking — a 70 to 80 percent yield is typical — while pickles and relishes can exceed 100 percent once brine is added. The second correction is headspace: every jar must be left partly empty so the contents can expand during processing and pull a vacuum on cooling, and that headspace is dead volume you cannot fill. Both corrections push the jar count up from a naive raw-weight estimate, which is why so many batches finish one jar short.
Finished volume
Finished ml = prepared kg x 1000 x yield%
Jars needed
Jars = ceiling(finished ml / (jar capacity x (1 - headspace / jar height)))
Quantity planning only, not a safe-preserving procedure. Home canning safety depends on correct acidity, the right processing method for the food (water bath for high-acid, pressure canning for low-acid), and correct processing times for your altitude. Follow a tested recipe from a recognised food safety authority — improperly canned low-acid food can cause botulism.
Around 6 mm for jams and jellies, 10 mm for most fruit and pickles, and 25 mm for low-acid foods that need pressure canning. Too little and the contents seep out past the lid and break the seal; too much and there is not enough steam to purge the air and pull a proper vacuum.
Single-use sealing compound only reliably seals once, and a small proportion of jars fail to seal on the first attempt and need reprocessing with a fresh lid. Ten percent spares covers a typical batch. Rings and jars are reusable indefinitely provided the rims are undamaged.