how much sugar is required to be added to fruit for jam making
Base ratio is equal weights of sugar and fruit.

Using less sugar is possible but requires considering the following points:


Sugar is a main aspect of preservation. Using less requires adding alternative substances such as sorbic acid to compensate.
Sugar is required for pectin to bond. This can be compensated with more pectin and/or longer cooking time while ensuring there is enough acidity (another factor for pectin bonding)
Canning protects the result better than simply jarring.


The sealing determines how difficult it is for germs to get in. The chemicals determine how well it resists to germs once they do get in.