I am often asked what the process time should be when home canning in a jar size not listed with a food.  Well, there is not just a formula to apply to change process times for sizing up or down a jar. (If there was, it would be easier to offer a wider range of jar sizes for all products.)  Different foods heat up at different rates and require different adjustments.

Therefore, the general rule of thumb is if you want to use a smaller jar than one listed with a tested process, use the process time for the next size larger jar than you have.  For example, if a procedure only lists pint jars as the smallest choice, and you want to use half-pint jars, you use the process time for pint jars. The product is likely to be even softer when processed in this manner, but it is the only choice for safety and you need to take that into consideration if desiring to use smaller jars. Do remember if you use too small of a jar compared to one studied, say a 4 oz instead of a pint jar (16 oz), you could end up with a very overcooked product and it may not be desirable to do so, even if safe.

Another example, since there are some 24- and 28-ounce jars now manufactured for home canning: These are between pint and quart size jars, so you would use the quart process time if one is available.  If there is only a pint and/or half-pint process listed with the procedure, then there is no tested process available for a 24-, 28- or quart size jar. So this leads to the next situation, there is no way to offer a known safe process for a jar larger than the largest one studied. If the largest jar size recommended is a pint jar, then there is not a formula to be able to say how to increase the process time for a larger jar.

To come up with a minimally safe and still optimum as much as possible process, one has to have the actual heat penetration (heat tracking data) from the food being heated in the jar size to be recommended in order to calculate a specific process to the jar size and type.  For more about how process times are researched, especially for all low-acid foods, there is more description here: https://nchfp.uga.edu/publications/nchfp/factsheets/heatprocessingbackgrounder.html

As you choose jars, in addition to size, please remember we recommend jars properly tempered for the temperature extremes of home canning, as well as those with closure systems recommended for home canning so you obtain good vacuum seals using recommended processing procedures. These jar choices will also be a limiting factor in sizes for which you need process times.

Please be safe in your home canning choices. The research-based processes available may seem very limiting for what you want to do; however, there has been little public funding for researching new recommendations in a long time and there are few labs set up and staffed to do home canning research. This often means when something new can get researched, there will not be multiple jar sizes tested.

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