Canning Headspace & Jar Yield Calculator
Estimate how many jars a batch of food will fill and get the recommended headspace for jams, fruits, tomatoes or low-acid foods.
How it works
Jars = ⌈food volume (fl oz) ÷ jar size⌉ · 1 cup = 8 fl oz · Headspace depends on food type and canning method
Headspace is the empty gap between the food and the jar lid, and getting it right is a safety and seal issue, not a cosmetic one. Too little headspace and food can boil out under the lid and break the seal; too much and there is too much air to vent, so a vacuum may not form. The standard allowances are about 1/4 inch for jams and jellies, 1/2 inch for fruits, pickles and tomatoes, and a full 1 to 1-1/4 inch for low-acid foods that expand under pressure canning.
Worked example
You have 8 cups of tomato sauce and pint (16 oz) jars. That is 8 × 8 = 64 fluid ounces, so you need ⌈64 ÷ 16⌉ = 4 jars, filled to a 1/2-inch headspace with no batch left over.
Frequently asked questions
Why does headspace matter so much?
It controls the vacuum seal. Too little and food siphons out during processing, fouling the seal; too much and trapped air keeps the lid from sealing tightly. Following the tested headspace for your food type protects both safety and shelf life.
How much does a cup of food fill?
One US cup is 8 fluid ounces, so it fills half a pint jar, and two cups fill a pint. The calculator rounds up to whole jars, so the last jar may be partly empty — process a partial jar or refrigerate it.
Is the headspace the same for water-bath and pressure canning?
No. Low-acid foods processed in a pressure canner expand more and need a larger 1 to 1-1/4 inch headspace, versus 1/4 to 1/2 inch for high-acid foods in a boiling-water bath. Always follow a tested recipe for your method.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.