Why seven, and why not eight
Equal circles inside a bigger circle is a problem with known best answers, and the answers are lumpy rather than smooth. Seven circles fit inside a circle exactly three times their diameter — one in the middle, six touching around it — and six circles need that same space, because the ring is what sets the size. So a pot that will take six will take seven for nothing, and taking eight needs a circle 3.30 times the jar rather than 3.00, a step of ten per cent in diameter for one more jar.
| Jars | Rack diameter, as a multiple of the jar pitch | Rack width for a 3.65 in pitch |
|---|---|---|
| 5 | 2.701 | 9.86 in |
| 6 or 7 | 3.000 | 10.95 in |
| 8 | 3.305 | 12.06 in |
| 9 | 3.613 | 13.19 in |
| 10 | 3.814 | 13.92 in |
| 12 | 4.029 | 14.71 in |
The pitch is the jar measured across its ring plus whatever clearance you leave, not the glass alone. A quart jar at 3.4 inches across the ring with a quarter inch of clearance is a 3.65 inch pitch, which puts seven of them in an 11 inch rack with a little to spare and leaves eight needing 12.06 inches.
Two things to hold lightly. These are the best packings anyone has found, and a rack you load by hand at speed will not always achieve them. And the canner manufacturer states a load figure for a reason, which is why the override field exists and why it wins when you fill it in.
The reheat is what costs the day
Add the times up honestly and the surprise is rarely the process itself. A load that processes for 25 minutes still ties the canner up for nearly an hour once you count bringing a full pot up on a domestic burner and letting it come back down. Four loads of that is close to four hours before anyone has washed anything.
Overlapping is where the time comes back. Packing the next load while the current one runs is free, because the canner is busy anyway, and it takes a whole packing cycle out of every load after the first. The calculator shows both ways so you can see what the overlap is worth on your numbers — and it will tell you when it is worth nothing, which happens when packing takes longer than the canner does.
The last load is a full load
A canner does not run faster with three jars in it than with seven. If your batch lands at 24 jars with a seven-jar canner, you run four loads and the last one is three-quarters empty, at full time cost. Having four more jars packed and ready is the cheapest capacity on the whole day. The harvest to jars calculator is the page for deciding how much produce that is, and the oven load and bake schedule calculator does the same clock arithmetic for rectangular pans on rectangular racks.
What is not here
No process time, no pressure, no temperature, no altitude adjustment and no vent or cool-down procedure. Every duration on this page is one you typed in from your own recipe, your own manual or your own stopwatch. Tested processes come from the USDA, the National Center for Home Food Preservation and your state extension service.
Questions people ask
How many jars fit in my canner?
Measure the flat of the rack the jars stand on and measure a jar across its ring, then add the clearance you want between jars. The calculator matches that against the best known packings of equal circles in a circle. If your canner manual states a figure for that jar size, put it in the override field and it will be used instead, because the maker knows the rack and the calculator only knows two numbers.
Why does going from seven jars to eight need such a bigger pot?
Because circle packing goes in steps. Seven jars fit a circle three times the jar pitch, with one in the middle and six around it. Eight need 3.30 times the pitch — the arrangement has to change completely, and there is no in-between. It is the same reason six and seven need the same pot.
Where does the process time come from?
From you. This page supplies none and has no way to check yours. Copy it off the tested recipe you are following, adjusted the way that recipe tells you to adjust it, and the calculator does arithmetic on it and nothing else.
Is it worth packing the next load while one is running?
Usually, and the calculator shows the difference on your own times. The saving is one full packing cycle for every load after the first, so it grows with the number of loads. It disappears if packing a load takes longer than the canner needs, in which case the packing bench is the bottleneck and a second pair of hands helps more than a second canner.
Does it tell me anything about safety?
It gives none, on purpose. This page is quantity and container arithmetic. Process times, pressures, headspace, altitude adjustments and anything about acidity belong to a tested recipe from the USDA, the National Center for Home Food Preservation or your state extension service, and those are the only places to get them. Where the calculator asks for one of those numbers, it is because you are copying it in from the recipe you are already following.