Why the same volume looks different in two bottles
Volume divided by cross-section is height. That is the entire mechanism, and it is the reason a fill line behaves so differently depending on where you put it. In the straight body of a bottle three inches across, the surface area is about seven square inches, so a fluid ounce of product raises the level by roughly a quarter of an inch. Up in a three-quarter-inch neck the area is under half a square inch, and the same ounce raises the level by more than four inches — except the neck is not four inches tall, so in practice a small overfill takes the level from the bottom of the neck to the brim.
This cuts both ways and neither way is wrong. A target in the body hides variation, which is comfortable and means you cannot see a problem by looking. A target in the neck shows variation, which is uncomfortable and means you can. Choosing which one you want is a real decision, and most people make it by accident when they pick a bottle.
Measuring the bottle so the numbers mean something
| Dimension | Where it starts and stops | Common mistake |
|---|---|---|
| Body diameter | Inside face to inside face | Using the outside diameter, which on thick glass overstates the volume badly because it is squared |
| Body height | Inside of the base to where the taper starts | Measuring from the outside of the base and counting the base thickness or the punt as product |
| Shoulder height | End of the straight body to the start of the neck | Guessing it, which is usually fine because the shoulder holds a small share of the total |
| Neck bore | Inside the finish, at the narrowest point | Measuring the outside of the thread, which is a lot bigger |
The model is a cylinder, a cone frustum and a second cylinder. Real glass is rounder than that in three places, so treat the brim capacity it gives you as an estimate with a known way to check it: fill one container to the brim and measure what went in. Compare, and the difference is a constant you can carry.
Where this sits next to the other container pages
If you are packing solids in liquid and want the liquid a packed jar takes rather than the level it reaches, that is the canning liquid calculator, which works from a brim capacity and a headspace given by a recipe. If you want the weight the scale should read rather than the height the liquid reaches, that is the fill weight and tare calculator. For a plain cylinder with no shoulder — a tank, a length of pipe — the pipe volume calculator is the shorter route.
What this page is not
It has no rule in it about how much a container has to contain, how that has to be declared on the outside of it, or what counts as an acceptable variation across a run. Those are regulated, they differ by product type and by state, and they are for the agency that covers your category and for a lawyer. This page converts between a height and a volume.
Questions people ask
Why does my measured capacity not match the number here?
Because a real bottle is not a cylinder plus a cone plus a cylinder. The base is rounded inside, the shoulder curves rather than tapering straight, and many bottles have a punt pushing up into the base. Fill one to the brim, measure what went in, and you have the correction for that mould. It will be consistent across bottles from the same mould, which makes it useful rather than annoying.
Should I put the fill target in the body or the neck?
It depends on whether you want variation visible. In the wide part of the body a fluid ounce moves the level a fraction of an inch, so bottles that differ noticeably in content look identical. In the neck the same difference is obvious at arm length. If you are checking a hand-filled run by eye, a target in the neck tells you something and a target in the body does not.
Can I use it for a jar or a can with no shoulder?
Yes. Set the shoulder height to 0 and give it the same diameter for the body and the neck, and it reduces to a plain cylinder. If you enter a neck wider than the body it clamps the neck to the body, which produces the same thing.
What does it do if I ask for more than the container holds?
It cuts the fill back to the brim capacity and says so. The level cannot be above the brim, so the alternative would be a height that does not exist. Usually it means one of the dimensions is wrong rather than the volume.
Does this tell me how full my containers have to be?
None at all, and that is deliberate. Labelling is one of the most heavily regulated corners of small manufacturing, the rules differ by product type and by state, and they move. This page has no rule about what goes on a label, how net contents are declared, what counts as an acceptable fill, or how anything may be shipped. It measures containers, labels, cases and station times. The rules come from the agency that regulates your category and from a lawyer.