The millimetres that decide whether a pen cup tips

A cup tips when the load leans further out than the base is wide. Height, base diameter and weight decide it, and only one of the three is usually printed.

A pen cup has one job and one common failure. It falls over. Whether it does is not luck — it is the relationship between how tall it is, how wide its base is, and how much the empty cup weighs.

The ratio to look for

As a rough working rule, a cup taller than about two and a half times its base diameter will tip when a long pair of scissors leans against the rim. Squat, wide cups tolerate a lot; tall, narrow ones tolerate almost nothing once something long is in them.

Weight is the quiet variable

A cast metal cup at 380 grams will hold a leaning ruler that a 90-gram mesh cup will not. This is why we list the empty weight where the supplier gives it to us, and leave the field out where they do not, rather than estimating.

What a felt base actually fixes

Felt stops the cup scoring a polished desk and stops it sliding when you pull a pen out fast. It does not stop tipping. Anyone telling you a felt base makes a tall cup stable is describing a different problem.

The honest fix

If you keep scissors, a steel ruler and long brushes, use two containers — a wide low one for the long items and a taller one for pens. It costs less than replacing a desk lamp knocked over by a falling cup.