Travel mugs can be tricky, as clay shrinks during the firing and has to be thrown a specific amount larger than the size needed.
If your clay supplier gives you a shrink rate, then the calculation is as simple as [size needed] * [100% + shrink %]
These mugs have a lid diameter of 8cm and a shrink rate of 12.5%, so the calculation is simply
8 * 112.5%
Which gives a thrown size of 9cm to fit perfectly (in theory).
If you don't know your clay's shrinkage, you can make a ruler out of it when it's fresh out of the bag. If you fire it to your normal temperature it shrink by the same amount your pieces will, and you can check your measurements against it to see quickly the size needed.
![](https://i.ytimg.com/vi/vwd30btYLEk/maxresdefault.jpg)