I weigh Timmy and Roz about every 6 months as long as they are eating normally. I weigh rescue torts when I first get them, and then every few weeks after that, to make sure they are thriving. Keep in mind that weight may differ a little depending on whether they just ate or defecated... I usually weigh my torts after a bath, before their meal.
I use a metric kitchen scale. It doesn't have to be a metric scale - although both medical dosage calculations and growth charts are metric, so depending on what your purpose of weighing your torts may be, you might have to do a little conversion math. It's a good idea to take note of the weight every time somehow.
|Roz gets weighed upside-down... he's half monkey, and wriggles out otherwise.|
Timmy: 855g (7.3 inches shell length) ~remember, Timmy is female
Roz: 505g (5.5 inches shell length)
Joe: 500g (6 inches shell length)
Mo: 580g (6 inches shell length)
Boo: 475g (5.5 inches shell length) ~ I just got Boo, he's a little on the light side, but still ok.
Roz has definitely packed on some weight since I last weighed him - he was 475g at the beginning of the year. Mo is an old old man, so it isn't surprising that he is so heavy - he is one dense tortoise!
If you are unsure if your tortoise is a good weight, HERE is a good calculator using Jackson's Ratio. All my tortoises are within the healthy range.
In comparison, Norbert (aka Stumpy) weighed only 275g when I first rescued him, and he is the same size as Roz! I'm working on writing his rescue story down right now...