any household/kitchen scale that can weigh in 1 gram increments is suitable.

THe easiest way to weigh a gecko (and most animals that don't sit still on their own) is to put them into a container of some sort. Before you put the animal in the container, weight it first, with lid, so you can 'zero' out the scale. Then place your gecko into the container, close the lid and the weight that shows on the scale is the weight of the gecko.

if the scale you buy doesn't have a 'zero out' feature (I am pretty sure they all do, whether its a digital scale or not) you can just substrate the container's weight from the total.
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PHLdyPayne