Your Guide to Heat Shrink Sizing
Logic RC Heat Shrink sizes are quoted as nominal Expanded, or pre-shrunk dimensions. The inside diameter is around 20% larger than its nominal size, so 6.0mm heat shrink tube can comfortably fit over a wire that's up to 6mm diameter. The shrink is around 3:1, so a 6.0mm tube will Recover or shrink to around 2.0mm. The Lay Flat size is the width of the tube when squashed flat, which is the internal diameter multiplied by 1.57 approximately.
So now you know.