How to measure a RT?

[Britannica]

I want to measure my Russian tortoise and I am wondering if the standard is to measure the carapace or plastron? Just curious because I hear they have an average size but I want to make sure I'm measuring the appropriate part.

 

[Yvonne G]

Place the measuring tape up against the wall pulling it out into the room at a right angle. Place the tortoise up against the same wall with its head in the shell. You want to measure the straight carapace length head to tail.

 

[RosemaryDW]

Similar to Yvonne’s but with picture: https://www.thetortoisetable.org.uk/tortoise-care/weighing-measuring/#.W_ypBFqIbmo.