Of all the sewing tools the tape measure is an essential for every dressmaker but please don’t use the old fabric one you found in you’re granny’s sewing basket – it’s probably stretched! A plastic one is just fine and one with inches on one side and centimetres on the other is best.
You will also need a good old fashioned ruler or even better a large acrylic set square. Some specialist pattern drafting ones will have curves incorporated into them, this is useful for armhole and neckline drawing. The other measuring device I use all the time is a yard stick, probably a metre stick now! Useful for all sorts of things including measuring hemlines from the ground up.