Rulers aren't the only ways to measure height. Really, a ruler is just an object that has a known definite length, so a ruler can be any object with a known length. You can find out your height without a ruler by substituting a variety of objects. Take an object you or someone around you is bound to have to use as a makeshift ruler.
Take off your shoes and stand up straight with your back pressing up against a wall or other tall, flat surface.
Mark your height by holding the pencil to the top of your head and lightly drawing a line.
Take a £5 note and measure from the floor how many lengths there are up to the mark of your height. Every £5 note length is 13.5 cm (5 1/2 inches).
Fold the note over to make divisions of measurement. Folding it once lengthwise makes a 6.75 cm (2 3/4 inches) measure.
Use this system to accurately gauge your height if you don't have a ruler or tape measure to hand.