Set up a flat measuring point
Stand on a hard, level floor with your back near a flat wall. Remove shoes and bulky hair accessories. Look straight ahead rather than tilting your chin up or down, and let your heels rest naturally on the floor.
Ask someone to help if possible. A second person can keep the book or right-angle object level while you stay in position, which is usually more reliable than trying to read the measurement yourself.
Use the same posture each time
Keep your legs comfortably straight without forcing your knees, and stand tall without stretching upward. Your shoulders should be relaxed. Touching the wall is helpful, but do not push your body into an uncomfortable position just to create more contact.
Place a flat book or similar object against the wall so it makes a right angle with the wall and touches the highest point of your head. Mark that point, then measure from the floor to the mark with a tape measure.
Record the result honestly
Measure twice, preferably at the same time of day, and record the result in centimeters or inches. If the two measurements are different, repeat the process and use the result that you can reproduce consistently rather than choosing the taller number.
Standii can display either centimeters or feet/inches. Keeping the original centimeter measurement is useful because it avoids rounding too early when you compare people whose heights are close together.