Thigh, Rise, Waist
Measure your baby without a diaper on
Measuring Your Baby's Rise
Start the tape measure at your baby's waist in front,
which should be about 1" below your baby's belly
button.  Now take the tape measure through your
baby's legs and end on his back, where his waist is at
on his back.  Measure your baby's rise with no diaper
on.  If you are measuring the rise for a diaper cover,
then measure the rise of your baby over a diaper.

This is the most critical measurement, because the
rise of most diapers is not adjustable (whereas the
thighs and waist are somewhat adjustable).

Measuring Your Baby's Thighs
Wrap a tape measure around your baby's thigh.  You
want to measure where the leg elastic of a diaper
would normally be.  This means the tape measure will
typically be at an angle.

Measuring Your Baby's Waist
Measure around your baby's waist.  This
measurement is usually not critical because the waist
of every diaper is very adjustable.