First and foremost, one of the things a person should never be without (regardless of their size) is a measuring tape. The reason for this being that most brands have what we call "vanity sizing" (a ploy from stores to make customers feel better, and therefore shop at their stores more than their competitors) - So even though you may truly be a size 16, in some stores you may be a 14, or even a 12 in vanity sizes.
DO NOT FALL FOR THIS TRICK. Learn your true size that way you aren't crushed when you find out your size is a lot larger than you may have thought it was. Its just a number, and it isn't changing your actual size. Rather than trying to squeeze yourself in to a smaller outfit and end up looking terrible, know your true measurement and avoid this problem all together.
The second reason for knowing your measurements is for ease of buying - More often than not, brands carry what they call "Extended sizes" (ie. plus sizes) online only. Without being able to try these things on, the only thing you have to go on is measuring charts.
For ease of measurement, have someone help measure you (boyfriend/husband, mom/sister), because trying to self-measure is difficult. If you do try and measure your self, I recommend adding an extra inch to the measurement. To begin, wear a comfortable every-day bra and panties. No shapewear, no minimizing bra, and definitely no push-up bras! All of those things will give you incorrect measurements. To measure, keep the tape level, and taut, but not so tight that you're "squeezing" anything.
Bust:
Place the tape measure just under your armpits and measure across the fullest part of your breasts (usually level with your nipples).
Band:
This measurement is important if you're trying to fit for a bra. Measure just under your breasts (on your ribcage).
Doing a write up on bra's would be a completely separate entry, so I will link a guide I found to be quite accurate and informative, since they have all of the bases covered.
007 Bras
Waist:
Traditionally, this is measured at your "narrowest point", but for plus sized ladies, your narrowest point is often your ribcage. So, for the waist measurement, I recommend measuring across your belly button.
Hips:
Measure the widest part of your lower half. For me, this fell across the lower part of my stomach (just above my butt); but for others the widest part may be across your butt and thighs.
Inseam:
This is important to know for measuring pants. Starting at your crotch, measure all the way to the floor. If you wear heels with your pants, measure with your heels on, to just above the floor. You want your pants to hit just above the heel, so you wont tread on the fabric.
**Addendum from my mother, the seamstress: "Also measure for pants by the outseam. Just in case the crotch is longer or shorter, the outside seam is a good measurement." (Referring to the RISE of the pants).**
