1. The first thing you will want to measure is your waist. Some jeans come sized in inches (such as 24, 25, 26), while others are sized in numbers (2, 4, 6). Knowing your waist size will allow you to buy jeans from any brand, no matter what the sizing system is that they use. It will also be helpful information to know if you are shopping for jeans in Europe.
Start by taking a soft measuring tape and measure around where your natural crease is. To find the natural crease, bend at the waist to your side. Remember not to hold the tape too tight as you’ll want to get jeans with a comfortable fit. Jeans that are too tight will be uncomfortable and result in the dreaded "muffin top."
2. The next part you will want to measure is your inseam. The inseam is the distance between your crotch and your ankle. Inseams can vary from one brand to the next, which is why sometimes you’ll see jeans sized as “petite” and other times they will be sized as “tall.” Many denim brands also offer jeans in standard waist sizes with varying length options. This is especially true when shopping for men's jeans.
If you already own a pair of perfect-fitting jeans, lay them flat on a hard surface and using the tape measure, measure the distance from the crotch seam to the ankle hem. If you don’t own a pair of perfect-fitting jeans to measure this, ask a friend to help you measure this area on your body. It is best to do this in lightweight clothing to get the most accurate measurement.
3. The last thing you may want to keep in mind when shopping is what type of rise you are most comfortable with. The rise is how far up the jeans will sit on your waist. Do you prefer a more low rise jean? These jeans typically sit 2-3 inches below the belly button, sometimes more. A mid-rise jean usually fits right around or immediately under the navel and a high rise jean will sit at or a few inches above the belly button. Experiment with the tape measure to determine what type of rise you like best.
4. Whether you're shopping online for jeans or shopping in a store, ask if the shop has their own size chart, as many vary greatly. Stores like Ann Taylor and Abercrombie & Fitch, for example, are known for "vanity sizing" while shops like Forever 21 tend to run a bit small. European sizing is different as well and varies in the UK, France, Italy and the rest of Europe.