Denier confuses a lot of people, but fear not. It's super simple to understand. Once you start working with fabrics often you'll be able to gauge denier by touch.
In my SUPER short research I found that the word denier stems from the Latin word "Denarius" - which was the standard Roman coin from 211 BC to 313 AD. Weird fact, I know. I have no idea how this relates 🤷🏻♂️
How Fabric is Made
ANYWAYS... When you look at fabric construction there are three main phases you can reference. The fiber, which is spun into a thread, which is woven into a fabric. Denier applies to the Fiber level... so like, the smallest piece that makes up the fabric.
How Denier is Measured
The way they measure the denier isn't by thickness - it's by weight. And the amount they measure is bonkers. 9,000 meters (that's 9km... or 5.6 miles).... Wait, wut... WHY?? 😐 At any rate, when you weigh the fiber in grams, you arrive at it's denier. I'm assuming they actually measure 9 meters and multiply that by 1,000. Because math. But I dono. I'm sure they have some super fancy method these days.
If a fiber is less than 1D, then this is considered a micro-fiber. BOOM 🤯
Not All Deniers Are Created Equal
If you were shopping for a fabric and saw two 600D options - perhaps one was Nylon 6 and the other was Nylon 6,6 - these fabrics are not equally as strong. Same goes for all other fiber makeups. Polyester, Nylon, Cotton, Dyneema, etc.
Each fiber will have it's own strength based on it's composition. Even if you had two 600D Nylon 6 fibers from different manufacturers, they would not be equal. The quality of the Nylon 6 itself plays a roll.
This is where we start to get into the weeds and consider way too much info for one article. What's the difference between Polyester and Nylon? What fiber is best for abrasion resistance? What denier and fiber makeup is best for ultralight bags? The list goes on and on and on and on...