What is yarn weight? All your questions answered here.

What is yarn weight?
Yarn weight refers to the thickness of yarn. It would have been more literal to call it yarn thickness, but instead the word weight is used. There are 8 categories of yarn weight, that range from Weight 0 (thinnest yarn) to weight 7 (thickest yarn).
How do I find out the weight of a particular yarn?
Yarn brands put a helpful guide on the label of every skein of yarn. That guide includes the weight of that yarn. Look for the symbol of a skein of yarn with a number from 0-7 on the label. If you are buying yarn online, the yarn weight should be listed somewhere in the item description. Read below to learn more about each weight of yarn.

The most common times its helpful to understand yarn weight:
Scenario 1: You want to follow a particular pattern, but don't love the yarn recommended for whatever reason. Maybe its too pricey, doesn't come in colors you like, is hard to find, or discontinued. Many times you can substitute that yarn for a yarn of equal weight. Just make sure the yarn you are choosing isn't an entirely different texture than the recommended yarn. For example, don't substitute a sleek wool blend yarn for a fluffy yarn. It is also important to check the gauge in this scenario, especially if you are making something where size or fit is important, like a garment.
Scenario 2: You've decided you are going to make a certain project solo, without a pattern. It will help you to select a yarn for your project if you understand which weights of yarn most commonly correspond with different projects, how they work up, and drape.
For example, you wouldn't choose a weight 4 yarn to crochet a doily. It would be better to choose a weight 0 yarn, which appropriately falls into the category "lace".
It is also very helpful in this scenario to understand the corresponding gauge and hook or needle size for every weight of yarn.

Standard Yarn Weights

Weight 0/ Lace
Type of yarn: fingering, 10-count crochet thread
Lace weight is the lightest and smallest weight of yarn available. It is often used to crochet or knit delicate openwork doilies, shawls, and lace.
Recommended Knitting Needle Size: US 000-1 or 1.5mm-2.25mm
Recommended Crochet Hook Size: Steel 1.6mm-1.4mm or Regular 2.2
Common Gauge over 4 inches
Knitting Stockinette: 33-40 stitches
Crochet Single Crochet: 32-42 stitches
Note: When it comes to lace weight yarn, gauge is difficult to determine as lace weight yarn is often worked with larger hooks or needles.

Weight 1/ Super Fine
Type of yarn: sock, fingering, baby
This yarn is often referred to as fingering weight. It is used for similar openwork projects as lace Weight yarn but it is a bit more substantial. Super fine weight yarn can be great for openwork shawls, socks, and delicate garments, especially for baby.
Recommended Knitting Needle Size: US 1-3 or 2.25mm-3.25mm
Recommended Crochet Hook Size: B-1 to E-4 or 2.25mm – 3.5mm
Common Gauge over 4 inches:
Knitting Stockinette: 27-32 stitches
Crochet Single Crochet: 21-32 stitches

Weight 2/ Fine
Type of yarn: sport, baby
Weight 2 yarn is often referred to as sport weight yarn and sometimes also baby. Weight 2 yarn is great for garments with a lovely drape, cozy socks, and to balance stitches that create a thicker fabric such as cables.
Recommended Knitting Needle Size: US 3-5 or 3.25-3.75mm
Recommended Crochet Hook Size: E-4 to 7 or 3.5mm – 4.5mm
Common Gauge over 4 inches
Knitting Stockinette: 23-26 stitches
Crochet Single Crochet: 16-20 stitches

Weight 3/ Light
Type of yarn: DK, light, worsted
The terms DK or Double Knit yarn refer to Weight 3 yarn. This yarn is also sometimes tossed into the "worsted" category, although worsted is a term more commonly reserved for weight 4 yarn. Weight 3 yarn is wonderful for many projects from garments to blankets. This yarn is probably the most popular weight of yarn for sweaters. Being just a bit lighter than the ever popular weight 4 yarn, weight 3 can be used for many of the same projects but provides a better drape than weight 4 yarn, albeit working up slower.
Recommended Knitting Needle Size: US 5-7 or 3.75mm-4.5mm
Recommended Crochet Hook Size: G-6 to I-9 or 4.5mm – 5.5mm
Common Gauge over 4 inches:
Knitting Stockinette: 21-24 stitches
Crochet Single Crochet: 12-17 stitches
Patterns:
The Petal Scarf Crochet Pattern
The Basketweave Baby Blanket Crochet Pattern

Weight 4/ Worsted
Type of yarn: worsted, afghan, aran
Weight 4 yarn, often referred to as worsted weight yarn, or aran weight yarn, is arguably the most widely used yarn in crochet and knitting. Weight 4 yarn is neither notably light or thick. It can be used for projects from garments to afghans to home decor. It is also recommended that beginners start with a weight 4 yarn in a lighter color as it is the easiest to work with and see.
Recommended Knitting Needle Size: US 7-9 or 4.5mm-5.5mm
Recommended Crochet Hook Size: I-9 to K-10 ½ or 5.5mm – 6.5mm
Common Gauge over 4 inches:
Knitting Stockinette: 16-20 stitches
Crochet Single Crochet: 11-14 stitches
Patterns:
The Bobble Throw Crochet Pattern
The Seersucker Baby Blanket Knitting Pattern
The Eyelet Throw Crochet Pattern

Weight 5/ Bulky
Type of yarn: chunky, craft, rug
Bulky weight yarn is one step up from worsted weight yarn, but twice as thick! Weight 5 yarn is most widely used for thick scarves, chunky blankets that work up quickly, and other home decor projects such as rugs and baskets.
Recommended Knitting Needle Size: US 9-11 or 5.5mm-8mm
Recommended Crochet Hook Size: K-10 ½ to M-13 or 6.5mm – 9mm
Common Gauge over 4 inches:
Knitting Stockinette: 12-15 stitches
Crochet Single Crochet: 8-11 stitches
Patterns:
The Boho Coaster Crochet Pattern
The Klaus Christmas Stocking Crochet Pattern

Weight 6/ Super Bulky
Type of yarn: super bulky, roving
Super bulky yarn works up very quickly and creates very chunky or sturdy projects. It is often used for chunky scarves, throws, or home decor projects.
Recommended Knitting Needle Size: US 11-17 or 8mm-12.75mm
Recommended Crochet Hook Size: M-13 to Q or 9mm – 15mm
Common Gauge over 4 inches:
Knitting Stockinette: 7-11 stitches
Crochet Single Crochet: 7-9 stitches
Patterns:
The Cordate Throw Knitting Pattern
The Bondi Basket Crochet Pattern
The Maxwell Scarf Crochet Pattern

Weight 7/ Jumbo
Type of yarn: Jumbo, Roving
Jumbo yarn works up very quickly and can be used on large needles, hooks, or for arm knitting. This is currently the thickest weight of yarn on the market. Note that there can be a lot of variation in thickness of yarn within this category, so check your gauge.
Recommended Knitting Needle Size: US 17 and larger or 15 mm and larger
Recommended Crochet Hook Size: Q and larger or 15mm and larger
Common Gauge over 4 inches:
Knitting Stockinette: 6 stitches and fewer
Crochet Single Crochet: 6 stitches and fewer
Pattern: The Carova Throw Knitting Pattern