standard yarn weights

  #6: BULKY (aka super-bulky; roving; polar)
6 wpi or less
2.5 sts per inch (2.5 cm) / 10 sts per 4" (10 cm) or less
60 yards or less per 50 gm (1.75 oz) ball
120 yards or less per 100 (3.5 oz) ball
Needle size: 9mm (13US) and up
For lace: 10 - 12mm (15 - 17US) and up
  #5: CHUNKY (aka bulky; craft; rug)
7 - 8 wpi
2.75 - 3.75 sts per inch (2.5 cm) / 11 - 15 sts per 4" (10 cm) 
70 - 85 per 50 gm (1.75 oz) ball
140 - 170 yards per 100 (3.5 oz) ball
Needle size: 6 - 8mm (10 - 11US)
For lace: 9 - 10mm (13 - 15US)
  #4: ARAN (aka medium; heavy worsted; 12-ply)
8 - 9 wpi
4 - 4.5 sts per inch (2.5 cm) / 16 - 18 sts per 4" (10 cm) 
85 - 100 per 50 gm (1.75 oz) ball
170 - 200 yards per 100 (3.5 oz) ball
Needle size: 5 - 5.5mm (8 - 9US)
For lace: 6 - 8mm (10 - 11US)
  #4: WORSTED (aka medium; afgan; 10-ply)
10 - 11 wpi
4.75 - 5.25 sts per inch (2.5 cm) / 19 -  21 sts per 4" (10 cm) 
100 - 150 per 50 gm (1.75 oz) ball
200 - 250 yards per 100 (3.5 oz) ball
Needle size: 4.5 - 5mm (7 - 8US)
For lace: 5.5 - 6mm (9 - 10US)
  #3: DK (aka light; double knitting; light worsted; 8-ply)
12 - 14 wpi
5.5 - 6 sts per inch (2.5 cm) / 22 - 24 sts per 4" (10 cm) 
120 - 140 per 50 gm (1.75 oz) ball
240 - 280 yards per 100 (3.5 oz) ball
Needle size: 3.75 - 4mm (5 - 6US)
For lace: 4.5 - 5.5mm (7 - 9US)
  #2: SPORT (aka fine; baby; heavy fingering; 6-ply)
15 - 17 wpi
6 - 7 sts per inch (2.5 cm) / 24 - 28 sts per 4" (10 cm) 
145 - 180 per 50 gm (1.75 oz) ball
290 - 360 yards per 100 (3.5 oz) ball
Needle size: 3.25 - 3.75mm (3 - 5US)
For lace: 3.75 - 4.5mm (5 - 7US)
  #1: FINGERING (aka super fine; sock; 4-ply)
18 - 20 wpi
7 - 8 sts per inch (2.5 cm) / 28 - 32 sts per 4" (10 cm) 
185 - 230 per 50 gm (1.75 oz) ball
370 - 460 yards per 100 (3.5 oz) ball
Needle size: 2.25 - 3.25mm (1 - 3US)
For lace: 3.5 - 4mm (4 - 6US)
  #0: LACE (aka cobweb; gossamer; 2-ply)
21 wpi or more
8.5 sts per inch (2.5 cm) / 34 sts per 4" (10 cm) or less
275 per 50 gm (1.75 oz) ball or more
545 yards per 100 (3.5 oz) ball or more
Needle size: 2.25mm (1US) or smaller
For lace: 2.75 - 3.5mm (2 - 4US)