There are two types of bars - Olympic and Standard. Therefore there are two types of plates to fit them!
Olympic plates have a 2# hole. They are meant to fit Olympic bars - which come with oversized rotating sleeves to allow the gripped portion of the barbell to stay still while the plates rotate. Useful in explosive lifting. These barbells have a large collar, so the central hole on the Olympic-sized weights is equally large. You'll find these at all Olympic training centers and most "hardcore" gyms.
Standard plates have a 1" hole. These are meant to fit bars lacking the rotating sleeve. These are the kind of weights you'll find most often at sporting goods stores, gyms, and garage sales.
Plates aren't all the same size. Standard sizes depend on your country...my readers are mostly in the USA so I'll use the US standards primarily and put the equivalent plate in metric in parenthesis.
2.5 pounds (1.25 kg) - These are the smallest commonly available plates, although some gyms may carry microplates (sub-2.5 pound plates). You'll usually find only a pair or two of these, because you don't need more than one set before it's easier to go up by 5s.
5 pounds (2.5 kg) - The smallest plate you'll find in bunches on a weight rack.
10 pounds (5 kg) - Another common size, a bit bigger than the 5 pound plates and thicker as well.
25 pounds (10kg) - At 2.5 times the weight of a 10, they're also wider and thicker. Like the 2.5 you generally find one or two pairs of these, since once you've gone past a 25, a 10, and a 5, you can just use a 45 instead.
35 pounds (15 kg) - these are not terribly common. You simply don't need them that often once you are able to lift 45 pound plates, and you can add up 35 pounds with a 25 and a 10, then swap them off for a 45 for the next big jump. Therefore you don't seem many of these.
45 pounds (20 kg) - in gym speak, these are plates. If someone says "I squatted 2 plates for 10" they mean they squatted with two 45 pound plates per side for 10 reps. With an Olympic bar (also 45 pounds) this means a total of 225 pounds. So next time someone asks you to load up "a plate and 25" they mean putting a 45 and a 25 on each side of the bar for a load of 140 pounds.
100 pounds - not common, but you'll sometimes find gyms that have these for high-end lifters. The diameter of these is the same as a 45, so they'll fit on the same plate-loading machines and raise the bar the same height off the floor.
Additionally, you'll sometimes see large rubber-coated plates. These are called bumper plates, and they cushion the impact of the bar on the floor. These are almost exclusively found in Olympic versions, since it is the O-lifts that require you to be able to drop the bar from the top without breaking the floor.
Why not 50 pounds instead of 45? Probably because 20kg is about 44 pounds, so pound-measurement plates are made to closely match the sizes of internationally accepted Olympic standards while fitting to easily addable numbers. It's easier to see 2 x 45 pound plates and think "90" than to have two 20kg plates and figure it's about 88.1 pounds or so...
大会情報：5/28 JBJJF 第1回北海道柔術オープントーナメント 中島体育センター
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