WEIGHTCLASS

SPREAD

LBS.

KILOS

Flyweight:

10 Lbs

125 lbs. & Below

56.81 kg & Below

Bantamweight

10 Lbs

125.1 to 135 lbs.

56.82 kg - 61.36 kg

Featherweight

10 Lbs

135.1 to 145 lbs.

61.37 kg - 65.90 kg

Lightweight

10 Lbs

145.1 to 155 lbs.

65.91 kg - 70.45 kg

Welterweight

15 Lbs

155.1 to 170 lbs.

70.45.1 kg - 77.27 kg

Middleweight

15 Lbs

170.1 to 185 lbs.

77.28 kg - 84.09 kg

Light Heavyweight

20 Lbs

185.1 to 205 lbs.

84.10 kg - 93.18 kg

Heavyweight

60 Lbs

205.1 to 265 lbs.

93.19 kg - 120.45 kg

Super Heavyweight

Unlimited

265.1 lbs. & Above

120.46 kg & Above

"CATCH WEIGHTS"
This is when say a Lightweight fighter at 150 lbs and a Welterweight at 160 lbs fight.
What you need to look at in this case is the most narrow weight spread, which is always in the lower weight class of the two.
For Lightweight, the weight spread is 10 lbs where is as in the Welterweight class, it goes to a 15 lb spread.
A fight can only happen inside the limits of the weightclass the lightest fighter. So for this example, (Lightweight at 150 lbs and a Welterweight at 160 ) this would be OK as a catch weight.
However if the Lightweight fighter was 149 and the Welterweight at 160, that's 11 lbs, and it wouldn't be allowed. The same if it were a pound the other way, where the Welterweight would weigh 161, again, out of the limits of the lower spread so no fight.

CONVERTING LBS. TO KG. AND BACK
For those looking to convert lbs to kg. If you have a 65 kg fighter and want to know how many lbs. that is, multiply it by 2.2. So, 65 kg X 2.2 = 143 lbs. If you have a 145 lb fighter and want to know the weight in kg, divide it by 2.2. So 145 / 2.2 = 65.90 kg.