While most dumbbells come equipped with collars so that the weight doesn't slip off the bar, using a belt as a replacement collar can be equally effective. Because leather belts are both thick enough to withstand the pressure of being pulled very tight and possess a dry surface that will grip a dumbbell bar, there aren't many ways to create a homemade dumbbell collar more effective than a leather belt.
Wrap the leather belt around one end of your dumbbell. Insert the belt into its buckle and tighten it as far as you can. Even though there are no holes this far down on the belt, use the pin of the belt to mark a spot.
Make a dark mark where the pin of the belt buckle is.
Use a sharp tool such as a knife or scissors to poke a small hole at the mark you made. The hole should be only large enough so that the pin of the belt buckle can fit through.
Add weight to your dumbbell. Wrap the dumbbell in the belt and fasten it at the hole you just created. If the belt is tight enough, weight should not slide off the dumbbell.
Pull the remaining length of belt so that it is straight. Leave four to five inches of slack and cut the rest off. Too much leftover belt will get in the way of your workout, but not enough will make it difficult to fasten the belt around the dumbbell.
Repeat Steps 1 through 5 with a different leather belt on the other end of the dumbbell.
Feel free to try another type of belt if you have one laying around, but leather is a dry material that creates a grip that makes it difficult for the dumbbell to slide when pushed by large amounts of weight.
If you find that your first try at creating a new hole in your belt didn't make the belt tight enough around the dumbbell, you might want to try again on a different belt. Punching another hole on the first belt could rip it.