I needed a medicine ball for a few plyometric workout routines. The only one I found in town was $25 for a 15 pounder, but the quality was poor and I didn't think it would last long being repeatedly thrown as hard as I could throw it against a wall twice a week. Basically it was an inflatable rubber ball with sand in it. I thought to myself, "I could do better than this," so I did. I had everything I needed except the sand, making the cost about $5.

Step 1: Supplies

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Old inflatable ball. I used a worn out size 5 soccer ball that still holds air, which is important.

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Exacto-knife or other sharp blade to open a part of the seam and pierce the inner rubber bladder of the ball.

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Sand

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Funnel- for adding the sand.

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Basin- to steady the ball and catch any sand that spills.

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Bicycle tube patch kit, or rubber cement and an old inner tube will do the trick- to seal the hole in the bladder

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Duct Tape- for closing up the cuts. You could resew the ball, but mine was so worn out, this was really the only way to go.

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Air pump- for inflating the ball and making fart noises with you belly button.

Step 2: The First Incision

Inflate your ball to make sure it holds air, give you something to cut against, and let you know if you go to deep and puncture it.

Making sure you only cut the laces, carefully start to cut through a line of them. You want to pick a small panel that can be put back together easily and not show that you've been operating on it. Do not cut out the entire panel. One or two sides and maybe part of an adjacent panel should be enough to get into it.

In this ball and probably most others, the inner bladder adheres to the outer skin, so it's really close to wear you're cutting. It separates easily simply by pushing it with your fingers. If you do cut all the way through, though, and hear the rush of air coming out, don't worry, this will become your bladder hole in the next step.

Step 3: Pierce the Membrane

Once you've opened up enough of the ball to work in, make a slit large enough to hold the small end of your funnel.

Step 4: Sand Transplant

How much do you want your ball to weigh? A size 5 soccer ball, like this one, filled with sand ends up weighing about 16 pounds. If this is too heavy for you, pre-weigh your sand and/or use a smaller sized ball. I think a smaller ball would be the route to go if you want less weight as the sand will shift around less in a smaller container. Soccer balls come in sizes 3,4, and 5.

Insert the funnel into your slit and place the ball in the corner of the basin. This will stabilize the ball as you pour and catches any that may spill out.

Fill 'er up or add your pre-measured weight.

Step 5: Start Closing

Clean off any sand and prep the surface just like you would to patch a punctured bike tube.

Rough up the surface
Size and cut a patch
Apply glue to patch and bladder surface, making sure you cover the entire area.
Allow to air dry for 1-2 minutes before affixing patch to bladder.
Work out any air bubble that may be around.

At this stage I put a little air into the ball so I had something to push against. Not much, mind you, you don't want to blow out your patch before it sets.

Step 6: Close the Panel

Now do the same thing to glue the inside of the skin to the bladder. Glue on both surfaces, dry, then stick. At this point you could try to restitch, but I fell back on the old standby- duct tape.

Step 7: Success!

Now you can pump it up to the recommended pressure. Mine's going to be taking some abuse so I taped all around it, making sure to leave room to air it up again if it needs it. Final weight was 15.8 pounds.