Calculate the formula for the weight required to achieve neutral buoyancy at the depth you desire while freediving by accounting for your body's fat percentage, the capacity of your lungs, the buoyancy of the clothing and equipment you are wearing and the type of water -- fresh or salt. For the average person, fresh water will require weights of about five percent of your body's to achieve neutral buoyancy at 15 feet. For salt water, add three percent more. Another method is trial and error.
Put on your diving mask and fins. Check your depth gauge. Make sure it is calibrated correctly. If you are in freshwater, your depth will be three percent greater than indicated if your gauge is calibrated to read the barometric pressure of saltwater. Three percent is not a great difference, but accuracy is critical for freediving.
Test your buoyancy in small weight increments. It is safer to achieve neutral buoyancy at a shallower depth and add weight on your second test than sink too quickly and struggle to return to the surface. Do not add more than five pounds at a time when testing for neutral buoyancy.
Add weight, dive, find neutral buoyancy and check your depth gauge. Add weight until you reach a neutral buoyancy of 15 feet. If you cannot reach 15 feet by adding weight, settle for neutral buoyancy at a shallower depth. Depending on what you are wearing, you may reach neutral buoyancy at 15 feet with as little as 3 pounds or it may require more than 25. If you are an experienced freediver and have a reason to add to the depth of your neutral buoyancy, make sure your partner knows what your farthest depth rate is.