Thanks to Catherine for this Frugal Baby Tip!
Wash and dry your diaper at least 3 times , if it's still new.
The hard part is you will need a scale that will weigh small amounts - in ounces, grams or millilitres. A food or postage scales will do.
Weigh the dry diaper, and write the down the weight. Submerge the diaper in 4 cups (1000 ml or 1000 grams) of water for one minute. Then remove the diaper from water, do not wring or twist. Simply hang to drip for 15 minutes. Best to do this over the bowl, or sink.
Now weigh the wet diaper. The absorbency capacity is calculated by subtracting the dry weight of the diaper, from the wet weight. (Weight of 1 ml of water = 1 gram. 250 ml = 8 ounces approx.)