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you can approximate. watermelon is almost all water. 1 fluid ounce is approximately equal to 1.0425 ounces. Multiply by 21 ounces required by the recipe (1 lb =16 oz +5) which = approximately 20 oz pureed. I would use a touch less than 20 oz becaue the density of watermelon is a little higher than water but not by much so maybe use 19 oz or so.
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Your recipe probably uses weight rather than volume because watermelon varies greatly in water content from one melon to another. I wouldn't try substituting volume for weight unless you have a very good feel for what consistency you're looking for in the mix. You want a sorbet, not a slush.

