Hi Brandy. Grams are a measurement of mass (similar to weight) and percent is just a ratio or a fraction, so these "units" don't really compare. It's kind of like saying "How do you convert 10:30 AM to a fraction?"

Perhaps you meant to write something like this though:
How do I convert 1.5g of fat (such as in a serving of some peanut butter) to the percentage recommended daily amount?

For that kind of question, you need another number: the amount (in grams) of fat that is recommended. Then you can use percentages, because percentages are ratios or fractions and so they compare two numbers like how much fat you consume compared to how much you are recommended to consume in a day. For example, if the recommended daily amount is 45 grams and I eat 1.5 grams, then that converts to a percentage like this: 1.5 ÷ 45 = 0.0333 = 3.33 %.

Hope this helps,
Stephen La Rocque