I'm creating a rogue character and I've looked through all the equipment I can get with my class and decided to get the burglar's pack. Within this, I get 10 feet of string. I'm calculating my equipment's weight at the same time. I've looked through the D&D handbook 5e for the weight of string, but there is nothing. Does anyone know what the weight of string is?
String weighs a negligible amount.
"String" is a bit of a vague category, but I wouldn't say it would weigh any more per foot than, say, twine. Looking at a couple of listings online, 328' of twine is 2.89oz and 500' of another brand of twine is 6.3oz. That's a weight of ~0.0088 - ~0.013 ounces per foot, or up to an eighth of an ounce for 10' of twine. That's well below what would be modeled for encumbrance.
10 feet of string are about 3 meters. Common Sense tells you that such an amount of string has a weight that is totally negligible. Even 3 meters of metal wire are in that area.
Many small items are without weight because you won't get encumbered by having said items in your pocket. 10-feet of string is one of these items.
If you really must have a weight: Yarn for knitting can be measured in "meters per 100 grams". The thickest available wool is 40 meters per 100 grams, resulting in about 7.5 grams for that thickness, but you will most likely have a less thick yarn, but even if you had such super thick yarn, you are facing less than 10 grams/a third of an ounce for the 10 feet.