Potions are usually 1 liquid ounce, plus the weight of the container, so they would weigh the same as a vial of ink, which is "no weight worth noting" according to p. 108 of the PHB. Same thing with rings and scrolls (signet ring and paper are both listed on the same page). Under the description of wands on p. 206 of the DMG, they are noted as weighing "under one ounce".
If it starts getting ridiculous, just fall back on the standard 10=1 pound, but for the most part you can ignore the encumbrance of such tiny items. If someone has so many potions that weight becomes an issue, they likely have a bigger problem of keeping track of which one does what.