Is 100gsm paper enough for fountain pens?

For most everyday fountain pens with fine to medium nibs, 100gsm is usually enough to prevent bleed-through, and specialist subreddit threads on r/fountainpens generally back this up for brands like CAGIE and Rettacy. Heavily saturated inks, broad nibs, or brush pens may still ghost on the reverse side. If fountain pen friendliness is your top priority, the 120gsm Leuchtturm1917 Bullet Journal Edition 2 is the safer bet.