Since agar agar is likely the substitute most people are actually trying to understand, it's worth giving it its own dedicated comparison.
Source: Gelatin comes from animal collagen; agar agar comes from red algae.
Texture: Gelatin sets into a soft, elastic, slightly bouncy gel that melts near body temperature — which is part of why gelatin desserts feel like they "melt in your mouth." Agar agar sets into a firmer, more brittle gel that doesn't have that same melt-in-mouth quality.
Activation: Gelatin is typically bloomed in cold water before being dissolved into a warm liquid. Agar agar generally needs to be brought to a boil to activate properly, and often requires a slightly longer dissolving time than gelatin.
Heat stability: This is one of agar's biggest practical advantages — it holds its set at room temperature and even in warmer conditions, while gelatin-based desserts will soften or melt if left out of the refrigerator too long.
Taste: Both are largely flavourless on their own, though agar agar can carry a very faint mineral or oceanic undertone in some preparations that gelatin doesn't have.
Common uses: Gelatin is common in Western-style jellies, mousses, and marshmallows. Agar agar is a staple in East and South Asian desserts and has become the standard gelatin substitute in vegan and vegetarian cooking more broadly.
Substitution considerations: It's worth being direct here — there isn't a universal 1:1 substitution ratio between gelatin and agar agar. The exact amount needed depends heavily on the specific recipe, the strength of the agar product being used, and the texture you're trying to achieve, since agar generally sets firmer than gelatin at an equivalent measured quantity. America's Test Kitchen's own side-by-side testing found that agar flakes and gelatin didn't behave identically even at carefully measured amounts, and that agar required a distinctly different preparation method — soaking, then boiling — to activate properly. Testing in small batches before committing to a full recipe is the safer approach.