How many letters does the top answer for “concave on both sides” have?

The most common answer is biconcave (9 letters).

Crossword clue: concave on both sides

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

biconcave 9 letters wn-def

concave on both sides

concave or hollow on both sides; double concave. 60% similar → concavoconcave

flabellate on both sides. 50% similar → biflabellate

supported by both sides 50% similar → bipartisan