וַיַּ֥עַשׂ אֶת־הַיָּ֖ם מוּצָ֑ק עֶ֣שֶׂר בָּֽ֠אַמָּה מִשְּׂפָת֨וֹ אֶל־שְׂפָת֜וֹ עָג֣וֹל ׀ סָבִ֗יב וְחָמֵ֤שׁ בָּֽאַמָּה֙ קֽוֹמָת֔וֹ וְקָו֙ שְׁלֹשִׁ֣ים בָּֽאַמָּ֔ה יָסֹ֥ב אֹת֖וֹ סָבִֽיב׃
Also he made the molten sea of ten cubits from brim to brim, round in compass, and the height thereof was five cubits; and a line of thirty cubits did compass it round about.
Rashi on II Chronicles
ten cubits from brim to brim Ten by ten in its center, circular around.
Rashi on II Chronicles
and a line of thirty cubits did encompass it round about Whatever has a circumference of three handbreadths has a diameter of one handbreadth.