noun, noun, used as a suffix
Refers to a plan, scheme, or arrangement. Often used in compounds like 計画 (plan) or 計略 (stratagem). As a standalone noun, it can mean a plan or design, but is more common in compound words.
彼の計は失敗に終わった。
His plan ended in failure.
この計は長期的な視点で立てられた。
This plan was made with a long-term perspective.
noun, noun, used as a suffix
meter; gauge; measuring device
Used as a suffix to indicate a measuring instrument or meter, e.g., 温度計 (thermometer), 速度計 (speedometer). Also used as a standalone noun for a measuring device.
温度計で室温を測った。
I measured the room temperature with a thermometer.
この計は非常に精密だ。
This gauge is very precise.
prefix
Prefix meaning 'total' or 'in total'. Used in words like 計数 (total count) or 計量 (measurement/weighing). Not freely productive; mostly appears in established compounds.
計10名が参加した。
A total of ten people participated.
計量カップで小麦粉を量る。
Measure the flour with a measuring cup.
計画 is the common word for 'plan' or 'project', while 計 alone is more abstract and often used in compounds.
計器 specifically means 'measuring instrument' or 'meter', whereas 計 as a suffix is more general and attaches to various nouns.
The kanji 計 combines 言 (speech) and 十 (ten), originally meaning 'to calculate' or 'to plan'. The reading けい is the on'yomi. The word is used in many compounds related to planning and measurement.