Karat is gold purity (24K = pure). Carat is gemstone weight (1 ct = 200 mg). Same pronunciation, different measurements.
Karat (spelled with K in US) measures gold purity on a 24-point scale: 24K is pure gold, 18K is 75% gold / 25% alloy, 14K is 58.3%, 10K is 41.7%. US law requires 10K minimum to be called "gold."
Carat (spelled with C) is a unit of weight for gemstones: 1 carat = 200 mg = 0.2 g. A 1-carat diamond is not a standard size — it is a weight. Diamond size depends on cut proportions, though a 1-carat round brilliant is typically ~6.5 mm across.
14K ring
58.3% gold by mass, rest alloy (typically copper + silver)
1-carat diamond
200 mg, roughly 6.5 mm for a round brilliant cut