Korean has two parallel number systems, and learners must know when to use each. The Sino-Korean numbers are based on Chinese characters and are used for dates, money, phone numbers, addresses, minutes, and any number above 100. They are 일 (il, 1), 이 (i, 2), 삼 (sam, 3), 사 (sa, 4), 오 (o, 5), 육 (yuk, 6), 칠 (chil, 7), 팔 (pal, 8), 구 (gu, 9), and 십 (sip, 10). Larger units follow a distinctive pattern: 100 is 백 (baek), 1,000 is 천 (cheon), and 10,000 is 만 (man). Notably, Korean groups large numbers by ten thousands rather than thousands, so 10,000 — not 1,000 — is the next major unit after 만.
The native Korean numbers, in contrast, are used for counting objects (with counters), hours, age, and quantities between 1 and 99. They are 하나 (hana, 1), 둘 (dul, 2), 셋 (set, 3), 넷 (net, 4), 다섯 (daseot, 5), 여섯 (yeoseot, 6), 일곱 (ilgop, 7), 여덟 (yeodeol, 8), 아홉 (ahop, 9), and 열 (yeol, 10). Before counters, these numbers shorten: 하나 becomes 한, 둘 becomes 두, 셋 becomes 세, 넷 becomes 네, and 스물 becomes 스무. Common counters include 분 (bun) for people in polite contexts, 명 (myeong) for people in casual contexts, 개 (gae) for general objects, 병 (byeong) for bottles, and 마리 (mari) for animals. For example, 두 분 (du bun) means "two people" politely, and 고양이 세 마리 means "three cats."
The days of the week are formed by combining the Sino-Korean numbers with 요일 (yoil, "day"), and each weekday is named after one of the five traditional elements plus the sun and moon: 월요일 (woryoil) for Monday (moon), 화요일 (hwayoil) for Tuesday (fire), 수요일 (suyoil) for Wednesday (water), 목요일 (mogyoil) for Thursday (wood), 금요일 (geumyoil) for Friday (gold/metal), 토요일 (toyoil) for Saturday (earth), and 일요일 (iryoil) for Sunday (sun). Months follow the same Sino-Korean pattern with 월 (wol, "month"): 일월 (irwol, January) through 십이월 (sibiwol, December).