The big picture
Japanese counts in two parallel systems — one borrowed from Chinese (Sino-Japanese, いち、に、さん) and one native (Wago, ひとつ、ふたつ、みっつ). The Sino set runs the whole range from zero to trillions; the native set tops out around ten and is mostly used with everyday objects, ages, and counters.
The two systems, side by side
Native numbers only go to 10 in everyday speech. They typically pair with counters or stand alone for small amounts ("two apples", "three of them"). Sino numbers do everything else.
Loaned from Chinese around the 8th century. Used for math, money, time, dates, and anything past 10.
Used for objects 1–10, especially with the generic counter 〜つ (-tsu). Ages 1–10 use a variant: issai, nisai…
Zero to ten
Memorize the Sino column. The native column is useful but a smaller commitment.
The tens (20 – 90)
Perfectly regular: digit + jū. 40 and 70 follow the yon / nana preference.
Compound numbers
Stack the parts. No "and", no hyphens, no spaces in writing — and no glue word like Portuguese e. 四十二 = "four-ten-two."
Hundred, thousand, ten-thousand
Japanese groups numbers in tens of thousands (万 · man), not thousands. Comfortable counting up to ichi-oku (一億 · 100 million) is a meaningful milestone.
A note on counters
You almost never use bare numbers to count objects. Japanese uses counters (classifiers) — small suffixes that depend on the kind of thing you're counting. 一冊 · is-satsu = one book; 一匹 · ip-piki = one (small) animal.
Things to remember
The four rules that will save you from the most common mistakes.
You'll hear it before you can read it.
The companion iOS app reads random numbers aloud in Japanese with Sino readings. Train the ear before the eye.
Get the app