Numbers

The system of Japanese numbers based on Chinese numerals. Therefore the grouping of large numbers made in chunks of 10,000 (tenth of thousands) and not in thousands as in European countries.

Basic numbers

Number	Reading	Kana
0	zero/rei	ゼロ/れい
1	ichi	いち
2	ni	に
3	san	さん
4	shi/yon	し/よん
5	go	ご
6	rorku	ろく
7	shich/nana	しち/なな
8	hachi	はち
9	ku/kyū	きゅう/く
10	jū	じゅう
100	hyaku	ひゃく
1,000	sen	せん
10,000	man	まん

To construct numbers higher then ten just combine the numbers you know. Example:

Number	Reading	Kana
12	jū ichi	じゅう いち
20	ni jū	に じゅう

There are some exceptions in pronunciation:

Number	Reading	Kana
300	sanbyaku	さんびゃく
600	roppyaku	ろっぴゃく
800	happyaku	はっぴゃく
3,000	sanzen	さんぜん
8,000	hassen	はっせん

Tip: Starting with 10,000, you should add number of tenth thousands from the first, one. That is: 100=hyaku, 1,000=sen but 10,000=ichiman.
Tip: In Japanese culture, the numbers 4 and 9 considered the unlucky ones. 4(shi) is a homophone for death. and 9 (ku) is a homophone for suffering.

The way of writing
The numbers in Japanese almost always written in kanji or numerical, because the kana is longer to write.
Another thing is, the formal numbers in economics, official documents etc. written in more complex kanji, known as daiji numbers. This way it's easier to prevent fraud. Even the banknotes of Japanese yen have formal numbers on them.

Number	Common	Formal
0	〇	零
1	一	壱
2	二	弐
3	三	参
4	四
5	五
6	六
7	七
8	八
9	九
10	十	拾
100	百
1000	千
10,000	万	万, 萬

Counting things
The most peculiar way the Japanese counting differs from English, is the counter words. These words are classifiers, used to count different things: objects, events, actions. Different types of objects has their own counters. Example: counter for flat objects, for people, for minutes etc. I'll write a separate post, dedicated for the counters.