noun-modifying (irregular)
six-year; sexennial; recurring every six years
Used as a prenominal modifier (adj-f) in phrases like 六カ年計画 (six-year plan). Describes a period or cycle of six years.
The government announced a new six-year plan.
This road construction will be carried out over a period of six years.
六年間 is a noun/adverb meaning 'for six years' (duration), while 六カ年 is a prenominal modifier meaning 'six-year' (attributive).
六年 is the simple number 'six years', used as a noun or counter. 六カ年 is more formal and often used in compound terms like 六カ年計画.
Commonly used in official documents and plans; the カ is a small katakana 'ka' used as a counter marker.
Variant with small katakana 'ke', also widely seen.
Hiragana variant; less common in formal writing but still used.
Using Arabic numeral 6; common in tables and abbreviated contexts.
Formal kanji 箇; mostly seen in older or very formal documents.
Variant with small katakana 'ka' using the alternate small-ka character; less common.
Full-size katakana 'ke'; occasionally seen but nonstandard.
Arabic numeral with small ke; included for search coverage.
Arabic numeral with alternate small ka; included for search coverage.
Arabic numeral with hiragana; included for search coverage.
Arabic numeral with full-size ke; included for search coverage.
Arabic numeral with formal kanji; included for search coverage.
Compound of 六 (six) + カ (counter marker, from 箇) + 年 (year). The reading ろっかねん results from the gemination of ろく (six) before the counter か.