七百(ななひゃく)

七(なな) + 百(ひゃく). Both parts keep their normal shape.

Pronunciation

The alternative しちひゃく is possible but rarely used; なな is preferred throughout the hundreds because しち is easily misheard as いち. The same preference shows up in 七時(しちじ) versus 七人(しちにん)／七人(ななにん), where speakers often choose なな for clarity.

Sound changes in the hundreds

Only 三百(さんびゃく), 六百(ろっぴゃく), and 八百(はっぴゃく) change; 七百(ななひゃく) and the rest are regular.