How does one write out "1km" -- in hiragana or katakana or both?

Question

This sentence on NHK's Easy News got me wondering:

Uの字【じ】のような形【かたち】をしていて、直径【ちょっけい】は1kmぐらいです

It is shaped like a U and is about 1 km in diameter.

https://www3.nhk.or.jp/news/easy/k10013205991000/k10013205991000.html

At this website

https://www.wasabi-jpn.com/how-to-speak-japanese/how-to-measure-length-weight-and-area-in-japanese/

I found いっきろ

but shouldn't it be

いっキロ

or possibly even

いッキロ

As always in language learning, there is little point for a learner in trying to think about what the correct answer to this question should be; one just needs to know what native speakers do.

Here are three more examples from Jim Breen's wwwjdic:

村の郵便局まで１キロ足らずだった

It was less than one kilometer to the village post office.

この川の１キロほど川上に橋があります

There is a bridge about one kilometer up this river.

ガソリンスタンドは１キロ先にあります

A gas station is one kilometer ahead.

(As always, the wwwjdic does not provide readings)

Answer 1

The standard way to write 1 km in kana is いっキロ, いっキロメートル, いちキロ or いちキロメートル. Since キロ(メートル) is a loanword from English, it should be written in katakana.

That site (wasabi-jpn) seems to have chosen the all-hiragana style in that page because kana is used like pure phonetic symbols. I guess they thought switching the types of kana depending on the origin of the words may be distracting for beginners when they want to focus only on the pronunciation. They could have chosen the all-katakana style as well. But this is a rather exceptional case where all-hiragana or all-katakana is permitted.