They are unrelated. Onto is a live preposition meaning movement to a position on top of something: "the cat jumped onto the table". Unto is archaic and means to — it survives almost only in scripture, hymns and deliberately old-sounding prose: "do unto others". If you are writing modern English, you almost certainly want onto.
The core difference
These look like a pair and are not. The similarity is a spelling accident.
- onto — movement ending on a surface: climbed onto the roof
- unto — an old form of to: Give unto Caesar
Unto is archaic, not formal
This is the mistake worth avoiding. Writers reach for unto believing it is the elevated version of to, and it is not — it is the obsolete one. In modern prose it reads as either a biblical quotation or an error. The King James Bible is why it survives at all, along with fixed phrases like do unto others and unto this last.
The test for onto
Say the sentence with on alone. If it still works, onto is right and you are describing a position or a movement to one: "he stepped onto the platform" → "he stepped on the platform".
Watch for the case where on belongs to the verb rather than the noun — "hold on to the rail", "log on to the site". There the on is part of hold on and log on, so the words stay separate.
Onto meaning "aware of"
Onto has a second, informal sense: to be onto someone is to have realised what they are doing — "the police are onto him". It is idiomatic and never written as two words.
Frequently asked questions
Is "unto" just a formal version of "to"?
No — it is archaic rather than formal. In modern writing it reads as a biblical echo or a mistake. Use to.
When is "on to" two words?
When on belongs to the verb: hold on to, log on to, move on to. Test by removing to — if "hold on" still makes sense on its own, keep them separate.
Is "onto" acceptable in formal writing?
Yes. Onto is standard modern English in every register.