The difference between aloud and out loud is that aloud used to be the only proper option and today aloud is still the more formal option, whereas out loud is somewhat more informal. Use aloud in a sermon or an academic paper, but it's fine to use out loud in a blog post or Facebook post.
To read out loud and to read aloud mean the same thing, but in the old days, aloud was the only cultured option.
In the early 1900s, usage guide writers looked down their noses at out loud and called it “colloquial.” Today, out loud and aloud are both fine, although aloud still feels more high-brow and formal.
Which word people use seems to vary depending on the context and the exact phrase. For example, in published books that Google has scanned, aloud is more common; but in Reddit comments, out loud is more common, which doesn’t surprise me since published books are much more formal documents than Reddit comments.
But even in published books, in some phrases, out loud is more common. For example, I said that out loud is more common than I said that aloud. It seems like when people are expressing the feeling I can’t believe I said that, they stick with out loud.
The quick and dirty tip is that you can use either aloud or out loud—whichever sounds more natural to you—but if you want to be a stickler, aloud is probably better for solemn or formal occasions such as asking someone to “read aloud” in church.
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