Wes Davis, often mentioned on this site, used the expression on top of this post in an email the other day and parenthetically added, “(as Paul Hollywood says).” I’m not proud to say I had to Google to find out that Hollywood is a judge on “The Great British Bake Off.” But I didn’t really have to consult any sources to realize “If I’m honest” is a characteristically British expression. It just sounds like one.
Google Books Ngram Viewer confirmed the impression. It shows the expression coming on the scene in about 1990 and always being much more popular in Britain than in the U.S.:
Thoughtfully doing my work for me, someone on the Quora site asked, “Is the phrase ‘if I’m honest’ used outside the U.K.?” Three people responded, most pithily Andrew Humphrey, who said, “Wherever it is used, it is a pointless affectation. People in the UK are very fond of such redundant and pretentious words and phrases. They use these phrases to give their hackneyed or cliched pronouncements some fake importance or profundity.”
But more helpful was Luke Proctor, who dug up examples of two American using it, thus securing NOOB status:
If I’m honest I don’t believe the world would miss me if I never acted again.
Jamie Lee Curtis, actress
Because if I’m honest, people in the white world might be appalled, but in the black world they’re making myths out of me. And I know that ain’t the life
John Singleton, director
I also found, amazingly, no fewer than eight popular songs called “If I’m Honest”: by Blake Shelton, Missy Higgins, Brendan Murray, Julia Gargano, Jay Denton, the group All That Remains, and Kaitlyn Bristow of “The Bachelorette.” I know Shelton is American and assume Bristow is; I’ll leave it to you lot to sort out the nationality of the rest.
In a post on her blog, Separated by a Common Language, linguist Lynne Murphy did some investigating and found out that not only “If I’m honest,” but also the similar expressions “If I’m being honest” and “To be honest,” are used far more in the U.K. than the U.S. She goes on to muse:
One has to wonder: why are these such popular idioms in BrE? And then one has to wonder: is it because most of the time people are expected NOT to be honest, so it has to be marked up where people are being honest? There may be something to that — the British, after all, have an international reputation for not saying what they mean.
Of the three expressions, the one that sounds most familiar to my American ears is “To be honest.” So I plugged it in to Ngram Viewer and found this:
That is to say, it was roughly equally popular in both countries for a long time, and was used markedly more frequently in both between about 1980 and 2000. After that, it skyrocketed in Britain.
Why? If I’m honest, I have no idea.
notoneoffbritishisms.com 
I’m with you. I would automatically say that “To be honest” is American and “If I’m honest” is British. Small sample size in my life, I guess.
I say “to be honest” fairly often, and it reflects more of a warning that I’m not going to sugar-coat the next thing I say when I speak. eg: “To be honest, I’d really rather not work an extra shift this week.”
I’d agree. I use it as a pre-warning that the next thing may be uncomfortable to hear. “If I’m honest, I think you should just leave.” “To be honest, I don’t really care.”
tbh, I really don’t know.
Missy Higgins = Australian
I’m Dutch (but I use English fairly often).
If necessary (yes, by way of putting in a preliminary warning) I would say “To be honest” .
Are ‘to be frank’ or ‘frankly’ more common US equivalents?
Google Ngram Viewer shows “to be frank” more common in U.K. than U.S. (and it sounds odd to me). “Frankly” (as the opening of a sentence) was roughly equivalent till about 1920 when it shot up in U.S. and stayed ahead for eight decades–the Rhett Butler effect, I’d say. (“Frankly, my dear, I don’t give a damn.”) Then, like apparently all these phrases, it shot up in the U.K. post-2000. https://books.google.com/ngrams/graph?content=Frankly%3Aeng_gb_2019%2CFrankly%3Aeng_us_2019&year_start=1800&year_end=2019&corpus=26&smoothing=3
This has reminded me very amusingly of when my husband and I bought our house 35 years ago. We had dealings with a young estate agent who used the phrase ‘to be honest’ every other sentence. In the end we referred to him as ‘T’be honest’ as a nickname.
He is still an estate agent in our small town, now with his own thriving business and very much a stalwart citizen (he sponsors the cricket club). We know him pretty well but still occasionally refer to him by his sobriquet.
Brilliant!
To be honest, if I’m honest are often used in Australia, especially where there are links to immigrants from England…
I’ve always love when my UK friends start a sentence with “if I’m honest”. Makes me think “are you?”
And if someone sometimes adds it, are they being dishonest when they omit it? I don’t think I ever say it.
I’m not really familiar with “If I’m honest” in the UK: “To be honest” is what I’d expect to hear. Another possibility is: “I’ll be honest with you”.
It is simply a marker that one is about to give a candid opinion.
Andrew Humphrey appears to be either xenophobic or generally sour.
I haven’t heard of “if I’m honest”, but I use “honestly speaking” (and to a lesser extent “to be honest”) ALL the time. Maybe it’s because I’m Chinese, and 實話來說 (literally “truth be told/honestly speaking”) is a really common term? Does “if I’m honest” mean EXACTLY the same as “truth be told”? Honestly speaking it’s my first time to encounter the expression “if I’m honest” (no pun intended)
I would say ‘if truth be told’ it’s pretty much the same.
I concur. “Truth be told” sounds a little bit…formal…like something I’d expect a spinster or an elderly minister to say, but it carries the same meaning for me.
I think you also have to consider the phrase “to tell the truth” here. Except for a couple of periods (including most recently in the last two decades), that seems to be more American than British.
I think a better variation is ‘If I’m being honest’, since it only applies to what is being said, not to the whole person. I would be more likely to say that.
What makes “If I’m honest..” statements striking is that they contain two elisions, that is: “If I’m [being] honest, [I would say that] that shirt looks terrible.” “To be honest…” and “If I’m being honest …” only have (the second) one.
I think it’s a stigmatised usage here in the UK. No-one I know uses it, and if I hear it I automatically mark the speaker down a bit (which I shouldn’t do). I’m a middle-aged Englishman, by the way. On the other hand the phrase, ‘to be honest’ is often heard from speakers of English as a second language. Oh, and Andrew Humphrey is obviously trying to be offensive.
Have a look at Kate Fox’s book “Watching the English”, where she describes in detail the English culture of humour. Everything is saturated in subtle shades of humour, or at least potential humour. It’s the water these fish swim in, not always noticed by them.
“If I’m honest”, “to be honest”, “truth to tell”, etc. are signals that what’s coming next is NOT humour, it’s serious and not exaggerated and not joking around, and the listener shouldn’t joke as a response. This sort of signal is vital in English culture, not nearly so much in American culture.