Why did nine-pound hammers become a standard, rather than eight- or ten-pound hammers?
Without knowing anything about hammers I would guess it was due to market forces. That was the size that the hardware stores wanted, because that was the size that their customers wanted. Over time it became the standard.
Are you talking about a Johnny Cash song lyric?
ETA: Had to check but this is the supposed weight of the hammers used by John Henry. That would mean it wasn’t standard but instead too heavy for the average man.
Used in the lyrics of many songs.
Nine pounds seems rather light to me. For breaking rocks etc in the UK, we used to use a 14lb sledge. For close work, a 4lb “lump” hammer was favourite.
The kind we used in the army was always referred to as a “5-Kilo Hammer”. That makes it, what, 11 pounds?
I just Googled “9lb hammer” and the whole first page, as well all the suggested search terms were Marijuana related. Now I’m really confused.
The most common versions of John Henry do not mention the nine pound hammer; one has him “swinging 30 pounds from my hips on down,” and I am sure the usual wags will shortly have something to add for our delight and delectation. FYI, John Henry is not driving spikes or breaking rocks, he’s driving a handheld drill bit that is some few feet in length, boring holes for the insertion of explosives.
There is a song recorded in 1927 called 9 Pound Hammer, but it’s very different from John Henry. In I think 1951 Merle Travis published a different song called 9 Pound Hammer that is probably the one people know, as it has been covered by all kinds of people, including Johnny Cash on his 1963 album Blood, Sweat and Tears, where he also recorded John Henry, a likely possibility for the conflation of the two.
The story began to change somewhere along the way. The George Pal short ‘John Henry and the Inky-Poo’ had him driving spikes in the ties. However, as you say, the legend is about driving a stone drill and racing against a steam drill.
I own an 8# hammer/sledge and have used 10#, 12#, 13#, & 16# ones. Outside the song, I have never heard of a 9# hammer. What makes you say it is a standard?
What happens if you have to pound ten times to drive in the nail? 
Then you’re not doing it right. Maybe get someone to hold the nail in place for you while you take a two-handed swing…which is how they drove spikes on the railroad.
Has a 9-pound hammer ever been a well-known ordinary item anywhere? Or has it ever been the biggest one normally available for some purpose?
Or was it originally just a phrase that sounded good to a songwriter?
There are no nine pound hammers. I checked tool suppliers and they make from 6 to 20 in 2 pound increments.
Dennis
So could the song have just as easily been called “Three-Dollar Bill”?
Get a 1 pound hammer to drive it home.
In the metal fabrication shop I once worked in, if you had to hit something repeatedly, soon would follow cries of “Get a bigger hammer, idiot!”
And interestingly enough, a 9 lb hammer is 4.08 kilos.
I have my doubts that it’s a 9 lb = 4 kg kind of thing, but it’s an interesting coincidence.
And FWIW, the vast majority of the first two pages of Google hits for 9 lb Hammer are for some strain of weed named that.
And results for “none pound hammer” come up with links for a musical group in some style called “cowpunk”.
OP joined yesterday, has made just this one post. Methinks we are being trolled…
Not trolling, I’ve just been curious ever since I first heard the song back in the '60s. “Nine just sounded good” doesn’t seem right, since “ten” strikes the ear (sorry) harder than nine, both because of the initial “t” and because of its phonetic proximity to “ton.”
I’ve tried researching the question a few times over the years, but most searches return hits (sorry again) for the song, not the hammer. So, maybe the popularity of the song is misleading, and nine-pound hammers aren’t really prevalent IRL.
Funny when the designation of your favorite music genre strikes someone else as unusual enough to enclose in “”! Next you’ll be saying you’ve never heard of psychobilly! 