LONG vs. SHORT VOWELS!
If you're like me, you constantly forget if a kanji's ONyomi is a long or a short vowel. In Kanjidamage, I tried to deal with this through the mnemonics; the LONG ONyomi (SHOU, KOU, etc.) are given regular keywords, but their short counterparts (SHO, KO, etc.) are given acronym keywords to set them apart (Smell HIs Organs, Knocked Out, respectively).
But even with my insulting acronyms, the long-vs-short battle is far from won. So here's another resource you can use to help you guess the pronunciation, when you inevitably forget.
STATISTICS!
Don't worry, you don't have to do them because I already did.
Following is a list of all the long-short ONyomi, presented as a sporting contest between Team Long and Team Short. Important caveat; It's not a total of ALL kanji, just the 1,700 in the KD system. You know, the useful ones.
Without further ado, here's the winners of our fictional contest:
bou vs. bo
9 long, 0 short
chou vs. cho
10 long, 1 short
chuuu vs. chu
7 long, 0 short
fuu vs. fu
long 1, short 6 - THE UNDERDOG WINS!
gou vs. go
3 long, 3 short TIE
gyou vs. gyo
3 long, 0 short
hou vs. ho
9long, 1 short
hyou vs. hyo
5 long, 0 short
jou vs. jo
10 long, 3 short
juu vs. ju
3 long, 2 short
kou vs. ko
22 long vs. 11 short
kuu vs. ku
1 long, 4 short - THE UNDERDOG WINS!
kyou vs. kyo
4 short, 11 long
kyuu vs. kyu
8 long, 0 short
mou vs. mo
1 long, 1 short TIE and isn't it odd there aren't more of those mo/mou kanji?
nou vs. no
4 long, 0 short
nyuu vs. nyu
2 long, 0 short
ou vs. o
6 long, 1 short
rou vs. ro
2 long, 1 short AMBIGUOUS
ryou vs. ryo
4 long, 1 short
shou vs. sho
19 long, 8 short
shuu vs. shu
10 long, 5 short
sou vs. so
2 long, 4 short - THE UNDERDOG WINS!
tou vs.to
10 long, 3 short
you vs. yo
7 long, 0 short???
yuu vs. yu
4 long 3 short TIE, ALMOST
zou vs. zo
2 long, 0 short.
TOTAL
The longs won 22 of 27!
8 of those victories were total; that is to say, there are NO short versions, so you don't even have to worry about it. The shorts just vanished, just like your moms' shorts did on a live taping of American Idol.
The shorts won a whopping 3 of 27.
2 ties.
PUT ANOTHER WAY
OK, you might say, but some of these ONyomi - like KOU/KO or SHOU/SHO are much more common than other ONyomi, such as MOU/MO or BOU/BO. Yes, that's a valid point.
Well, so let's calculate just using raw numbers of words then.
Of the 288 kanji included in this list, LONG is like 228, and SHORT is like 60.
SO, shorts are around 21 PERCENT of the total.
Long ones are 80 percent. Ish.
WHAT WE LEARNED
tl;dr, just guess the long version.
Also, instead of memorizing all 288 kanjis' long/short ONyomi, if you're in a rush, it might pay you to simply memorize the list of ONyomi where SHORT is more common.
FU, KU, SO. That's it. You're welcome.
Also, don't forget to memorize the ones that are TIED (equal chance of it being either): GOU/GO, MOU/MO, YUU/YU