I’ve been trying to figure out how many women could be in the next U.S. Senate.

Right now, 17 senators are women.

Two of them are retiring: Olympia Snowe of Maine (R), and Kay Bailey Hutchison of Texas (R). So that’s -2.

But in Hawaii, where Daniel Akaka is retiring, the next senator from Hawaii will definitely be a woman, because both major party candidates are women: Mazie Hirono (D) and Linda Lingle (R). So that’s +1.

In California and New York, the incumbent and the challenger are both women: Dianne Feinstein (D) is being challenged by Elizabeth Emken (R), and Kirsten Gillibrand (D) is being challenged by Wendy Long (R). Feinstein and Gillibrand should have easy wins; in any case, that’s +0.

So far, that’s net -1.

What about other races?

There are three races where women are running and are not likely to win (according to Nate Silver): in New Mexico, Heather Wilson (R) vs. Martin Heinrich (D); in North Dakota, Heidi Heitkamp (D) vs. Rick Berg (R); and in Maine, Cynthia Dill (D) vs. Charlie Summers (R) vs. Angus King (I). So, that’s +0.

That leaves six races.

Five of those six races could increase the number of women:

Nebraska: Deb Fischer (R) vs. Bob Kerrey (D). Deb Fischer is way ahead and will likely win, according to Nate Silver. So that’s a likely +1.

Connecticut: Linda McMahon (R) vs. Chris Shays (D). Nate Silver says Shays is likely to win, so that’s a likely +0. (My instinct says: who knows with this one, but I’ll trust Nate.)

Three of those five appear to be tossups right now:

Massachusetts: Elizabeth Warren (D) vs. Scott Brown (R): 0/+1.

Nevada: Shelley Berkeley (D) vs. Dean Heller (R): 0/+1.

Wisconsin: Tammy Baldwin (D) vs. Tommy Thompson (R): 0/+1.

And there’s one race that could decrease the number of women:

Missouri: Claire McCaskill (D) vs. Todd Akin (R). Despite Akin’s implosion, Nate Silver gives McCaskill just a 65% chance of winning — which is still pretty high, but who knows. So, -1/0.

Tallying this all up, and I don’t know if I’m doing this right, but:

Women in the Senate now: 17
ME (Olympia Snowe retirement): -1
TX (Kay Bailey Hutchison retirement): -1
HI (Hirono vs. Lingle): +1
NE (Fischer): +1
MA (Warren), NV (Berkelely), WI (Baldwin): +1.5 (based on probabilities?)
MO (McCaskill): -0.33 (based on probabilities?)

That works out to 18 plus a fraction. So it seems like there could be a net gain of at least one Senate seat, and maybe even two or three. The next Senate will most likely have 18-19 women. If McCaskill and all three tossups lose (which is possible), there will be just 16. If McCaskill and all three tossups win (which is also possible), there will be 20.

If all women running against men lose their races, the next Senate will have just 16 women; if they all win, even the long shots (CT-McMahon, NM-Wilson, ND-Heitkamp), the next Senate will have 24 women.

So the possible number of women in the next Senate is 16 to 24, with 18-19 most likely.

[Update: I forgot about Debbie Stabenow (D) in Michigan and Amy Klobuchar (D) in Minnesota, who are both heavy favorites for re-election. If they somehow lost along with all the other women, there would be only 14 women in the next Senate, but that’s not gonna happen.]