The Tinseth equation is the best of the bunch... and it's not perfect either, as another pointed out that it was based on whole hop cones, not pellets. But for lack of anything better, it's the best, and is what I use and what I recommend.
For a ballpark number, you can use this approximation that I developed that follows pretty closely with Tinseth (try it and you'll see) AND which adds IBUs for hop stand / whirlpool / whatever you want to call post-boil hop additions:
IBU = oz * AA% * [sqrt(5*Boiltime)/V + sqrt(2*HStime)/V]
where
V is post-boil volume,
Boiltime is in minutes, and
HStime is post-boil hop stand time in minutes that it hangs around between about 150-190 F.
For multiple hop additions, put each one in separately, run the calc, then add up the total IBUs. It's not perfect, but it's pretty darn close for standard gravity worts of 1.050-1.060. At higher gravity (e.g., >1.080), change the sqrt 5 to 4 instead, and sqrt 2 to 1.5. At low gravity (e.g., 1.035-1.040), change the 5 to a 6, and the 2 to 2.5. Pretty close, and not too hard to memorize for those of us who like to formulate recipes on the fly on the back of napkins and such, that's the real reason why I took time to figure this out... that, and I'm a math nerd.