I have a 21 speed bike, and I'm thinking of purchasing a 1 speed. Does anyone know what gear on the 21 would be for a 1? I'm trying to see how well my bike does on that gear in my area. Thanks!
We can't tell without more information about your gears.
However, you can figure it out for yourself quite easily, by looking at the ratio of chainring teeth to rear sprocket teeth on the single speed bike (the most common I've seen is 44/18 = 2.44).
Then you just need to figure out the size of your current bikes chainrings, the size of the sprockets on your current bike, and calculate all 21 gear ratios, and find which is closest to the single speed ratio.
If I had to guess, I would say it would most likely be equivalent to your middle chainring (39 teeth?) and somewhere near the third smallest rear sprocket (16 teeth?) because 39/16 = 44/18.
Of course, you can almost always pick and choose the gearing on a single speed bike, so realistically you yourself get to pick "what gear on the 21 would be for a 1".
There are numerous gear ratio calculation tools out there. Once you know which gears work best in your area, you can use these tools to find the ration you need. Something like this for example: http://www.bikecalc.com/gear_ratios
Or you can do it on paper :)