That is an open-ended question; it depends upon the size of the slice, the fat content, etc. but for your normal packaged grocery store version, it between 40-50 calories for one slice of bacon ... which I believe is one strip piece.
Also, it depends on how you serve it of course - grilled or fried.
I have found that the low salt bacon tastes just as good as the regular and would have added benefits as you wont take in as much fluid from salt retention. So a good rule of thumb--- 40-50 calories a slice.
For thicker fried slices it might be 150-200 per slice.