- you can do this in any DAW program.
Step one: pull out the calculator and divide 60 (as in seconds) by the the target BPM. for example 60sec./125bpm=.48sec=480ms. this will find the length of one beat.
step two: multiply the number of beats in the loop by the beat length. example: 4beats*480ms=1.92sec= 1920ms
step three: pitch adjust the loop to the length you detirmined.
step four: if the individual beats are off and you want to correct them you can do this a number of ways
step four method one: you can visually locate the beginings of sounds, so highlight from the begining of a beat to the next drum hit and correct the length with pitch shift, an eigth note in the previous examples would be 240ms, a sixteenth would be 120ms.
step four preferred method: sometimes the above method can make the loop sound "wobbley", a better way is to make up for differences in timing by inserting silences (if the silence is noticable, you can use a reverb over the last snippet of sound to give it a better sense of continuity), or by selectively cutting out sections of sound, the best parts to cut out are the (more-or-less empty) spaces between beats.