With the help of the chart below, you can choose an audio file with bpm (beats per minute) very close to the needed lenght.
In your editing software, you now can pitch your file for generating the perfect loop. Therefore you need to right click your audio clip, choose „change clip speed“. In Adobe Premiere you can type in the exact lenght you want the file to be.

So why the chart? Because this operation will change the pitch of the audio, too. (In fact there is an option for keeping the pitch the same while changing the lenght, but this always ends up in bad results.)
For best sounding results it’s advisable to choose a close bpm.

You don’t know the bpm of your track? Just try a simple tap tool like all8.com*.
On Soundfluencer.com you can find royalty free tracks in all genres. Bpm information included.

 

 

*(As the content of the website, that links leads to is not under our control, we cannot assume any liability for such external content. In all cases, the provider of information of the linked websites is liable for the content and accuracy of the information provided. At the point in time when the links were placed, no infringements of the law were recognisable to us. As soon as an infringement of the law becomes known to us, we will immediately remove the link in question.)