If your song’s peak volume is 0, meaning the loudest point in the song is hitting 0 (or higher) you’ll want to do a little mixing before sending it off to mastering.
The easiest way to get your mix levels right for mastering is to drop the volume level on your faders by -6. So for example, if you’re bass is coming in at -8, lower the volume on it until it is – 14.
If you’re vocals are sitting pretty at -3, drop it to -9. Basically you want to take whatever level each channel is at when you’ve got the song mixed and sounding good and drop it by -6.
Here’s a fancy math formula which will either make me look smarter than I really am or just confuse you even more.
Volume of channel – 6 = Happy Mastering Engineer
Why -6? You can really choose any number below -3, but I find -6 is easy to subtract from any number and at least helps provide some headroom for mastering (it is certainly better than 0).
When it is all said and done you should have a song that has a peak volume of -6 (assuming that your songs peak volume was 0 before), which will make it suitable for high quality mastering -thus making your mastering engineer very happy because they can than do a really good job for you.