Mixing turns many recorded parts into one balanced piece. Mastering takes that finished piece and prepares it to work everywhere it will be heard. They are sequential, not interchangeable.
What mixing does
Mixing is where the relationships are decided: how loud each element is, where it sits left to right, how much space is around it, and which parts lead at each moment. It includes cleaning, tonal shaping, dynamics control and effects. Almost every creative sound decision belongs here.
A mix is finished when each element is audible and the listener's attention goes where you want it.
What mastering does
Mastering works on the finished stereo file. It sets overall tone, controls dynamics, ensures consistent level between tracks in a release, catches problems the mixer stopped hearing, and produces the delivery formats required by distributors, broadcast or podcast platforms.
Mastering is measured in small adjustments. If a master is changing the balance of the song, the mix was not finished.
What mastering cannot fix
A buried vocal, a harsh cymbal, a bass that dominates, an edit that is out of time. All of these live in the mix. Sending a problem mix to mastering and hoping is the most common false economy in audio.
Do you need both
For commercial music release, yes. For a podcast, a single combined process handled by one editor is usually appropriate: cleaning, balancing voices and meeting the loudness target platforms expect. For video, a mix to a broadcast standard may be required by the platform or the client.
Fresh ears matter
The strongest argument for a separate mastering engineer is not equipment; it is that they have not heard the song two hundred times. A second opinion catches what familiarity hides.
Preparing to send to mastering
Send the mix without limiting on the master bus unless it is part of the sound, leave headroom, use the original sample rate, and include a note about the intended feel and any references. Provide any alternative versions you need at the same time, such as instrumental or clean edits.
Know when to stop
Both stages have diminishing returns. The mix that is ninety five percent right and released beats the mix that is endlessly revised and never finished.