Before anything else: mastering cannot rescue a mix. It is the last five per cent, applied to a finished stereo file, and if the mix underneath it is muddy then what you get back is a louder muddy mix. Everything below assumes the mix is as good as you can currently get it. If it is not, that is the job, and it is a different page.
The route that costs nothing and genuinely works
BandLab's online mastering. You open a web page, drag in your stereo mix, pick one of a handful of presets, listen to the result, and download it. No installation, no account gymnastics, no charge, no watermark on the file.
It is an automated process and it is not going to make the decisions a mastering engineer would make. What it will do is get your track to a sensible level with a sensible tonal balance, which is what most self-released songs actually need. For a first release, it is the correct answer, and the alternative it is competing against is not a great mastering engineer — it is you uploading an unmastered mix at the wrong volume.
Several other automated services exist and most of them let you hear the result before you pay. LANDR will process your track and let you listen to the master for nothing, then charge around $10 to download it. Hearing a second opinion at no cost is genuinely useful even if you download the other one.
The numbers that matter
Every major platform normalises playback volume, which means the loudness war is over and you lost nothing by not fighting it. Spotify, YouTube, Tidal and Amazon all turn loud tracks down to a common level. Push your master to be as loud as possible and the platform will simply attenuate it — and you will have squashed the life out of the song for no gain at all.
The targets worth knowing:
Integrated loudness: around -14 LUFS. That is the level the main platforms normalise to, and landing near it means your track is heard as you made it.
True peak ceiling: -1 dBTP. This leaves headroom for the distortion that appears when your file gets converted to the compressed formats streaming actually delivers. Amazon asks for -2 dBTP, so if you want one number that satisfies everyone, use -2.
If your track is quieter than -14, nothing bad happens — quiet tracks are turned up, not penalised. It is the loud ones that get hurt.
How to check, without installing anything
Your recording software almost certainly measures this already. Reaper, Logic, Ableton and Cubase all have loudness metering built in, and it is generally hiding in the master channel's metering options rather than in the plugin list. Find it before you go looking for a download.
Play the loudest section of the finished song from start to end and read the integrated value, not the momentary one. The integrated figure is the average across the whole piece and is the number the platforms use.
When an automated master is not enough
Automated mastering handles overall level and broad tonal balance well. It handles specific problems badly, because it cannot tell a problem from a choice. A harsh sibilant vocal, a bass note that is twice as loud as the others, a section that needs to feel different from the section before it — these need somebody who can hear intent.
So the honest line is: a good mix mastered automatically will beat a bad mix mastered by a human, every time. Pay for mastering when you have run out of things you can fix yourself, not before, and not on your first release.
The order of operations
Finish the mix. Leave it a day. Listen on the worst speaker you own and in mono. Fix what that reveals. Export a stereo file with a few decibels of headroom left — do not put a limiter across the master bus and squash it before sending it. Master that file. Compare the master against the mix at matched volume, because louder always sounds better for the first ten seconds and that is how people talk themselves into worse masters.
Then stop. The version you have is almost certainly fine, and the next four hours of adjustment will not be.
Platforms turn loud tracks down. The loudness war ended and nobody told the people still fighting it.
Three pages on this site that carry it further: recording the thing in the first place, the recording software that costs nothing, and what you are listening on.
The Rest of the Recording Chain
Mastering is the last step of a chain that starts with a room and a microphone. The guides cover every stage of it, written for people working with what they already own.