What is dBFS? Decibels relative to full scale, explained

dBFS (decibels relative to full scale) is the unit for level inside a digital audio system, measured against the highest level that system can represent, which is called 0 dBFS. Nothing in a fixed-point digital signal goes above it, so every other level is written as a negative number: -6 dBFS, -18 dBFS and so on.

How it works

The dBFS reference defines it as a unit of amplitude in digital systems, with 0 dBFS assigned to the maximum possible digital level. Push a fixed-point signal past that point and there are no larger numbers to store it in, so the waveform is cut off: that is digital clipping.

There is a wrinkle for RMS readings. The same reference, citing the AES17 measurement standard, notes that the convention sets the RMS value of a full-scale sine wave as 0 dB FS. Because the reference is a sine, a full-scale square wave can read as much as about +3 dB FS on an RMS meter without any sample going over the limit.

dBFS is not a voltage

0 dBFS is a number, not a fixed analog level. What it becomes on the wire depends on the converter. RME's Fireface UCX II manual lists the phones output at +19 dBu at 0 dBFS on its High setting and +4 dBu at 0 dBFS on Low, and gives the minimum input level for 0 dBFS on the rear line inputs as +1 dBu. The same digital peak, two very different analog levels.

What we think

Most confusion about dBFS comes from treating 0 as a target rather than a ceiling. A digital meter counts down from the top, so "how loud" questions belong to loudness units, and dBFS answers only "how close to the edge". Leave headroom while recording and mixing, and check the interface's spec sheet so you know what 0 dBFS actually means in your room.