Inside your DAW every meter is in dBFS, and what counts is keeping peaks under 0 dBFS wherever the audio meets a converter or gets exported to a fixed-point file. dBu and dBV come in wherever audio travels as a voltage: between pieces of analogue gear, and in and out of your interface.
There's no fixed conversion from dBu to dBFS. Look up your interface's maximum input level in dBu; that's the voltage that reads 0 dBFS, and everything else follows from it.
At a glance
| Property | dBFS | dBu | dBV |
|---|---|---|---|
| What it measures | Sample values in a digital file | Voltage on a cable | Voltage on a cable |
| What 0 means | The largest value the file can hold | 0.775 volts RMS | 1 volt RMS |
| Can it go above 0? | Not in a fixed-point file or a converter | Yes, up to wherever the gear clips | Yes, up to wherever the gear clips |
| Normal operating level | About −18 to −20 dBFS, set by the converter | +4 dBu on pro gear | −10 dBV on consumer gear |
| Where you see it | DAW meters, plug-ins, exports | Mixing desks, outboard, interface spec sheets | Hi-fi, some semi-pro gear and interfaces |
| Convert to dBu | Add the converter's maximum input level | Already dBu | Add 2.2 dB |
What is dBFS?
dBFS stands for decibels relative to full scale. A digital file stores audio as numbers, and full scale is the largest number it can store, so 0 dBFS is the top of the range and every level under it is negative. A sample at half of full scale is −6 dBFS, and one at a tenth is −20 dBFS.
Nothing in a fixed-point file, such as a 16 or 24-bit WAV, can go above 0 dBFS. A sample that tries is stored as the maximum instead, the top of the wave comes out flat, and that's clipping. A converter works the same way: once the voltage at its input would need a number bigger than full scale, it writes full scale. Inside most DAWs the mix runs in 32-bit floating point, which can hold levels far over 0 dBFS, so a channel reading +3 dBFS does no harm until it reaches a converter or a fixed-point export.
dBFS has no voltage attached to it. A file peaking at −6 dBFS could come out of your speakers at almost any volume, depending on the interface and the volume knob, which is why the analogue side needs units of its own. The decibels lesson covers the maths behind all three, and the bit depth lesson covers what sits at the bottom of the dBFS scale.
What is dBu?
dBu measures voltage against 0.775 volts RMS. RMS is a kind of average for a voltage that keeps swinging up and down, and it's how audio voltages are quoted on spec sheets. 0 dBu is 0.775 V, +6 dBu is about twice that and −20 dBu a tenth of it.
The odd reference comes from telephone lines. Old audio systems were built around 600 ohm circuits, and 0.775 volts across 600 ohms delivers one milliwatt, which is 0 dBm, a power unit. Modern gear doesn't run at 600 ohms, so engineers kept the voltage and dropped the power. The u is usually explained as unloaded, meaning the figure doesn't depend on a 600 ohm load.
Professional line level is +4 dBu, which works out at 1.23 volts. That's the nominal level, the level the gear is designed to run at most of the time, and on a desk or a piece of outboard with a VU meter it's normally what 0 VU reads. Pro gear can usually go a long way above it before it clips, which is the analogue version of headroom.
What is dBV?
dBV measures the same thing, voltage, against a round 1 volt RMS. Consumer and some semi-pro gear is rated in dBV, and its nominal level is −10 dBV, which is 0.32 volts. Home hi-fi and the RCA phono sockets on lots of gear tend to run around there.
Because 1 volt is a little more than 0.775 volts, any voltage reads 2.2 dB higher in dBu than in dBV. 1 volt is 0 dBV and +2.2 dBu; 0.775 volts is 0 dBu and −2.2 dBV.
How to convert between dBu and dBV
Add 2.2 to a dBV figure to get dBu, and take 2.2 away from a dBu figure to get dBV. The exact offset is 20 × log10(1 ÷ 0.775), which is 2.21 dB.
To get from either one to volts, divide the decibel figure by 20, raise 10 to that power and multiply by the reference. +4 dBu is 0.775 × 104/20, about 1.23 V. −10 dBV is 1 × 10−10/20, about 0.32 V.
The sum people get wrong is the gap between pro and consumer line level. +4 against −10 looks like 14 dB, but the two numbers use different references. Put −10 dBV into dBu and it becomes −7.8 dBu, so the real gap is 4 − (−7.8) = 11.8 dB. In volts, 1.23 against 0.32, the pro level is a little under four times the consumer one.
How dBu relates to dBFS
A converter turns voltage into numbers, so somewhere in its design one voltage has been picked to become 0 dBFS. Spec sheets call it the maximum input level, and it's normally printed in dBu. If an interface's line input has a maximum of +24 dBu, then +24 dBu reads 0 dBFS, +4 dBu reads −20 dBFS and 0 dBu reads −24 dBFS: subtract the maximum from any dBu level and you have its dBFS reading.
Nothing in physics fixes that figure, so studios line converters up to a standard. SMPTE RP155, used widely in the US, puts a +4 dBu reference tone at −20 dBFS, which makes full scale +24 dBu. EBU R68, the European broadcast recommendation, puts 0 dBu at −18 dBFS, so full scale is +18 dBu. Both leave 18 to 20 dB between the reference level and the point of clipping.
Steps 1 to 3 in the lab show both. The pro gear's average and its +4 dBu tone stay at exactly the same voltage the whole time; only the dBFS ruler slides when you change the converter's maximum. That's also where −18 dBFS comes from as a target for gain staging, since many plug-ins modelled on analogue gear treat −18 dBFS as their 0 VU.
An input gain knob changes the same figure. At minimum gain an interface might take +24 dBu before it clips; turn the gain up 12 dB and +12 dBu is enough to reach 0 dBFS. That's why the lab's Max input knob stands in for both the converter's design and its gain setting.
+4 dBu vs −10 dBV gear
Plug a −10 dBV device into a +4 dBu input and it arrives 11.8 dB lower than the input expects. Nothing is damaged and nothing distorts, but the recording comes in quiet, and turning it up later raises the input's own noise along with it. In the lab, the consumer gear's mix peaks at −18.4 dBFS on a +24 dBu input. Turning the gain up 12 dB brings it to −6.4 dBFS, within a fifth of a decibel of where the pro gear sat.
The other way round causes real trouble. A +4 dBu output into an input set up for −10 dBV gear arrives about 12 dB hotter than it was designed for, and the peaks of the music can run out of room. Step 6 does that: the same gain that suited the consumer gear sends the pro gear's peaks 5.4 dB past full scale. Press A/B there to hear the clipped file against a copy with no ceiling.
Many interfaces have a switch on each line input marked +4 dBu and −10 dBV, which changes the gain by about the right amount, and some outboard has the same switch on its outputs. Whether a connection is balanced is a separate matter. Pro gear is usually balanced and consumer gear unbalanced, but the level switch only changes level and does nothing about hum.
Microphones and guitar pickups sit far below either line level. A microphone puts out somewhere around a millivolt to a few millivolts, roughly −60 to −40 dBu, which is why it needs a preamp to bring it up to line level before a converter can make good use of its range.
How much room above nominal level?
Nominal level describes the average, and music has peaks well above its average. The full mix in the lab has its highest peak 13.4 dB above its average, measured the way this lab measures RMS, and the drum loop has more than that. On a +24 dBu converter, pro gear at nominal leaves 20 dB above its average, so the mix's peaks clear full scale easily.
Step 7 turns the gear up 6 dB, to +10 dBu. The mix still fits, peaking at −0.6 dBFS. Step 8 keeps every setting and swaps in the drums, whose sharper peaks would reach +25.6 dBu, 1.6 dB past full scale, while their average sits 14 dB under it. You can't tell from the average where the next peak will land, so leave room when you record.
The lab measures average as AES17 RMS, the kind that reads a sine wave's average the same as its peak, so a tone sits on one line across every scale. A meter using plain RMS reads the same audio about 3 dB lower, so check which kind you have before comparing numbers. The LUFS vs RMS vs peak comparison has more on meter types, and the headroom lesson covers how much room to leave at each stage.
Common levels on every scale
Each row is one voltage, worked out on each scale. The last two columns are the reading on a converter lined up to EBU R68 (+18 dBu at full scale) and to SMPTE RP155 (+24 dBu at full scale).
| Level | Volts RMS | dBu | dBV | dBFS at +18 | dBFS at +24 |
|---|---|---|---|---|---|
| Microphone, roughly | 2.5 mV | −50.0 | −52.2 | −68.0 | −74.0 |
| Consumer line level, −10 dBV | 0.316 V | −7.8 | −10.0 | −25.8 | −31.8 |
| 0 dBu | 0.775 V | 0.0 | −2.2 | −18.0 | −24.0 |
| 0 dBV | 1.00 V | +2.2 | 0.0 | −15.8 | −21.8 |
| Pro line level, +4 dBu | 1.23 V | +4.0 | +1.8 | −14.0 | −20.0 |
| +18 dBu | 6.16 V | +18.0 | +15.8 | 0.0 | −6.0 |
| +24 dBu | 12.3 V | +24.0 | +21.8 | Over | 0.0 |
"Over" means the converter clips. Plenty of pro gear can put out +24 dBu, and a converter lined up to EBU R68 runs out of numbers 6 dB before that.
How to set levels between analogue gear and your interface
- Find the output levelCheck the gear's manual or the label next to its outputs. A +4 dBu or −10 dBV rating tells you its nominal level, and a maximum output figure tells you how hot it can get.
- Find the input's maximumLook up your interface's maximum input level for its line inputs, in dBu. Subtract it from the gear's nominal level and you know where that level reads in dBFS at minimum gain.
- Set the switch or the gainIf the input has a +4 dBu / −10 dBV switch, match it to the gear. If not, aim for the gear's average to land somewhere around −20 to −18 dBFS.
- Check the loudest partPlay the loudest passage you'll record and watch the peak meter. At 24 bits there's nothing to gain from getting close to 0 dBFS, so leave at least 6 dB clear.
- Line up with a tone if you canLarge-format desks and some outboard have a built-in tone oscillator. Send a 1 kHz tone at 0 VU and adjust the gain until it reads your reference, −18 or −20 dBFS; everything else at nominal level will land in the same place.
Going out of the interface works the same way in reverse. Its maximum output level tells you what voltage 0 dBFS becomes, so a DAW send peaking at −6 dBFS out of a +18 dBu output arrives at +12 dBu. That's fine for most pro outboard and too much for some consumer inputs. The signal flow lesson has more on routing audio out to hardware and back.
Common mistakes
Treating dBFS and dBu as one scale
A meter reading −6 dBFS tells you nothing about voltage until you know the converter. Two interfaces with different maximum levels give different dBFS readings for the same cable, and both are right.
Taking +4 and −10 as 14 dB apart
They're 11.8 dB apart, because one is in dBu and the other in dBV. Put both on the same reference before you subtract.
Mixing up dBu and dBV on a spec sheet
The difference is only 2.2 dB, so on its own the mistake rarely causes trouble, but it adds up when you're working out how much headroom an input has.
Feeding pro outputs into consumer inputs
A +4 dBu output into a −10 dBV input arrives nearly 12 dB hot. Turn the source down or set the switch before you blame the recording for crackling.
Fixing a low input with digital gain
If −10 dBV gear arrives quiet on a +4 dBu input, turning it up in the DAW brings the input's noise up with it. Turn the interface's gain up instead, so the converter gets a proper level to work with.
Recording as close to 0 dBFS as you can
An analogue source can swing past the converter's maximum on a peak you didn't see coming, and anything past 0 dBFS is gone for good. The clipping lesson shows what that does to a sound.
Training your ears to hear converter clipping
The quiz under the lab plays two recordings of the same loop with the screens covered. They're matched in loudness, but one was sent to the converter 2, 4 or 8 dB too hot and clipped, and you pick which. Clipping shows up first on the loudest hits, so listen to the front of each one. The 2 dB rounds are the hard ones, because only a handful of peaks lose anything.
Questions people ask
Is 0 dBu the same as 0 dBFS?
No. 0 dBu is a voltage, 0.775 volts RMS, and 0 dBFS is the largest number a digital file can hold. Which voltage becomes 0 dBFS depends on how the converter is set up. On a converter lined up to EBU R68, 0 dBu reads −18 dBFS; on one lined up to SMPTE RP155, it reads −24 dBFS.
What is the difference between dBu and dBV?
Only the reference. dBu counts from 0.775 volts and dBV from 1 volt, so any voltage reads 2.2 dB higher in dBu than in dBV. To convert, add 2.2 to a dBV figure to get dBu, or take 2.2 away from a dBu figure to get dBV.
How many dB apart are +4 dBu and −10 dBV?
About 11.8 dB. The numbers make it look like 14, but they use different references. −10 dBV is −7.8 dBu, and the gap from −7.8 to +4 is 11.8 dB. In voltage, that is 0.32 volts against 1.23 volts, a little under four times as much.
What does maximum input level mean on an audio interface?
It's the voltage, usually printed in dBu, that the converter turns into 0 dBFS at the lowest gain setting. An interface with a maximum line input of +24 dBu reads a +4 dBu tone at −20 dBFS. Turn the gain up and the maximum drops, so the same voltage reads higher.
Can I plug −10 dBV gear into a +4 dBu input?
Yes. Nothing gets damaged; the signal just comes in about 12 dB lower than pro gear would, so turn the input gain up or use the interface's −10 dBV setting if it has one. The other way round, pro gear into a consumer input, is the one that can clip.
Why is dBFS always negative?
Because it counts down from the top. 0 dBFS is the largest value a fixed-point file or a converter can store, so every level it records is some number of decibels below it. The 32-bit float mix bus inside a DAW is the exception: it can read above 0 dBFS without clipping. dBu and dBV count from a voltage in the middle of the range, so their readings can be positive or negative.