LUFS loudness meter

How the meter works

Your browser decodes the file (at its own sample rate whenever that can be read from the file header) and the measurement code runs on your device. The meter follows ITU-R BS.1770-5 for loudness and true peak, and EBU Tech 3342 for loudness range. It is tested against FFmpeg's EBU R 128 meter on calibration tones and noise programmes: integrated loudness agrees within 0.2 LU and true peak within 0.3 dB, and a 0 dBFS, 997 Hz sine on one channel reads −3.01 LUFS, the calibration point the standard defines.

Integrated loudness (LUFS)

The average loudness of the whole file. Each channel is K-weighted (a high-frequency shelf and a low cut that approximate how loud we hear different frequencies), squared and averaged over 400 ms blocks that overlap by 75%. Blocks below −70 LUFS are ignored, then blocks more than 10 LU below the average of the rest, so pauses and silence do not pull the number down. LUFS and LKFS are the same unit, and 1 LU is 1 dB.

True peak (dBTP)

The highest level of the waveform as it is reconstructed between samples, found by oversampling the audio four times. A file whose highest sample is −0.1 dBFS can still have a true peak above 0 dBTP, which may clip when it is encoded to MP3 or AAC or played through a converter. That is why delivery specifications set a true-peak ceiling such as −1 dBTP. A 4x meter can under-read by up to about 0.7 dB for content near the top of the audio band, so leave a little margin under any ceiling.

Loudness range (LU)

How much the loudness varies over the programme: the spread between the 10th and 95th percentiles of the 3-second short-term loudness, sampled ten times a second, after removing silence (below −70 LUFS) and passages more than 20 LU below the average. A steady single-voice show has a low range, while quiet interviews mixed with loud music beds give a higher one. EBU R 128 sets no maximum, and the value means little for clips shorter than a minute.

Sample peak and maximum short-term loudness

Sample peak is the largest absolute sample value in dBFS. Maximum short-term loudness is the loudest 3-second stretch, which helps you spot a loud intro, sting or advert.

Mono and stereo

BS.1770 measures a mono file as a single channel. The same audio copied onto both channels of a stereo file reads about 3 LU louder, which is why mono podcasts are commonly mastered to −19 LUFS rather than −16. Once you know your numbers, the gain calculator shows how far to move and whether a limiter is needed, and the delivery presets explain the targets LufsReady masters to.