Load an audio file and drag the gain slider to the number of decibels you want to add or remove. Check the predicted peak, then press Apply and download the new WAV.
Digital audio cannot go above full scale. If your chosen gain would push the loudest peak past 0 dBFS the tool warns you and shows the resulting peak, because clipped samples turn into audible distortion that cannot be undone later.
When enabled, samples that would exceed full scale are gently rounded off instead of being hard-clipped. It keeps a boosted track listenable, though for the cleanest result you should lower the gain until no clipping is reported at all.
Raising the gain raises the recording's own background hiss by exactly the same amount, because every sample is multiplied by the same factor. Volume change never adds new noise, it just makes existing noise more audible.