The day fine concept consists in imposing fines in such a structured way that the final amount of the fine is directly proportionate to offenders' means and to the offense's seriousness. This is achieved by multiplication of two quantities: the amount of one day fine and the number of day fines.
The amount of one day fine is set in proportion to offenders' means, while the number of day fines reflects the offense's seriousness. In some-especially Anglophone-countries the day fine is called either a structured fine, as there is a clear structure of how the fine is calculated, or a unit fine, as offenders' means serve as a basis for calculating one day fine unit.
The systematic way of assessing offenders' means is what differentiates day fines from fixed fines: by setting only the final amount, fixed fines might be proportional to offenders' means, yet such a relationship is unclear, unverifiable, and likely inconsistent across sentencers. There are several rationales for making explicit the relationship between offenders' means and a fine.
Consequentionalists (utilitarians) support day fines because they are supposed to similarly deter offenders of different wealth. Those less concerned with utilitarian theories support day fines because they better communicate the appropriate amount of censure via hard treatment to offenders than fixed fines.
Day fines further limit their unequal impact on disadvantaged groups upon default: if a fine is set in proportion to offenders' means, the ratio of poor offenders defaulting and serving a prison sentence is likely to decrease when compared with not setting a fine in direct proportion to offenders' means. Sentencing scholarship would prefer day fines over fixed fines as they are an expression of principled sentencing and they likely limit disparities in assessing offenders' means.
Day fines are used especially in Continental Europe and in Latin America. Even though the Anglophone world often strove to introduce day fines, and sometimes succeeded, the day fine concept was never widely accepted in the common-law systems.
The large amount of scholarship published in English thus retells Continental experiences; suggests ways of implementing day fines, especially in the United States, where they are neglected compared with Continental Europe; or discusses the pilot projects in Anglophone countries. Readers should be warned, however, that there is still little research on how day fines function in practice, and even on fines in general, even though they the most popular sanction.