Short answer. Days are deducted from the sentence for each month of good behaviour, and the rate rises the longer the person is detained. Be careful with older copies of Article 97: Republic Act No. 10592 (2013) replaced the figures printed there with substantially larger allowances, and those larger ones are the law today.
What the law says
During the first two years of imprisonment, he shall be allowed a deduction of twenty days for each month of good behavior during detention
Revised Penal Code, Article 97 — Allowance For Good Conduct. Read the full provision →
The current rates come from RA 10592
The mechanism has not changed: good behaviour earns a fixed number of days off for every month served, and the rate steps up in bands as the years accumulate. What changed is the numbers. Many reproductions of the Code still print the original schedule of five, eight, ten and fifteen days per month. Republic Act No. 10592, enacted in 2013, rewrote Article 97 and raised those to twenty days a month during the first two years, twenty-three days during the third to fifth years, twenty-five days from the sixth to the tenth year, and thirty days from the eleventh year onward. Any computation still using the old figures understates the credit by a wide margin.
Who may earn it, and for what
The allowance belongs to any prisoner in any penal institution, and the amending law extended the credit to persons still detained while their case is being tried, not only to those already convicted. It also added a separate deduction for each month spent in study, teaching or mentoring service, so schooling and vocational programmes inside the facility count on their own. Deductions are earned month by month for behaviour actually shown; they are not a discount announced at sentencing. Because the amendment is favourable to the accused, penal law principles allow it to benefit persons already serving sentence, subject to the rules the implementing regulations set.
What good conduct time does not do
It shortens the time served; it does not erase the conviction, wipe the record, or touch the civil liability the judgment imposed. It is also not automatic. Credit is granted by the authorised prison official on the basis of the institution's own records, and it can be forfeited or withheld — misconduct, prison offences and escape are the obvious triggers. The amending law expressly withholds the benefit from certain groups, including recidivists, habitual delinquents, escapees and persons charged with heinous crimes. Nor is the allowance the same thing as parole, probation or executive clemency, each of which has its own qualifications and its own process.
If you are computing a release date
Do not rely on a figure someone recalls, and do not rely on an undated printout of the Code. Ask the facility for the written record of credits actually granted, month by month, and check which schedule was applied. Errors run in both directions, and a wrong band or an omitted period of preventive detention can cost months. Where credit appears to have been denied or forfeited, there should be a documented basis for it, and that basis can be questioned. This page explains the article in general terms and cannot tell you what a particular sentence works out to; if a release date is in dispute, book a consultation and bring the commitment order and the prison records.