In the Philippine legal system, the determination of a penalty is not a mechanical process of matching a crime to a fixed term of imprisonment. Instead, it is a calibrated exercise governed primarily by the Revised Penal Code (RPC) and various special penal laws. The law provides specific grounds—ranging from the circumstances of the crime to the post-conviction behavior of the offender—that allow for the reduction of a sentence.

Understanding these grounds requires a distinction between the modification of the penalty during sentencing and the reduction of the actual time served through administrative or remedial means.

1. Mitigating Circumstances (Article 13, RPC)

Mitigating circumstances are those which, if present in the commission of the crime, do not entirely free the actor from criminal liability but serve to reduce the penalty imposed.

Ordinary Mitigating Circumstances

Under Article 13 of the RPC, several factors can lower a penalty within the periods prescribed by law:

Incomplete Justification/Exemption: When not all the elements necessary to justify the act or exempt from liability are present.
Under 18 or Over 70 Years of Age: The "minority" or "senility" of the accused.
Praeter Intentionem: When the offender had no intention to commit so grave a wrong as that committed.
Provocation or Threat: Sufficient provocation or threat on the part of the offended party immediately preceded the act.
Vindication of a Grave Offense: The felony was committed in the immediate vindication of a grave offense to the perpetrator, or their spouse, ascendants, descendants, or siblings.
Passion or Obfuscation: Acting upon an impulse so powerful as naturally to have produced passion or obfuscation.
Voluntary Surrender and Confession of Guilt: Surrendering to authorities before arrest or pleading guilty in open court prior to the presentation of evidence for the prosecution.
Physical Defects: Being deaf and dumb, blind, or otherwise suffering from physical defects that restrict means of action or defense.
Illness: Sufferings that diminish the exercise of will-power without depriving the person of the consciousness of their acts.

Privileged Mitigating Circumstances

Unlike ordinary mitigating circumstances, which typically lower the penalty to its minimum period, privileged mitigating circumstances allow the court to lower the penalty by one or two degrees.

Minority: Under the Juvenile Justice and Welfare Act (RA 9344), if a minor is found discernibly liable, the penalty is automatically lowered by two degrees.
Incomplete Self-Defense: When a majority of the conditions for self-defense are present.

2. The Indeterminate Sentence Law (Act No. 4103)

The Indeterminate Sentence Law (ISL) is the most common statutory vehicle for reducing the actual time a convict spends in prison. Instead of a "straight" sentence (e.g., exactly 10 years), the court imposes a sentence with a minimum and a maximum term.

The Maximum Term: Determined by the rules of the RPC, considering attending circumstances.
The Minimum Term: Falls within the range of the penalty next lower in degree to that prescribed by the Code.
Effect: Once the convict serves the minimum term, they become eligible for parole, subject to the discretion of the Board of Pardons and Parole.

3. Plea Bargaining

Plea bargaining is a process where the accused and the prosecution agree to a disposition of the case, usually involving the accused pleading guilty to a lesser offense which is necessarily included in the crime charged.

Following the landmark case of Estipona v. Lobrigo, plea bargaining is now even permitted in certain drug-related cases under RA 9165, provided the quantity of drugs involved falls within the thresholds set by the Supreme Court. This results in a significantly lower penalty than what the original charge carried.

4. Good Conduct Time Allowance (GCTA)

The duration of a sentence can be reduced after conviction through Republic Act No. 10592, which amended the RPC's provisions on time allowances. Convicts can earn deductions from their sentence through:

Good Conduct Time Allowance: Days deducted for every month of good behavior.
Loyalty Allowance: A deduction of one-fifth (or two-fifths in some cases) of the sentence for those who choose to stay or return to their place of confinement during calamities or mutinies.
Teaching/Mentoring Allowance: Additional credits for those who act as literacy workers or instructors within the correctional facility.

Note: Under current jurisprudence and implementing rules, those convicted of "Heinous Crimes" are generally excluded from the benefits of GCTA under RA 10592.

5. Executive Clemency

The President of the Philippines possesses the constitutional power to grant forms of clemency that reduce or eliminate sentences:

Reprieve: A temporary stay of the execution of a sentence.
Commutation of Sentence: The reduction of a heavier penalty to a lighter one (e.g., Reclusion Perpetua reduced to 20 years).
Pardon (Conditional or Absolute): An act of grace which exempts the individual from the punishment the law inflicts for a crime he has committed.

6. Probation (PD 968)

While not a "reduction" of a sentence in the traditional sense, Probation allows a person to serve their sentence outside of a correctional facility under the supervision of a probation officer. If the sentence imposed is a term of imprisonment of not more than six (6) years, the convict may apply for probation. If granted, the execution of the sentence is suspended.