Section 21 — Calculation of imprisonment term
English translation
In calculating the term of imprisonment, the day shall be counted as the unit. If the term is fixed in months, thirty days shall count as one month; if fixed in years, it shall be calculated by the calendar year. A person sentenced to imprisonment shall be released on the day following the day on which the term of imprisonment is completed.
This English translation is provided for reference only and has not yet been firm-verified — always rely on the Thai original.
Firm annotation
Section 21 is a technical general-part rule that fixes how imprisonment terms are measured and when release occurs. The mixed method matters: a term stated in months is 30 days each, so twelve months equals 360 days, whereas a term stated in years follows the calendar and can be 365 or 366 days. This distinction affects the exact length a prisoner actually serves and is a recurring point in appeals over sentence computation. The release rule, the day after completion, gives a bright-line end point that dovetails with the deduction of pre-trial detention in Section 22.
Why this matters in practice
This rule can change the real length of a sentence: because a month is only 30 days, a sentence expressed in months is slightly shorter in practice than the same nominal period expressed in years. When multiple counts are combined and reduced, the method of counting can affect the total, so it is worth checking the arithmetic in a judgment. Errors in sentence computation are a legitimate ground to raise, and a lawyer can verify whether the term and release date were calculated correctly.
Frequently asked questions
How many days is a one-month prison sentence?
Under Section 21, one month counts as thirty days, so a twelve-month sentence equals 360 days, which is fewer than a calendar year.
When is a prisoner released?
Under Section 21, a person is released on the day following the day the term of imprisonment is completed.