Section 21: Calculation of imprisonment term
Statutory text (Thai original)
ในการคำนวณระยะเวลาจำคุก ให้นับวันเริ่มจำคุกรวมคำนวณเข้าด้วย และให้นับเป็นหนึ่งวันเต็มโดยไม่ต้องคำนึงถึงจำนวนชั่วโมง ถ้าระยะเวลาที่คำนวณนั้นกำหนดเป็นเดือน ให้นับสามสิบวันเป็นหนึ่งเดือน ถ้ากำหนดเป็นปี ให้คำนวณตามปีปฏิทินในราชการ เมื่อผู้ต้องคำพิพากษาถูกจำคุกครบกำหนดแล้ว ให้ปล่อยตัวในวันถัดจากวันที่ครบกำหนด
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.
Cited in 25 Supreme Court decisions (1965 to 2024)
Selected citing decisions
- Decision 418/2567 (2024)
- Decision 3044/2565 (2022)
- Decision 2164/2561 (2018)
- Decision 415/2561 (2018)
- Decision 9174/2551 (2008)
- Decision 5347/2550 (2007)
- Decision 6632/2544 (2001)
- Decision 6396/2544 (2001)
This list is selected automatically, weighted towards judgments that turn on this section rather than ones that merely recite it when passing sentence. It has not yet been reviewed by the firm.
Most often cited alongside
- Section 91 22
- Section 78 18
- Thai CrPC s. 225 7
- Section 83 7
- Thai CrPC s. 195 6
- Section 29 5
Sections that appear in the same judgments as this one most often. The figure is the number of decisions.
Counted across the firm's corpus of 83,652 Supreme Court decisions, 1921 to 2026. The count is complete; the stated year range trims rare outliers so it describes where the citations actually sit. These figures are computed by us and are not official court statistics. How we count these, and what we measured
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.
Related guides on ThaiLawOnline
Cite this section
-
Plain citation
Penal Code, s. 21 (Thailand) -
Academic citation
Penal Code (Thailand), s. 21. ThaiLawOnline, https://www.thailawonline.com/thai-penal-code/section-21/ (accessed 17 August 2026). -
Thai citation
ป.อ. มาตรา 21 -
Permalink
https://www.thailawonline.com/thai-penal-code/section-21/ -
Embed on your site
<blockquote cite="https://www.thailawonline.com/thai-penal-code/section-21/"><p>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…</p><footer>Penal Code, s. 21 (Thailand) — <a href="https://www.thailawonline.com/thai-penal-code/section-21/">ThaiLawOnline</a></footer></blockquote>
The Thai text is authoritative. The English is an unofficial translation by ThaiLawOnline, free to reuse with attribution.