Flag of Burundi — construction sheet
Own work. Geometry based on the description in the Constitution of Burundi and dimensions provided by Decree 1/31 — September 27,1982 — Dimensions et autres caractéristiques du Drapeau national ainsi que les conditions de son utilisation

The flag of Burundi is tricolor: green, white, and red. It has the shape of a rectangle partitioned by a diagonal cross, having at its center a white disk stamped with three
six-pointed red stars which form a notional equilateral triangle inscribed in a notional circle having the same center as the disk and of which the base is parallel to the length of the flag.— Article 10 — Constitution of Burundi 2018

In standard format, the national flag measures 2.50 m by 1.50 m.
The other dimensions relating to the components of the drawing of the national flag as defined by article 7 of the Constitution of the Republic of Burundi are fixed as follows:

the bands of the cross (saltire) are 20 cm wide;
the central disc has a diameter of 85 cm;
the imaginary circles in which the three stars are inscribed have each a diameter of 20 cm;
the imaginary circle in which the imaginary equilateral triangle is inscribed formed of the three red stars has a diameter of 44 cm;
each star is bordered by a border 9 mm wide fimbriation.— English translation of Decree 1/31 — September 27,1982
Note: note there is a discrepancy regarding the number of the article in the constitution because the articles have been renumbered several times since 1982.

The law is vague regarding the sizes of the stars.  There are two plausible interpretations:

The 0.9 cm green fimbriations sit entirely within the 20 cm diameter imaginary circles.
The red stars are sized to fit precisely within the 20 cm imaginary circles, and then the 0.9 cm fimbriations are added to the outside of the red stars.
This construction sheet uses the first of these interpretations which is in agreement with both Flags of the World (FOTW) and Vexilla Mundi.
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