Opera propia. Basic geometry and dimensions from the description in Belarus Law No. 301-Z.

The national flag of the Republic of Belarus is a rectangular panel consisting of two horizontal colored stripes: the upper one is red and the lower one is green. The ratio of the width of the red and green stripes is 2:1. The ratio of the width of the State Flag of the Republic of Belarus to its length is 1:2. At the pole there is a Belarusian national ornament of red color on a white field, which is 1/9 of the length of the State Flag of the Republic of Belarus. The national flag of the Republic of Belarus is mounted on a staff (flagpole), which is painted in golden (ocher) color. The ratio of the width of the State Flag of the Republic of Belarus to the length of the staff is 1:3.

— English translation of Chapter 2, Article 4 Law 301-Z

The version of Law No. 301-Z linked above contains a small blurry image of the flag with no labeled dimensions. The image clearly shows a vertical red stripe along the hoist that is not mentioned in the text.

In addition to Law 301-Z, the Belarus State Committee on Standardization maintains standards for the flag in СТБ 911-2008. СТБ 911-2008 was updated in 2009 with text that can be seen at ru.wikisource.org.

The length of the flag is limited from the pocket of the flagpole by a vertical red stripe with a width equal to ¹⁄₂₁ of the ornament horizontally.

— translation of 2009 amendment to СТБ 911-2008

That text can be interpreted as a vertical red stripe between the hoist and the ornament; it is contained within the borders of the flag and is not part of the hem or pocket. Oddly enough, the specification drawing included with the text in the 2009 update for СТБ 911-2008 did not include the red stripe. Oops.

In 2012, the government amended СТБ 911-2008 again and added a new specification drawing that did include the red stripe: [1]. The specification drawing includes dimensions for the red stripe and the ornament. Sadly the specification drawing is low quality and presented to us only in JPG format which makes it impossible to recreate the internal geometry of the ornament with any degree of precision.

The government does provide an larger JPG image at mfa.gov.by/sgp/state_symbols. It is higher resolution than the image in СТБ 911-2008, but it is still a JPG and it is impossible to perfectly extract precise internal geometry for the ornament. Also, the width of the ornament in the JPG is about 4% wider than the the law dictates.... and the red vertical stripe is about 3% wider than the dictated by СТБ 911-2008. Oops again.

The red vertical stripe on this construction sheet is set at exactly ¹⁄₂₁ the width of the ornament as per СТБ 911-2008.

The ornament on this construction sheet is based on best effort measurements from the JPG image listed above. It has been squeezed to make its width ¹⁄₉ the length of the flag, and its internal coordinates have been snapped to 200×900 grid. Note that due the the low quality of the source JPG image, there is no justification to use a higher resolution than 200×900. Reflectional symmetry (horizontal and vertical) has been applied. For scale purposes, a 100x450 grid has been drawn on top of the ornament, but this is not to imply that the designer of the ornament used any grid at all. Attempting to determine the grid size (if any) that was used for the original design of the ornament is an exercise in complete and utter frustration.

The ornament can potentially improved by redrawing it on a grid with a small number of large rectangles as seen in File:Belarus_flag_pattern.svg, File:Flag_of_Belarus_(ornaments).svg, and, vexillographia.ru. In one the examples shown at vexillographia.ru, all the lines in the ornament are the same width as the vertical red stripe; it gives the flag a sharp and clean look. But alas, any such modifications are in violation of Chapter 1, Article 3 of the Flag Law 301-Z.

The State Flag of the Republic of Belarus and its image, regardless of their size, must exactly correspond to the standard of the State Flag of the Republic of Belarus.

— Chapter 1, Article 3 Law 301-Z