Add square in header text bound field gridview

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C#

ASP.NET

Ho do i add kg/m2 (kg/meter <sup>2</sup>)

Squre is not showing in header text ,its just show 2.
help me to solve this

<pre> <asp:BoundField DataField="BMI" HeaderText="BMI (kg)/m2" HeaderStyle-VerticalAlign="Top"
                    HeaderStyle-Width="100px">
                    <HeaderStyle VerticalAlign="Top"  Width="100px" />
                </asp:BoundField>

What I have tried:

² but not resolve. with sup tag .
I have tried to add htmlencode=true but not resolve to this also.