See the description of the Knoop Hardness test below.

The following equation is used to calculate Knoop Hardness number (HK):

 HK = 1.451*\frac{F}{D^2} .

For further information see [1].

References

[1] https://www.emcotest.com/en/the-world-of-hardness-testing/hardness-know-how/theory-of-hardness-testing/knoop/knoop-testing-procedure/