Nominal size of the unified threads

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The nominal sizes smaller than 1/4" of Unified threads are defined by a No. such as No.10-24 UNC but how can I calculate the basic major diameter of the No. sizes?
I understand the calculation for the fractional sizes, for example the basic major diameter of "1/4 UNC" is "25.4 mm ÷ 4", which is "6.35 mm". Please tell me how to calculate for a No. sizes.

The formula you used to calculate fractional Unified threads is correct.
For No. sizes this is a bit more complicated, but there is a formula to calculate the basic major diameter for No. sizes smaller than 1/4". The formula is as follows. It is easier and less likely to make a mistake if you look at the chart below.
It is good idea to remember this information as a reference.

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