Minor Diameter calculation

Many years ago my father used an equation to find the minor diameter of a thread. Unfortunately I have forgotten it and my father is no
I have looked at various equations, calculation programs and tabulated information on the internet and they all seem to produce different answers.
Does anybody know an equation which from experience will produce reasonably accurate result when calculating the minor diameter of a thread?
Thanks.
John H
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John wrote:

There is no universal equation.
There is a formula for BA threads only, because they are almost unique in being a mathematically derived series.
In general, you need to know the size and thread designation, and if not a standard thread, the thread form and pitch. Thread form and pitch will enable you to find the theoretical thread depth, and that gives you the minor diameter.
--
Charles Lamont

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If the threads are very old they may be Whitworth which was also formula driven.
wrote:

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mecc_smith wrote:

The relationship of pitch to diameter in Whitworth threads was determined empirically and does not follow a formula.
--
Charles Lamont

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for metric threads taking pitch from the designation gives a pretty good approximation, at least for tapping drills. E.g. M6x1 -> tapping drill 5mm
Dave