> If a switch contact is rated 0.5A @ 125VAC, what is the correct way to
> calculate its rating @ 5VDC?

Can't help with a specific calculation (but I'm sure it has a lot to do
with the resistance of the contacts and joules / energy)

Looking through the C&K and MEC catalogues, a typical example
for a small toggle switch is 0.25A 125VAC / 9W AC / 6W DC which
I infer from that  you de-rate by 1/3rd from AC W to DC W

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