What is the equation to convert millibars to inches of Mercury, please?

There are a lot of on-line unit conversion tools available for
problems like this. You can find them with a Google search:

://www.google.com/search?q=unit+conversion

One that I like to use is

http://www.onlineconversion.com/

which can convert just about anything to anything and seems to do it
more accurately than some others (if that matters to you).

For your particular question, using onlineconversion.com, you select
conversions for pressure and then select "bar" to "inch mecury (60F)"
to get:

1 bar = 29.613397 inch mercury (60 F)

Therefore 1 millibar = .029613397 inch mercury (60 F)

Note that there is a separate conversion factor given for inch mercury
(32 F). Mercury expands significantly with temperature, and there are
apparently two standard temperatures that are used to measure pressure
in inches of mercury.

I presume you are converting barometric pressure reports, since these
are two common units used in different places to report such data.
Have fun with your data!

Hi!
It is simple and straightforward, like this:

millibar x 0.029531 = inches of Mercury.

millibar x 0.7501 = millimter of Mercury.

http://www.campbellsci.ca/CampbellScientific/Pressure.html