It's not simple -- many variables. You may need a bundle of small wires or a flat ribbon.
Try a software tool such as Intusoft's Magnetics Designer, or maybe something here: https://www.smps.us/magnetics.html
Be sure the software considers proximity effect (transverse eddy current losses). If not, it's pretty much worthless.
Or maybe dig into a book such as "Soft Ferrites" by Snelling. That's a classic old book with lots of good theory, but it's not simple to use.
This collection of papers has been recommended by many folks. I'm not sure if you can see this forum:
"Magnetic Design for SMPS", by Lloyd H. Dixon
Many of Dixon's papers are included in TI/Unitrode's design seminars, especially the 2001 Magnetics Design Handbook:
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