if you have a king duvet cover, then order a king duvet. I find that bedding from well known stores (Ikea, Target, West Elm, etc) stay pretty standard with their sizing. so if your are sticking with major stores, you should be fine. (note: king and california king duvets and covers often tend to be lumped together as the same size.)

One time I ordered a super cheap duvet cover online, made by some unkown brand. it was several inches too big. I had to sew it to make it fit. so lesson learned: stick with the known brands.

Thank you

Just looked at Bed Bath Beyond dot com and they have several cal/king inserts.

Don't see any sizes mentioned, so buy one and see how it fits into the 3 of them.

Should fit into the Cal/king 92 x 108. If it's a little big for the other 2, just have them

taken in a little to fit your covers. Shouldn't be a big deal to lop off a few inches

each side - as needed.

I did days of research on inserts for our King duvet. Decided on Southern Living Year Round Warmth down insert from Dillard’s. 96x110. Fits Boll and Branch King duvet cover perfectly. 10/10

Thank you

I recently read that using the next size up to stuff a duvet cover greatly reduces/eliminates the problem of the duvet shifting around or not staying tied to the corners. Haven’t tried it yet, but I am planning to when it is time for new. However, I imagine that is tough with a California King.