I'm not so scientific. I think my 6” pans are 3”. Whatever they are, whether 6” or any other size, or deep mold like bundt, or loaf, I just make the batter, and fill to what looks right. If I have leftover batter, I just bake it off separately, either as a tester, or to use in something else. If I were baking commercially, I'd perfect the recipe amounts. As it is, I can't be bothered. I do have some anti-doming devices, but they don't work well enough, IMHO, to bother with most of the time. A cake fills best when perfectly flat, or even lightly dished. It's still going to need evening up. Many people prefer fewer taller layers (less “crust” to get rid of), and just slice them to the size they want. Others just make sheet cakes and cut their layers out of those.

I just realized. My “eyeball” measure is about an inch from the top of the pan. That's what the half full and 2/3 full are both telling you.

So the best way to get equal amounts of batter in each pan is to use a scale. For less formal, I just pour some batter in one pan and let it settle while pouring some in the other, then eyeball the height back and forth until the pans are filled properly. Usually once to start, a second time to divide the batter and a third to adjust and clean out the batter bowl.

Measuring batter by volume is a good way to compare the recipe with the how much your pans will hold, but not an easy way to fill them.

If you're worried your pans will overflow, you can line the sides with parchment paper or extenders made for the purpose.

Don't fuss over precision unless there are judges involved. It's just cake. It's supposed to be fun.

What's important is getting your batter in the pans promptly, a good whack to get out air pockets, and get them right in the oven. The moisture or acid acting part of the baking powder is already at work. If you leave the batter sitting around, it'll discharge and deflate. Some recipes do that on purpose for denser cakes, only lifted by the heat action and eggs, but if that's not your goal, make sure your oven has been at temperature for some time, so that the walls, not just the air, are hot, and you get your batter into the pans, and the pans into the oven promptly.

"Don't fuss over precision unless there are judges involved. It's just cake. It's supposed to be fun."

Great advice!

My 6" round pans are only 2" deep and I usually fill them half way. Filling 6x3 pans with about 3 cups batter each should work fine since that's a little less than 2/3 the volume (if my math is right :-/), but plllog's "eyeball" measure will work. Yes, you can use the same recipes you find for three 6x2 pans to fill your two 6x3 pans.

So 6 cups of batter sounds about right for either size pan. If it's too much for the 3" deep pans, I can always make a few cupcakes.

I'm used to cakes rising to the top of the pan and photos I've seen of the cakes baked in 6" pan have a lot of interior cake pan visible.

This is from Sally's Baking Addiction using 6x2 pans. I don't want a 1" layer!

Eileen, the rise in those 6x2 pans does look a little low. I've always found that filling mine (6x2) halfway gives plenty of rise. I suppose it can also depend on the recipe you use.

It doesn't seem 3 cups of batter would be too much for the 3" deep pans, but, yes, you can make cupcakes. :-)

I baked the recipe shown in the pans in one 9x1 1/2" round pan and it domed slightly over the pan. It makes 4 1/2 cups of batter.

It was a very good cake, scaled down from a King Arthur Flour recipe.

https://sallysbakingaddiction.com/6-inch-cake-recipes/print-recipe/73029/

Eileen, those look like she was making thin layers on purpose. Sometimes, there's call to make a single layer high out of three 1” layers, with only a smasl amount of jam or cream between. I wouldn't worry too much about what Sally did. It's a good site, but the plans you made for yourself sound just fine. It's just cake. So long as you remember the baking powder, it'll be fine. ;)

Recipes using less than 2 cups of flour seems to be the basic recipe for these 6" cakes.

Here's a similar recipe that calls for 2 1/4 cups of flour instead of the 1 1/2--1 3/4 cups of other 6" recipes I've found. Which reminds me--I did sub two whole eggs for three egg whites in Sally's recipe.

https://baranbakery.com/6-inch-vanilla-cake-recipe/

In the Q&A, she advises not to make it in two 6x3 pans but she doesn't say why. I remember reading somewhere that too deep of a batter in a pan can affect the rise, but that may be for only delicate white cakes. I also read that 3" deep cakes tend to be drier because the sides cook fast compared to the center. I do have baking strips though and they work well. So that's my concern--if there's a difference in the final product.

I keep waffling on whether to buy three 6x2 pans or two 6x3 pans. I plan to decorate the cake with roses and such so I really don't want two layers of filling too.

You just answered your question. You don't want two layers of filling, so you want a two pan cake. But you could buy three 3” pans to cover all your bases. There are tricks for making taller cakes cook more evenly, but you shouldn't have any trouble with a 6”, or even 8-9”, pan. It's the big ones that need intervention. You're not trying to bake it 5” tall after all!

Two tricks some pros use might help you. To make perfect sides and top, use a long, very sharp cake knife to remove the “crust”. To get a particular size and shape, bake in a sheet pan, and cut out the shape, removing the bits you don't want first, and a cake lifter for the “real” part. The excess cake can go into cake pops, trifle, parfaits, etc.

Remember, also, if you don't want to do these cutting things, frosting covers a lot of sins..,