Always look for the most efficient way to level equipment. Doing so will save a lot of time, equipment and coins. I have created an image showing the most efficient way to max a silver equipment [ring]. PM me on the discord if you want more help, im almost always on there, rarely here.

I only upgrade High Tier equipment of Rarity 9+
Its expensive and takes a long time to do, so anything less than lvl 9 item is not worth the trouble.
Also I used to have all my girls equipped but with the way the game is set up, and the items are not "taken out" of your communal inventory, I had to unequip all non-active knights.

Normally in any game that has inventory you have "active" and "Passive" - passive is the one where you carry stuff, active is the one where you equip stuff. When equipping item, it gets moved from INVENTORY into EQUIPMENT (any RPG, Crafting and Survival game out there). This game is an exception (a stupid one at that) that ANY equipped item is still counted as part of the inventory, thus it still counts towards the maximum number of items you can have.
With that being said - equip only your active girls, otherwise you will soon run out of space, and with no space you cant roll for new - maybe better Rarity - equipment.

The *only* things I upgrade are the lvl 10 silver ones and the personal earrings. Why upgrade a lvl 9 when it's going to be replaced with a lvl 10 eventually? So I rather upgrade a lvl 10 necklace than a lvl 9 ring. And I have far from enough spare jewelry to upgrade all my lvl 10 jewelry, so it's not hard to "only level what I won't replace".

Upgrading a necklace with spare necklaces and upgrading rings with spare rings boosts them a bit better, but just boosting a single piece with whatever 10 spare pieces you have is more cost efficient. I tend to go for the median of that. If most of my spare pieces are rings, I use it to upgrade a ring. If most of my spare pieces are wristbands, I use them to upgrade a wristband.

I've made a a guide based on my experiences with equipment leveling and after researching different ways for each equipment. I only focused on what i thought was important (RIP earring). Take it with a grain of salt or treat it as a trusty page to follow.
And this is my current ring collection

Ok, thank you for your input everyone.

As a final question, is there a noticable XP loss for 'banking' items? If not it doesn't really matter how you upgrade stuff as you never lose XP. You can then simply sacrifice your previousy upgraded crap item whenever you get +10 items.

Is 'banking' items adding a lot of lv 1 to the same equipment?
IIRC 'banking' a bracelet requires around 19-20 bracelets from level 1
ie. LV 1 + 10* LV 1 = LV 22 [10 x lv 1 upgrade cost]
LV 22 + 9 or 10* LV 1 = LV 30 [9 or 10 x lv 22 upgrade cost]

However if you dont 'bank' and combine equipment like in the guides you can save exp and coins
Back to the bracelet example
Make 8 Lv 1 + Lv 1 = Lv 10 or 11(if you get a great synthesis success) [8 x lv 1 upgrade cost]
Lv 1 + 7* LV 10 & 1 Lv 11 = Lv 30 [8 x lv 1 upgrade cost]

As you can see the bottom method, it is cheaper in terms of upgrade cost, as all you leveling is done at level 1 and not at lv 22. Also we are only using 16 bracelets to max it as opposed to dumbly sending 10 at a time all level 1 into the desired equipment which would need around 20 bracelets.

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Make em like this

I think most of this advice is great for endgame players, but is kind of beyond a fresh level 40 player. I wouldn't worry about waiting for +10 warrior rings, they could be weeks or months away, upgrading whatever you have at the time helps a lot. Don't try to max level any of it, but throwing 10 junk items onto +4 warrior rings or defense bracelets will be a nice, and cheap, boost in power.

Originally Posted by Mystic

...
Make 8 Lv 1 + Lv 1 = Lv 10 or 11(if you get a great synthesis success) [8 x lv 1 upgrade cost]
Lv 1 + 7* LV 10 & 1 Lv 11 = Lv 30 [8 x lv 1 upgrade cost]
...

Wow thats nifty. It will also allow me to 'upgrade' my crappy stuff (though just to Lv 10) without a second thought until I don't need them anymore. Just as Astasia said, for now I still have to use the medicore stuff.

After testing it a bit, it seems that the amount of sacrificed items is just as important as their level:
- Lv 1 + 2 x Lv 20 = Lv 21 (sacrificed: 22 x Lv 1)
- Lv 1 + 1 x Lv 20 + 9 x Lv 1 = Lv 25 (sacrificed: 21 x Lv 1)

This would mean that the most effective upgrade method is to create throw-away items with such a level, that you will need exactly 10 of them to max a high end level item in one try. Looking at your example that would be level 9 / 10 items for a final level 30. Understood!

Btw. would the same principle hold for upgrading the girls level using 2* girls as an alternative to manyus? Just combine 2* girls to level 8 and then fuse them with other high level girls? Or is it still not worth the coin cost?