Balance Scale

Tells you exactly how heavy a tool, weapon or piece of armour is. The tooltip already says head-heavy or heavy for free; the scale says how many.

Crafting

  R
I   I
I I I

R is a Stick and I is an Iron Ingot.

Using it

Carry it anywhere in your inventory. Every modular item's tooltip gains a line like "Balance scale: 3.4 Mu". It reads to a tenth.

Mass matters because a heavier head hits harder and swings slower, and because a head that's too heavy for its handle works the joint loose.