(This was posted by /u/AtomicSlumber on a site I refuse to link to; copying here since the info is really good.)

How fuse durability actually works

After hours of extensive testing and research, I think I finally understand how weapon durability in TOTK actually works.

We all know how fuse damage works, it’s simple addition. However, durability is a different matter.

For all weapons, fusing for the very first time will grant “bonus” durability. Subsequent fusions will not do anything to increase or decrease this bonus. The bonus durability is used before the weapon’s durability.

If you remove the fuse (or it breaks off on its own), the bonus will not be active anymore and the base weapon’s durability will be depleted instead.

However, if you re-fuse on that weapon again, then the bonus will continue where it left off (if there is any left). So long as the weapon is fused, the bonus will be active.

By default, fused weapons get +25 bonus durability. (For reference, 1 durability = 1 hit)

There are exceptions to this rule; like Tree Branches only receiving +10 instead of +25 bonus durability.

Here is a list of all the exceptions (along with their base durability).

+10:

Tree Branch (4)
Torch (8)
Soup Ladle (5)
Boat Oar (8)
Farming Hoe (6)
Farmers Pitchfork (12)
Wooden Mop (8)
Fishing Harpoon (12)
Forest Dweller’s Sword
Forest Dweller’s Spear (35*)
Rusty Broadsword (6)
Rusty Claymore (8)
Rusty Halberd (12)
Royal Guard’s Sword (10, 12*)
Royal Guard’s Claymore (11, 12*)
Royal Guard’s Spear (14, 15*)
Gloom Sword (15)
Gloom Club (14)
Gloom Spear (16)

+5:

Gerudo Scimitar (10, 14*)
Gerudo Claymore (10, 14*)
Gerudo Spear (15, 20*)
Scimitar of the Seven (60)

+3:

Bokoblin Arm (5)
Lizalfos Arm (5)
Moblin Arm (5)

Degraded weapons (without the shiny icon ✨*) affect its damage and durability, but not bonus durability. They have the same bonus durability as their nondegraded counterpart.

(Surprisingly, only the shiny Forest Dweller’s Spear is +10, while the degraded version (24) is +25. Also both Forest Dweller’s Swords (17, 27*) are also actually still +25)

There are two types of fusions. Fusions with materials and fusions with other weapons.

Fusion between a weapon and a weapon will still deplete the durabilities of both weapons, but only activate the bonus durability of the base weapon.

This means that it is possible for the end of the fused weapon to break off naturally, and leave you with the base weapon. But if the base weapon breaks before the end weapon, then both weapons will be broken, regardless of how “fresh” the end weapon is.

However, fusions with materials will never break off by default. The only exceptions to this are materials that are destroyed immediately after 1 hit.

Material Exceptions:

Brightbloom Seed
Giant Brightbloom Seed
Bomb Flower
Puff Shroom
Muddle Bud
Fire Fruit
Ice Fruit
Shock Fruit
Splash Fruit
Dazzle Fruit
Chuchu Jelly
Red Chuchu Jelly
Yellow Chuchu Jelly
White Chuchu Jelly
Gibdo Bone
King’s Scale
Bird Egg
Fresh Milk
Goron Spice
Monster Extract
Oil Jar
Ancient Blade*

*Interestingly with the Ancient Blade, it won’t break off until it actually hits an enemy (usually one-shotting them along with their loot)

Every successful hit reduces the weapon’s durability by 1, whether you hit an enemy, a blocked shielded hit, or the environment. It is possible to hit multiple things in one swing. A common example is jump attacking an enemy; reducing 1 for hitting the enemy, and reducing 1 for hitting the ground.

However, not everything you hit will reduce your durability. Sometimes, depending on what is being done, the weapon won’t lose durability.

Some actions that don’t reduce durability:

Hits that are non-reactive or have a small shine instead of the usual hit effect like…
- Hitting Zonai devices or shrine switches
- Hitting a tree or rock with a weapon that can’t damage it (Sticks, Bones, Spears, etc…)
- Hitting Villagers or Koroks :C
Hitting walls without collision (its like hitting air)
Cutting grass, small bushes, tree saplings, or other seemingly massless objects
Temporarily lighting your wooden weapon on fire (it will instantly break eventually, except torches)
Swinging a melee elemental weapon without hitting anything like…
- Windy weapons; like Rito weapons or weapons fused to a Korok-Frond or flat board
- Weapons fused to Lizalfos Horns; Dragon Horns, Fangs, Scales, Claws, and Shard Spikes
- Weapons fused to Gleeok Horns (which apparently have no elemental cooldown)
Hitting a Lynel while riding its back
Successful shield parries
Shield surfing on snow, sand, or rails (yes it’s real)

I’ve also come up with some examples below to help you better understand how all this fusion really works. (Numbers in parenthesis = base durability + bonus durability[attached weapon’s durability])

Example #1: No fusion

Tree Branch (4)
4 hits, weapon breaks

Weapon lasted 4 hits.

Example #2: Early Material Fusion (Recommended)

Wooden Stick (12)
Rock is fused to Wooden Stick (12 + 25)

37 hits, weapon breaks

Weapon lasted 37 hits.

Example #3: Late Material Fusion

Wooden Stick (12)
11 hits (1)
Rock is fused to Wooden Stick (1 + 25)
26 hits, weapon breaks

Weapon lasted 37 hits.

Example #3.5: Weak Material Fusion (+10 instead of +25)

Tree Branch (4)
Rock is fused to Tree Branch (4 + 10)
14 hits, weapon breaks

Weapon lasted 14 hits.

Example #4: Multiple Material Fusion

Wooden Stick (12)
Rock is fused to Wooden Stick (12 + 25)
8 hits (12 + 17)
Rock is removed (12)
4 hits (8)
Boulder is fused to Wooden Stick (8 + 17)
18 hits (7)
Boulder is removed (7)
Rock is fused to Wooden Stick (7)
7 hits, weapon breaks

Weapon lasted 37 hits.

Example #5: 1-Time Material Fusion

Wooden Stick (12)
Fire Fruit is fused to Wooden Stick (12 + 25[1])
1 hit, Fire Fruit breaks off (12)
12 hits, weapon breaks

Weapon lasted 13 hits.

Example #6: Continuous 1-Time Material Fusions

Wooden Stick (12)
Fire Fruit is fused to Wooden Stick (12 + 25[1])
1 hit, Fire Fruit breaks off (12)
Another Fire Fruit is fused (12 + 24[1])
1 hit, Fire Fruit breaks off (12)
Repeat steps 4 and 5; 34 times (1)
Another Fire Fruit is fused (1 + 0[1])

1 hit, weapon breaks

Weapon lasted 37 hits.

Example #7: Weapon Fusion

Wooden Stick (12)
Tree Branch (4) is fused to Wooden Stick (12 + 25[4])
4 hits, end breaks off (12)
12 hits, weapon breaks

Weapon lasted 16 hits.

Example #8: Inefficient Weapon Fusion

Tree Branch (4)
Wooden Stick (12) is fused to Tree Branch (4 + 10[12])
12 hits, end breaks off (2)
hits, weapon breaks

Weapon lasted 14 hits.

Example #9: Detrimental Weapon Fusion

Tree Branch (4)
Long Stick (18) is fused to Tree Branch (4 + 10[18])
14 hits, both weapons break

Weapon lasted 14 hits.

Example #10: Weapon Fusion Edge Case

Tree Branch (4)
Thick Stick (14) is fused to Wooden Stick (4 + 10[14])
14 hits, both weapons break

Weapon lasted 14 hits.

Example #11: Weapon “Repairing”

Wooden Stick (12)
9 hits (3)
Tree Branch (4) is fused to Wooden Stick (3 + 25[4])
4 hits, end breaks off (3)
Damaged Thick Stick (4) is fused to Wooden Stick (3 + 21[4])
4 hits, end breaks off (3)
Long Stick (18) is fused to Wooden Stick (3 + 17[18])
18 hits, end breaks off (2)
Rock is fused to Wooden Stick (2)
2 hits, weapon breaks

Weapon lasted 37 hits.

Example #12: Weapon Swapping

Wooden Stick (12)
9 hits (3)
Tree Branch (4) is fused to Wooden Stick (3 + 25[4])
4 hits, end breaks off (3)
Damaged Thick Stick (4) is fused to Wooden Stick (3 + 21[4])
4 hits, end breaks off (3)
Wooden Stick (3) is fused to Long Stick (18 + 25[3])
3 hits, end breaks off (18)
Rock is fused to Long Stick (18 + 22)
40 hits, weapon breaks
Weapons lasted 60 hits.

Hopefully this info will help you maintain your weapons’ durability more efficiently throughout your adventure.

(Also included below is a link to a very helpful datasheet of all the items in TOTK, including weapon damage, durability, and much much more)

https://docs.google.com/spreadsheets/d/18pNtDx3z-8CwGJRmlW574xbQ6VphQOkvpZhClpOEVDA

TLDR;

All fusions give a one-time “bonus” durability (usually +25) that is only active when the weapon is fused.
Fusions with “nonfragile” materials are always just as or better than fusions with other weapons.
All weapons are eventually doomed to die (except the Master Sword)

The only way to truly repair a weapon is with Rock Octorocks (Pelison’s Break-A-Part Shop doesn’t repair)

___``___