Spellslinger’s Shoes lists 20 magic penetration and 8% magic penetration. Those are two different stats. They do not combine into 28 flat penetration or 28% penetration, and neither number tells you the final percentage increase to your damage.
Magic penetration in LoL lets your magic damage ignore part of a target’s magic resistance, or MR. Percentage penetration applies before flat penetration. Against 150 MR, the current Spellslinger’s Shoes calculation is 150 × 0.92 - 20 = 118 effective MR. The target keeps its actual MR; your damage uses the lower value. Riot confirmed the boots’ current 20 + 8% stats in patch 26.16.
The item and resistance rules below concern standard PC Summoner’s Rift.
Turn effective MR into damage
"Effective MR" means the resistance your particular hit is calculated against after the relevant reductions and penetration. At zero or positive effective MR, the magic-damage formula is:
Damage after MR = raw magic damage × 100 / (100 + effective MR).
Raw damage is the amount before resistance. A spell dealing 400 raw magic damage against 100 effective MR deals 200 after MR. Against 50 effective MR, it deals about 266.67. The examples here isolate resistance: no shields, damage amplifiers or other modifiers, and displayed decimals are rounded after calculation.
Return to the boots example. The 400-damage spell against 150 MR would normally deal 160. With Spellslinger’s Shoes and no other penetration or reduction, 118 effective MR gives 400 × 100 / 218 = 183.49 damage.
Flat penetration is also not a same-sized percentage damage bonus. Against 60 MR, that spell deals 250. Add 20 flat penetration, and 40 effective MR gives 285.71 damage. The increase is 14.29%, not 20%. The penetration rules change the resistance in the denominator, rather than adding a percentage to the spell.
Flat or percentage penetration? Compare the target
Flat penetration ignores a fixed number of MR points. Percentage penetration ignores a share of the MR entering that step of the calculation. A percentage therefore removes more points against a higher-resistance target.
For this comparison, keep the spell at 400 raw magic damage and change only the penetration. These are isolated stat packages, not two complete item builds.
| Target MR | Damage with 20 flat penetration | Damage with 40% penetration |
|---|---|---|
| 30 | 363.64 | 338.98 |
| 60 | 285.71 | 294.12 |
| 120 | 200.00 | 232.56 |
| 240 | 125.00 | 163.93 |
The two packages ignore the same amount at 50 MR, because 20 / 0.40 = 50. Below that point, the flat package does more work; above it, the percentage package does more. This comparison assumes no other penetration or reduction.
Do not turn 50 MR into an automatic Void Staff purchase. The table compares penetration alone. In the shop, you also pay for AP, haste, movement speed and other effects, so a full-item comparison can give a different answer.
Do not wait for a conspicuous MR item before considering percentage penetration, either. Champions have base MR and gain more through levels. Check the resistance of the enemy you need to damage, including relevant temporary defenses, rather than counting only their defensive purchases.
Sorcerer’s Shoes and the current mid-lane upgrade
Current Sorcerer’s Shoes costs 1,100 gold and supplies 12 flat magic penetration plus 45 movement speed. It has no percentage penetration. Do not accidentally calculate ordinary Sorcerer’s Shoes using the larger numbers on its upgrade.
Boots can also help you reach spell range or leave it after a trade. That benefit is absent from a stationary damage calculation. Compare the movement speed with the AP component you could buy instead, especially when walking close enough to cast is already costing you health.
Spellslinger’s Shoes supplies 20 flat penetration, 8% penetration and 45 movement speed. It is the Sorcerer’s Shoes upgrade granted through the mid-lane role quest, not a normal upgrade every role can buy for an extra fee. Under Riot’s current quest system, completing the quest upgrades your tier-two boots; if you buy them afterward, they upgrade on purchase.
The upgrade itself is free. You still need to buy Sorcerer’s Shoes. Old instructions involving Feats of Strength, two legendary items or a paid upgrade do not describe this current route.
Combining Spellslinger’s Shoes with Void Staff
Different percentage-penetration sources combine multiplicatively, not by adding their percentages. With 40% from Void Staff and 8% from the boots, the share of MR left is:
(1 - 0.40) × (1 - 0.08) = 0.552
That is 44.8% total percentage penetration, not 48%. Apply the boots’ 20 flat penetration afterward. Against 100 MR with no reduction, the hit uses 100 × 0.552 - 20 = 35.2 effective MR. A fixed 400 raw magic damage then becomes 295.86 after MR.
The stacking and operation-order rules explain the result. You do not subtract 20 first, and you do not treat the boots as another 20 percentage points.
Void Staff and Cryptbloom are alternatives
Current Void Staff costs 3,000 gold for 95 AP and 40% magic penetration. When you can land your damage but MR is reducing too much of it, these are useful stats. The percentage applies to the magic damage you deal; it is not a debuff that lowers the target’s MR for your teammates.
At an objective, the enemy tank may be the only champion your spells can reach. Use that tank’s MR for the purchase decision. A flank changes the calculation if it gives you a credible shot at a lower-MR carry. Do not price a backline burst combo you cannot deliver, or ignore the frontline you will spend the fight hitting.
Cryptbloom also costs 3,000 gold, with 75 AP, 30% magic penetration and 20 ability haste. Its healing nova can trigger when an enemy champion you damaged within the previous three seconds dies. The current passive has a 60-second cooldown and cannot trigger while you are dead. These values and rules come from the normal item records and their English tooltip bindings.
Void Staff gives 20 more AP and 10 more percentage points of penetration; Cryptbloom gives haste and conditional team healing. Cryptbloom’s passive does not rescue a fight in which nobody dies before you do. Its haste may still be relevant, but do not budget a guaranteed heal into every engage.
You cannot buy both items together. They belong to the same one-item purchase group, along with Bloodletter’s Curse. The shop restriction still applies even though the formula can combine independent percentages.
Shadowflame has two separate damage effects
Current Shadowflame costs 3,200 gold and gives 110 AP and 15 flat magic penetration. Its Cinderbloom passive separately increases magic and true damage against enemies below 40% health by 20%.
The penetration is not restricted to low-health targets; the separate damage amplification is. That distinction matters when an enemy begins a trade healthy. Do not calculate the entire combo as if Cinderbloom were already active, and do not credit its true-damage amplification to magic penetration.
Flat penetration from different sources adds together. Spellslinger’s Shoes plus Shadowflame supplies 35 flat penetration, alongside the boots’ separate 8%. The damage you gain still depends on the target’s MR and on any other effects already applied.
Reduction changes the target; penetration changes your hit
Magic resistance reduction lowers the target’s actual MR. That can benefit allies dealing magic damage while the reduction is active. Magic penetration instead lets its owner ignore resistance for their own damage calculation. Buying Void Staff does not give your allied mage 40% penetration.
Current Malignance illustrates flat reduction: its ultimate-triggered Hatefog reduces the affected champion’s MR by 10. Bloodletter’s Curse illustrates percentage reduction: qualifying magic damage from abilities or passives applies stacks that reduce MR. Their current normal item records are distinct from the penetration stats on Void Staff. These reductions require their effects to apply; owning the item is not enough to give every spell the benefit in advance.
The full calculation order is:
- Flat MR reduction.
- Percentage MR reduction.
- Percentage magic penetration.
- Flat magic penetration.
Here is a hypothetical target with 120 MR, affected by 10 flat reduction and 30% reduction. The attacker has 40% penetration and 15 flat penetration. Assume the debuffs are already active; this is not a claim about which hit applies them.
120 - 10 = 110 MR
110 × 0.70 = 77 actual MR after reduction
77 × 0.60 = 46.2 MR after percentage penetration
46.2 - 15 = 31.2 effective MR for this attacker
The 400-damage hit becomes 400 × 100 / 131.2 = 304.88. An ally with no penetration would calculate their own magic hit against 77 MR, not 31.2.
Can magic penetration make MR negative?
No. Penetration cannot push the resistance used for your hit below zero. If the target has 10 MR and you have 20 flat penetration, the hit uses zero MR, not minus 10. Further penetration does nothing to that isolated hit once you have reached zero.
For a nonnegative MR value after reductions, the compact formula is effective MR = max(0, MR × (1 - total percentage penetration) - flat penetration). Enter 40% as 0.40. If you have several percentage sources, combine them multiplicatively first.
Flat reduction can make actual MR negative. Under the current specialist calculation rules, percentage reduction and both penetration types are ignored when the remaining MR is zero or negative.
For negative MR, use the other branch of the resistance formula:
Damage after MR = raw magic damage × (2 – 100 / (100 – MR)).
At minus 20 MR, a 400-damage hit deals 466.67. Adding more magic penetration does not increase that result. Do not put minus 20 into the positive-MR formula, which would incorrectly give 500.
Reaching zero effective MR also does not convert magic damage into true damage. It only removes MR’s reduction from that calculation. Magic penetration does not delete an enemy shield or guarantee a kill.
Check the order before opening the answer
A target has 80 MR. You have 40% magic penetration and 15 flat penetration, with no reduction effects. Does your hit use 33 MR or 39 MR?
It uses 33 MR: 80 × 0.60 - 15 = 33. Subtracting 15 before multiplying would produce 39, but that reverses the two penetration steps.
More AP can help damage that penetration cannot
AP changes raw damage when an ability has an AP ratio. Magic penetration helps the part of your output that deals magic damage. Read both the ratio and the damage type before choosing which stat to add.
Ahri’s Orb of Deception deals magic damage outward and true damage on its return. Magic penetration helps the outward hit, not the return’s true damage. Ezreal’s Mystic Shot has an AP ratio in its current normal calculation, but the spell deals physical damage. That AP contribution does not become magic damage just because AP increased it.
Even for an entirely magical spell, you must recalculate the raw hit when comparing AP purchases. Suppose an ability has 200 base damage plus an 80% AP ratio. At 250 AP, it deals 400 raw damage. Against 60 MR, that becomes 250 after MR.
Adding 50 AP raises the raw hit to 440 and the result to 275. Keeping the original AP but adding 20 flat penetration instead gives 285.71. The penetration package wins this isolated comparison, but these two hypothetical packages need not cost the same gold. A higher AP ratio, a different target or another spell in the combo can change the answer.
On your next purchase, start with the enemy you can realistically damage and the magic portion of your kit. Check their MR during that exchange and compare the complete items you can afford. If you can only buy components, use their current stats in your calculation instead of the finished item’s numbers. If you keep dying before your second cast, consider what would let you survive to use it. If you never reach spell range, more penetration will not make those missed opportunities deal damage.