How does Storm work in Magic?

When you cast a spell with Storm, you copy it once for each spell cast before it this turn. Your storm count is that number of prior spells. The original spell still resolves, so a storm count of 5 means the original plus 5 copies, six instances total.

Do lands, abilities, or triggers add to storm count?

No. Storm counts only spells cast this turn — anything that goes on the stack as a spell. Activated and triggered abilities, mana abilities, and playing a land do not increase your storm count, which is a common scoring mistake.

What storm count do I need to win?

It depends on the payoff. Grapeshot deals damage equal to one plus the copies, so beating a 20-life opponent needs a storm count of roughly 20 (one original plus 19 copies, dealing 20). Tendrils of Agony drains 2 per instance, so a storm count of 9 plus the original drains 20. The tool shows copies; compare against your payoff's per-copy effect.

How is the going-off probability estimated?

Casting a chain of cheap spells is modelled as a simple build-up: each spell in hand reliably advances the count unless it fizzles. The estimate uses your hand size and an average successful-chain length to approximate the chance of reaching your target. It is a rough planning aid, not an exact deck simulation.

Does the copy choose new targets?

Yes. Each copy can choose new targets independently when it is put on the stack, which is what lets a single Grapeshot finish multiple creatures or a Tendrils drain repeatedly. The copy count itself is fixed at the storm count regardless of targeting.

What is the MTG Storm Count Calculator?

Count the spells cast this turn to find your storm count and how many copies a Storm spell produces. Also estimates the probability of reaching a target storm count from a hand of cheap spells. For Magic: The Gathering combo players. It runs free in your browser on Gera Tools, with nothing uploaded.

MTG Storm Count Calculator

Name: MTG Storm Count Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Storm is one of Magic’s most explosive mechanics: cast a Storm spell and it copies itself once for every spell you have already cast this turn. This calculator tracks your storm count, shows exactly how many copies you will get, and estimates the odds of chaining enough cheap spells to “go off”.

How it works

Storm copies equal the number of spells cast before the Storm spell this turn. The original still resolves, so:

storm count = spells cast this turn (before the Storm spell)
copies       = storm count
instances    = copies + 1   (the original plus its copies)

For planning, the going-off estimate models a chain of cheap spells. With n spells available and an average successful advance, the chance of reaching a target count is approximated by treating each spell as an independent success at the per-spell rate implied by your average chain length:

P(reach target) ≈ (avg_chain / hand_size) ^ max(0, target − already_cast)

This is a rough planning aid, not a full deck simulation.

Worked examples by Storm payoff

Grapeshot deals 1 damage to a target for the original plus each copy. To kill a 20-life opponent, you need 20 instances, which means a storm count of exactly 19 (19 copies + the original = 20). If they have taken 2 damage earlier, a storm count of 17 is enough.

Tendrils of Agony drains 2 life per instance. Ten total instances (storm count 9) drains 20 life and is the common kill threshold. Each pair of instances gains you 4 life and costs them 4 — sequencing matters when racing a clock.

Mind’s Desire exiles cards equal to the storm count and casts them for free. A storm count of 6 lets you exile and cast 6 extra cards, which often generates even more spells and snowballs the count further.

Sequencing strategy

Getting to a high storm count is mostly about spell ordering:

Cantrips first. Cast Ponder, Preordain, and Gitaxian Probe before rituals to dig deeper and build the count cheaply.
Mana rituals after cantrips. Once you have enough cards to keep going, Dark Ritual and Desperate Ritual add count while generating mana.
Float mana before each payoff check. Confirm you can cast the payoff before tapping out; a counterspell on Grapeshot at storm count 0 ends the turn.
Only spells count. Mana abilities (tapping a land, Mox Opal’s ability), triggered abilities, and playing a land do not advance storm count. This is the most common miscount at a tournament table.

Reading the probability estimate

The going-off probability is a rough planning tool. It estimates the chance that the spells in your hand chain far enough to reach your target, given how reliably each one advances the count. A hand with five cheap cantrips pointing at a target of 8 has a high probability. The same hand aiming at 15 without mana rituals is much more uncertain. Use it to evaluate opening hands during playtesting rather than to make in-game decisions — a real game involves shuffling, opponent interaction, and cards drawn mid-chain that this simplified model does not capture.