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.

MTG Storm Count Calculator

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.

Example and tips

If you have cast 5 spells this turn and then cast Grapeshot, your storm count is 5, giving 5 copies plus the original — six instances dealing 6 damage total. To beat a 20-life opponent with Grapeshot you need a storm count near 19. Count carefully: only spells add to storm, so rituals and cantrips help while mana abilities and lands do not. Sequence your cheapest cantrips first to build the chain before committing your payoff.