What triggers a Wild Magic Surge in 5e?

By the standard rules, after a Wild Magic Sorcerer casts a sorcerer spell of 1st level or higher, the DM may have them roll a d20. On a 1 they roll on the Wild Magic table. That is a flat 5 percent chance per qualifying cast when the DM calls for the roll.

How is the cumulative probability calculated?

The chance that no surge happens in n casts is (1 minus per-cast chance) to the power n. The cumulative chance of at least one surge is one minus that. For the flat 5 percent rule, ten casts give about a 40 percent chance that a surge has occurred.

What is the escalating-DC house rule?

A popular variant starts the surge range at a 1 and widens it by one each cast without a surge (surge on 1, then 1 to 2, then 1 to 3, and so on), resetting after a surge. This tool models that growing per-cast chance and the cumulative odds across the streak.

Does Tides of Chaos affect surges?

In the standard rules, after you use Tides of Chaos the DM can have you roll for a surge after your next spell, and on a surge you regain the Tides use. The trigger chance itself is unchanged; this calculator focuses on the d20 surge probability, not the feature interaction.

Does cantrip casting count?

No. Only sorcerer spells of 1st level or higher can trigger the standard surge roll, so cantrips do not count toward the streak. Enter only your leveled spell casts since the last surge for an accurate cumulative figure.

D&D Wild Magic Surge Probability Calculator

The Wild Magic Sorcerer’s signature chaos comes from a small chance of a surge on every spell — but how likely is a surge after a string of safe casts? This calculator gives the per-cast chance under both the standard rule and the popular escalating-DC house rule, plus the cumulative probability that a surge has already struck.

How it works

Under the standard rule a surge triggers on a natural 1, a flat 5 percent per qualifying cast. The chance that no surge happens across n independent casts, and the cumulative chance of at least one, are:

P(no surge in n)  = (1 − p) ^ n
P(at least one)   = 1 − (1 − p) ^ n

For the escalating house rule, the surge range grows each cast without a surge — surge on a 1, then on 1–2, then 1–3, and so on — so the per-cast chance is k / 20 on the k-th cast of the streak, and the cumulative no-surge chance is the product of each (1 − k/20).

Example and tips

Under the flat 5 percent rule, after 10 leveled casts the cumulative chance a surge has fired is 1 − 0.95^10 ≈ 40 percent; it reaches about 50 percent around 14 casts. Under the escalating rule, the streak rarely survives long — by the time the range is 1–10 the per-cast chance is already 50 percent. Reset your streak counter to zero each time a surge actually triggers, and count only 1st-level and higher sorcerer spells.