**Radicool21's answer is only correct in the second-generation games.** If there is both an accuracy modifier and an evasion modifier in the later games, *the evasion modifier will be subtracted from the accuracy modifier, the result is capped at 6/-6 if it exceeds one of them, and the multiplier corresponding to that modified stat stage is used*. In other words, the result is exactly the same whether the accuracy is -6 and the evasion is 6, the accuracy is -6 and the evasion is 0, or the accuracy is -3 and the evasion is 3.

So if the opponent's move's accuracy is 100, the modified accuracy will be simply 100 * 1/3 = 33. 1/3 is the multiplier corresponding to an accuracy/evasion stat stage of -6/6. You can find the rest in the Bulbapedia article Radicool21 linked to.

If you want it laid out in equations, first figure out the modified accuracy stat stage a:

```
a = min(6, max(-6, n - m))
```

Then the final accuracy of the move can be calculated as follows:

```
p * max(3, 3 + a)/max(3, 3 - a)
```