Need to know what model calculator you have...
Below are examples for my TI-84 Plus C Silver and my Casio ƒx-115ES Plus
The formula is...
r² = x² + y²
tanΘ = y/x
On my TI-84 Plus C Silver...
I press 2nd, Angle (the Apps button)
Choose option 5, R►Pr(
This gives you the "r" value
---OR---
Choose option 6, R►PΘ(
This gives you the "Θ" value
Using options 7 and 8, it changes from Polar to Rectangular.
On my Casio ƒx-115ES Plus...
I press Mode, 2 for complex
Enter the number in rectangular form
Press ENG (i) button
Press 2, then 3, ►r∠Θ
HOPE THIS HELPS!