How to calculate current into GPIO pin after voltage division
It is wrong. 







simulate this circuit &ndash; Schematic created using CircuitLab

As I understand the GPIO is in the Input mode (or analog mode). (Output mode when connected to the circuit which actively drive the pin can damage the pin - so be careful). In this mode pin has a very high impedance - lets say much more than 10MOhm (R1). R1 &amp; R2 are in parallel and its resistance is 32891.458hms. The current sourced from the 5V is 5V / (18k + 32891.458) is 98.24uA. The voltage drop across the R1 &amp; R2 is 98.1uA * 32891.458Ohms = 3.231V So the current which flows through the Raspberry PI pin (R1) is 3.231V / 10MOhm = 0.32uA

In the low power uC this current is much lower as the PINs input impedance is much higher than 10MOhm