Measure the power consumption of a circuit with Arduino
You'll get very poor results using this methodology.


Since you are sensing current in the low side the signal you create will subtract from the sensor/MCU Vdd.
The signal you develop across the sense resistor is in series with any signals between the sensors and the MCU ...for example the I2C interface.
The Vdd for the sensors and MCU will vary because of the signal developed across your sense resistor so measuring the battery voltage is inaccurate.


I'd suggest if you're using a separate MCU for the power measurements that you use an INA219 or its three channel variant the INA3221 to measure the currents using a high side sense resistor. 

There are breakout boards readily available on Ebay/Adafruit, and you can easily modify the sense resistor(s) to measure much lower currents.

 

If all you want is an estimation of the battery lifetime, then you don't need to measure each individual portion of your project.
You could use a single high side sense resistor and an op-amp or better still an INA169 to feed you ADC.
You could even use a sensitive multimeter and a large filter cap on the Vdd to measure the current (and use a power supply instead of the battery so you can increase the voltage). This would allow you to make an estimation. 

If all you have is an Arduino Nano and some resistors (from your comment), then the only way to get any worthwhile result is to program the ADC to use the Internal 1.1V reference. This at least will give you a better range on the ADC.
Refer to the Nano schematic and the ATMega328p datasheet, section 28.