As KIIV mentioned you need to do the math:
Use this online voltage divider calculator to calculate your maximum input.
- Keep in mind you shouldn't exceed Arduino 5V.
- Use a multimeter to measure the resistors actual resistance.
- Do the math for your example assuming R1 is exactly 100K and R2 10K you have to multiple the voltage by 11.
- remember any error is going to multiple by 11 too, so make sure the first reading is accurate/stable.
ADS1115 is actually 15bit which will give you resolution of 32768, with default PGA setting you get:
6.144 / 32768 = 0.0001875V per bit.
Voltage = (your ADC reading) * 0.0001875V;
Voltage = Voltage * 11;
Take a look at This question/answer to see how I did reduced Arduino ADC noise, some steps can be done for ADS1115 as well.