The calculation of how a resistor reduces Voltage (pressure), is very important in electronics. The value of the correct Resistor for LEDs in Series is the Supply Voltage (pressure) minus the total pressure consumed by all off the LEDs (Voltage Drop Across a Single LED times by the total Number of LEDs), that answer is then divided by the LED Current (electron flow) required by the circuit.

R = ( VS - ( VL x n )) / I

Calculated Limiting Resistor Ohms

Resistors do not come in every value of ohms, the answer you get from the formula may not be available in real life. You will need to select the next nearest higher value resistor based on the number calculated above (Real Resistor Values).

Nearest Higher Rated Limiting Resistor

Supply Voltage ( VS )
This is the Voltage of the power supply. Volts

Voltage Drop Across a Single LED - Load ( VL )
For most low power LEDs this is 1.7 Volts. Volts

Desired LED Current ( I )
For most low power LEDs this is 20mA. Amps
(Quantities and Units)

Number of LEDs ( n )
How many leds connected?