Zener Regulator Resistor Calculator
A Zener diode holds a fairly constant voltage across its terminals once it is biased into reverse breakdown, which makes it a simple voltage reference or shunt regulator. To use it you place a series resistor between the supply and the diode, and that resistor sets how much current flows. This calculator sizes that series resistor from three numbers: the input supply voltage, the Zener voltage you want to hold, and the current you want flowing through the circuit. The resistor value equals the voltage dropped across it, which is the input voltage minus the Zener voltage, divided by the current. Choosing the current is a design trade-off: it must be enough to keep the diode in breakdown and supply the load, but not so high that the resistor or diode overheats. The calculator gives the ideal resistance; in practice you pick the nearest standard value and check the power rating of both the resistor and the diode. Engineers, hobbyists and students use this every time they build a basic reference or protect a logic input. Every figure here is computed deterministically from the formula below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
The series resistor equals the voltage across it over the current: R = (Vin - Vz) / I. With Vin = 12 V, Vz = 5.1 V and I = 0.02 A, the resistor is 345 ohms.
Zener Regulator Resistor formula
R = (Vin - Vz) / I
R = series resistor in ohms
Vin = input supply voltage
Vz = Zener (regulated) voltage
I = series current in amperes
The resistor drops the difference between the supply and the Zener voltage. Dividing that drop by the chosen current gives the resistance. Always check the resistor power rating, which is the drop times the current.
Worked example
Size the series resistor for a 5.1 volt Zener reference from a 12 volt supply at 20 milliamperes.
- Voltage across resistor = 12 - 5.1 = 6.9 volts
- Convert current: 20 mA = 0.02 amperes
- R = 6.9 / 0.02 = 345 ohms
The series resistor is 345 ohms. These are the calculator's default inputs, so the result above matches the widget exactly.
Resistor value at common currents (Vin 12 V, Vz 5.1 V)
R = (12 - 5.1) / I = 6.9 / I.
| Current (mA) | Resistor (ohms) |
|---|---|
| 5 | 1,380.00 |
| 10 | 690.00 |
| 20 | 345.00 |
| 50 | 138.00 |
| 100 | 69.00 |
Electrical units and reference data: US National Institute of Standards and Technology (NIST).
Zener Regulator Resistor Calculator: frequently asked questions
What does the series resistor do in a Zener regulator?
It limits the current flowing into the Zener diode and the load. Without it the diode would draw unlimited current and fail. The resistor drops the difference between the supply voltage and the Zener voltage, and its value sets how much current flows for a given supply.
How much current should I choose?
Enough to keep the diode firmly in reverse breakdown plus whatever the load draws, but low enough that nothing overheats. A few milliamperes above the load current is common for a light reference. The right figure depends on the diode datasheet and the load, so the calculator leaves the current as an editable input.
What power rating does the resistor need?
The resistor dissipates the voltage across it times the current through it. For 6.9 volts at 20 milliamperes that is about 0.14 watts, so a quarter-watt resistor is fine with margin. Always rate the resistor at least double the calculated dissipation.
Why pick a standard resistor value?
Resistors are sold in standard series such as E12 and E24, so the exact calculated value rarely exists off the shelf. Pick the nearest standard value, usually rounding up slightly to keep current within limits, then verify the resulting current and dissipation.
What is the formula?
The series resistor equals the input voltage minus the Zener voltage, divided by the current: R = (Vin - Vz) / I. For 12 volts in, a 5.1 volt Zener and 0.02 amperes, that is 6.9 divided by 0.02, which is 345 ohms.
Official sources
- Measurement units, physical constants and computational reference data: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.