Electrical Engineering

Diode Calculator

Calculate diode series resistor, forward current, diode power, resistor power, E24 resistor value, or Shockley diode current and voltage.

Formula R = (V_s − V_F) / IReviewed Sep 7, 2026

This diode calculator covers the two jobs people usually mean by a diode calculation: practical series-resistor sizing for a real supply and advanced PN-junction current-voltage estimates with the Shockley diode equation. Use the simple modes for silicon, Schottky, germanium, rectifier, protection, and indicator-diode circuits; use the Shockley modes when you know saturation current, ideality factor, temperature, and diode voltage/current.

Calculation Bench

Mode

01Supply Voltage

V_s · DC source feeding one diode-resistor branch

02Forward Voltage

V_F · Hint: silicon ~0.7 V, Schottky ~0.2-0.4 V, germanium ~0.3 V

03Current

I · Target or solved forward current

Solution

Enter diode circuit values to size a resistor, check current/power, or run the Shockley equation.

R = (V_s − V_F) / I

Forward Voltage Starting Points

Silicon signal/rectifier: ~0.7 VSchottky: ~0.2-0.4 VGermanium: ~0.3 VLEDs: use color/current datasheet

Formula Sheet

R = \dfrac{V_s - V_F}{I}

I = \dfrac{V_s - V_F}{R}

P_D = V_F I,\quad P_R = (V_s - V_F)I

I_D = I_S\left(e^{\frac{V_D}{nV_T}} - 1\right)

V_D = nV_T\ln\left(\dfrac{I_D}{I_S}+1\right)

V_sSupply Voltage
V_FForward Voltage
RSeries Resistance
ICurrent
I_SSaturation Current
V_DDiode Voltage
nIdeality Factor
TTemperature
V_TThermal Voltage

Variables & Units

Symbol	Variable	Description	Common Units
V_s	Supply Voltage	DC source voltage feeding the diode branch.	V
V_F	Forward Voltage	Approximate diode forward voltage at the intended current.	V, mV
R	Series Resistance	Current-limiting resistor in series with the diode.	Ω, kΩ
I	Current	Forward current through the diode branch.	mA, A
I_S	Saturation Current	Shockley reverse saturation current parameter from model data.	A
V_D	Diode Voltage	Voltage across the diode junction in the Shockley model.	V
n	Ideality Factor	Dimensionless model factor, commonly between 1 and 2.
T	Temperature	Junction temperature used to calculate thermal voltage.	°C, K
V_T	Thermal Voltage	Thermal voltage kT/q, about 25.7 mV at 25 °C.	mV

How to Use This Calculator

01Choose Required Resistor when you know the supply voltage, diode forward voltage, and target forward current.
02Choose Check Current when you already have a resistor and want current, diode power, and resistor power.
03Choose Shockley Current to estimate diode current from diode voltage, saturation current, ideality factor, and temperature.
04Choose Shockley Voltage to estimate the diode voltage needed for a target current under the same model assumptions.
05Use the forward-voltage hints as a starting point only: silicon is often about 0.7 V, Schottky about 0.2-0.4 V, germanium about 0.3 V, and LEDs vary heavily by color and current.

How the Formula Works

In a simple DC series circuit, the resistor drops whatever voltage is left after the diode's forward drop: V_R = V_s - V_F. The required resistor is R = V_R / I. If the resistor is already known, current is I = V_R / R.

Power is computed separately because it drives real component choice. Diode power is P_D = V_F × I, while resistor power is P_R = I²R or equivalently V_R × I. The UI recommends checking resistor wattage rather than treating resistance alone as enough.

For resistor sizing, the calculator also rounds up to the next E24 value and reports the actual current with that higher standard resistor. Rounding up usually lowers current slightly, which is safer than rounding down for a current-limited diode branch.

The Shockley equation models an idealized PN junction: I_D = I_S(e^(V_D/(nV_T)) - 1), where V_T = kT/q. It is useful for learning and semiconductor modeling, but it does not replace datasheet curves for power diodes, LEDs, Zeners, or thermal design.

Worked Example 01

5 V supply, silicon diode, 20 mA target

Known

Supply Voltage: 5 V
Forward Voltage: 0.7 V
Current: 20 mA

Formula

R = (V_s − V_F) / I

Substitution

R = (5 − 0.7) / 0.02 = 215 Ω

Result

Use 220 Ω E24; current is about 19.5 mA

The exact resistor is 215 Ω. Rounding up to the nearest E24 value gives 220 Ω, which slightly lowers current and keeps the resistor dissipation near 84 mW.

Worked Example 02

12 V supply, Schottky diode, 1 kΩ resistor

Known

Supply Voltage: 12 V
Forward Voltage: 0.3 V
Series Resistance: 1 kΩ

Formula

I = (V_s − V_F) / R

Substitution

I = (12 − 0.3) / 1000 = 0.0117 A

Result

11.7 mA, resistor power about 137 mW

A quarter-watt resistor has margin in this example, but the exact rating should still account for ambient temperature and part derating.

Worked Example 03

Shockley current at 25 °C

Known

Saturation Current: 1e-9 A
Diode Voltage: 0.6 V
Ideality Factor: 1.8
Temperature: 25 °C

Formula

I_D = I_S(e^(V_D/(nV_T)) − 1)

Substitution

V_T ≈ 25.69 mV, I_D = 1e-9 × (e^(0.6/(1.8×0.02569)) − 1)

Result

I_D ≈ 0.431 mA

This is a model result, not a guaranteed real diode current. Datasheet curves and heating matter for real component selection.

Applications

01Sizing a resistor for a silicon, Schottky, germanium, or indicator diode branch
02Checking diode current and component power for an existing resistor value
03Comparing fixed-forward-voltage estimates with Shockley-model diode behavior
04Teaching diode I-V behavior, thermal voltage, saturation current, and ideality factor

Assumptions

01Simple resistor modes assume one diode and one resistor in a DC series branch.
02Forward voltage is treated as a fixed estimate in practical modes, so use the diode datasheet at the intended current when accuracy matters.
03The nearest standard resistor output rounds up to E24 to avoid increasing current above the target.
04Shockley modes assume an ideal PN-junction model with constant saturation current and ideality factor at the entered junction temperature.

Where This Model Stops

01Does not model Zener breakdown, reverse leakage limits, surge current, reverse recovery, capacitance, or thermal runaway.
02Does not replace an LED driver calculation for high-power LEDs; use a constant-current driver when the LED power/current requires it.
03Shockley results can differ greatly from real datasheet curves because real diodes include series resistance, heating, manufacturing spread, and package limits.

References

[1]
Diode Calculator (Resistor, Current & Power)
Power4All
Competitive reference for resistor sizing, forward current, diode power, resistor power, and typical forward-voltage hints.
[2]
Ideal Diode Current and Voltage Calculator
All About Circuits
Reference for the Shockley diode equation variables, thermal voltage, ideality factor, and temperature-dependent PN-junction model.
[3]
IEC 60063 Preferred Numbers
International Electrotechnical Commission
Basis for preferred resistor-value series such as E24 used for practical component selection.

Frequently Asked Questions

How is this different from the LED Resistor Calculator?

The LED Resistor Calculator is optimized for LED strings, LED count, driver drop, and LED-specific guidance. This Diode Calculator is broader: it handles general silicon, Schottky, germanium, rectifier, and signal-diode resistor checks, plus Shockley equation current/voltage modes.

What forward voltage should I enter?

Use the datasheet value at your intended current whenever possible. As rough starting points, silicon diodes are often about 0.7 V, Schottky diodes about 0.2-0.4 V, germanium diodes about 0.3 V, and LEDs vary from roughly 1.8 V to 3.6 V depending on color and current.

Why round the resistor up instead of to the nearest lower value?

For current limiting, rounding up usually makes the current slightly lower than the target, which is safer for the diode and resistor. Rounding down can push current and power above the intended design point.

Is the Shockley equation accurate for any diode?

It is useful as a model and teaching equation, but real devices deviate because of series resistance, temperature rise, leakage, manufacturing spread, package limits, and reverse-breakdown behavior. Use datasheet curves for final design.

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