ElektroBox Tool
Electronics Calculator for Circuit Design and Learning
Use this browser-based electronics calculator for common circuit calculations: Ohm’s law, power, voltage dividers, LED resistors, resistor networks, RC filters, reactance, op-amp gain and resistor color codes.
Ohm’s law & power
Enter any two values. The calculator solves voltage, current, resistance, and power.
How this calculation works
Ohm’s law links voltage, current, and resistance. Power is then derived from the solved values.
- V = I × R
- I = V / R
- R = V / I
- P = V × I = I²R = V²/R
Enter any two known quantities. The calculator uses the matching formula set to compute the missing values.
Voltage divider
Calculates an unloaded two-resistor voltage divider.
How this calculation works
A voltage divider creates a lower output voltage from a higher input voltage using two resistors in series.
- Output voltage: Vout = Vin × R2 / (R1 + R2)
- Divider current: I = Vin / (R1 + R2)
- Voltage ratio: Vout/Vin = R2 / (R1 + R2)
This simplified calculator assumes that the output is not loaded by another circuit. For accurate results, the input impedance of the next circuit should be much higher than R2.
LED series resistor
Calculates the series resistor and resistor power for one LED.
How this calculation works
The resistor drops the voltage that is left over after the LED forward voltage is subtracted from the supply.
- LED forward voltage Vf: the voltage drop across the LED when it is on. Typical values are about 1.8–2.2 V for red LEDs and about 2.8–3.4 V for many blue/white LEDs.
- LED current: the desired operating current through the LED. Small indicator LEDs often use 5–20 mA.
- Resistor voltage: VR = Vs − Vf
- Series resistor: R = VR / I
- Resistor power: P = VR × I
The suggested power rating is chosen above the computed resistor dissipation for a safer design margin.
Series / parallel resistors
Enter resistor values in ohms, separated by commas, spaces, semicolons, or line breaks.
How this calculation works
- Series: Req = R1 + R2 + R3 + ...
- Parallel: 1/Req = 1/R1 + 1/R2 + 1/R3 + ...
Series resistors add directly. Parallel resistors always produce an equivalent value lower than the smallest individual resistor.
RC time constant & cutoff frequency
Calculates τ = R · C and fc = 1 / (2πRC).
How this calculation works
- Time constant: τ = R × C
- Cutoff frequency: fc = 1 / (2πRC)
- Charging law: Vc(t) = Vs × (1 − e−t/τ)
At 1τ, a charging capacitor reaches about 63.2% of its final voltage. Exact 100% charge is theoretical because the capacitor approaches the final voltage asymptotically. In practice, 5τ is commonly treated as almost fully charged, about 99.3%.
The cutoff frequency is the frequency where a simple RC filter output has dropped to about 70.7% of its low-frequency value, also called the −3 dB point.
Capacitive / inductive reactance
Calculates Xc = 1 / (2πfC) and XL = 2πfL.
How this calculation works
- Capacitor: Xc = 1 / (2πfC)
- Inductor: XL = 2πfL
As frequency increases, capacitive reactance decreases, while inductive reactance increases. These values behave like frequency-dependent AC resistance.
Op-amp gain
Calculates non-inverting and inverting amplifier gain from feedback and input resistors.
How this calculation works
- Non-inverting gain: Av = 1 + Rf / Rg
- Inverting gain: Av = −Rf / Rin
If you enter an input voltage, the calculator also estimates the output voltage. If a supply limit is entered, the displayed output is clipped to the chosen ± supply rail.
Resistor color code
Supports 3, 4, 5, and 6 band resistors.
How this calculation works
The color bands encode the significant digits, multiplier, tolerance, and optionally the temperature coefficient.
- 3 bands: 2 digits + multiplier
- 4 bands: 2 digits + multiplier + tolerance
- 5 bands: 3 digits + multiplier + tolerance
- 6 bands: 3 digits + multiplier + tolerance + tempco
Example: brown, black, red, gold means 1, 0, ×100, ±5%, which equals 1 kΩ ±5%.
How to use it
Select the calculator that matches your circuit question
Choose a tool from the left menu, enter the known values and press Calculate. The calculator keeps the formulas visible below each tool, so the result is easier to understand and verify instead of being only a black-box number.
Start with units
Most fields use base SI units such as volt, ampere, ohm, farad and hertz. Where smaller values are common, unit selectors are included for µF, nF, pF, mH and µH.
Check practical limits
For LED resistors, op-amps and dividers, the result should still be checked against component ratings, power dissipation, tolerances and the real load connected to the circuit.
Use it for learning
Each tool includes the underlying formula, making the page suitable for project work, homework checks and quick electronics reference notes.
Typical examples
Common electronics calculations
5 V / 0.02 A = 250 ΩUseful for checking current draw, resistor values and power dissipation.
Vout = Vin × R2 / (R1 + R2)Useful for sensor scaling and simple reference voltages when the output load is high impedance.
fc = 1 / (2πRC)Useful for first-order low-pass and high-pass filter estimates.
R = (Vs − Vf) / IUseful for selecting a safe series resistor and estimating resistor wattage.
Reference
What each electronics calculator is for
| Tool | Use it for | Important note |
|---|---|---|
| Ohm’s law & power | Voltage, current, resistance and power relationships | Power results should be checked against the component wattage rating. |
| Voltage divider | Estimating unloaded divider output voltage | A real load changes Vout; use a buffer or lower resistor values if needed. |
| LED resistor | Calculating a series resistor for one LED | Use the LED datasheet for forward voltage and recommended current. |
| Resistor network | Series and parallel equivalent resistance | All resistor values must be positive and in the same unit. |
| RC time constant | Charge timing and first-order filter cutoff | Capacitor tolerance can strongly affect the real timing result. |
| Reactance | Frequency-dependent impedance of capacitors and inductors | Real parts also have ESR, leakage, parasitic capacitance and tolerance. |
| Op-amp gain | Ideal inverting and non-inverting gain estimates | Real output swing depends on supply voltage, load and the op-amp type. |
| Resistor color code | Reading 3, 4, 5 and 6 band resistors | Measure with a multimeter when precision matters. |
FAQ
Electronics calculator questions
Can I use this calculator for real circuit design?
Yes, for first estimates and quick checks. For final designs, verify the result with datasheets, tolerances, thermal limits and measurements on the real circuit.
Why do some results differ from measurements?
Real components have tolerance, temperature drift and parasitic effects. A resistor marked 10 kΩ may not be exactly 10 kΩ, and capacitors can vary significantly depending on type and voltage bias.
Why is the voltage divider marked as unloaded?
The formula assumes that no current is drawn from the output node. When another circuit is connected, its input impedance appears in parallel with R2 and changes the output voltage.
What does 5τ mean for an RC circuit?
After five time constants, a charging capacitor is practically close to its final value, about 99.3%, although mathematically it approaches the final voltage asymptotically.
Use this electronics calculator to solve common circuit values quickly. Calculate Ohm’s law, power, voltage dividers, LED resistors, series and parallel resistors, RC time constants, reactance, op-amp gain, and resistor color codes.