CircuitCalc
Ohm's law, LED resistor sizing, voltage dividers, resistor color codes, and RC time constants. No signup. Works on your phone at the bench.
Ohm's Law Calculator
Calculate voltage, current, resistance, or power. Enter any two values to find the rest.
LED Resistor Calculator
Find the right resistor for your LED. Enter supply voltage, LED forward voltage, and current.
Voltage Divider Calculator
Calculate output voltage or find resistor values for a voltage divider circuit.
Resistor Color Code Calculator
Decode 4-band or 5-band resistor color codes to find the resistance value.
RC Time Constant Calculator
Calculate the time constant, charge time, and cutoff frequency for an RC circuit.
About CircuitCalc
CircuitCalc is a collection of free electronics calculators for hobbyists, students, and engineers. Every tool runs entirely in your browser — no data leaves your device.
Whether you're sizing an LED resistor for a project, checking Ohm's law, designing a voltage divider, decoding resistor bands, or calculating RC time constants, these tools give you instant answers.
All calculations can be shared via URL — copy the address bar after calculating to share your exact inputs and results.
Ohm's Law: V = I × R (voltage = current × resistance). The three-way form: I = V/R and R = V/I. For a 12V circuit through a 470 Ω resistor: I = 12/470 = 25.5 mA. Power: P = V × I = I² × R = V²/R. The same 12V/470Ω circuit dissipates P = 12 × 0.0255 = 0.306 W. Resistors must be rated above actual power dissipation — use a 0.5W or 1W resistor in this case, not a standard 1/4W resistor (0.25W).
Resistors in series and parallel: series resistance R_total = R1 + R2 + R3 + ... (same current through each). Parallel resistance: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... For two resistors in parallel: R_total = (R1 × R2) / (R1 + R2). Two 1kΩ resistors in parallel = (1000 × 1000)/(1000 + 1000) = 500Ω. Parallel resistors always yield a total resistance lower than the smallest individual resistor.
Capacitor charging time constant: τ (tau) = R × C (resistance in ohms × capacitance in farads). After one time constant, a capacitor charges to 63.2% of supply voltage. After 5τ, it's considered fully charged (99.3%). For R = 10kΩ and C = 100μF: τ = 10,000 × 0.0001 = 1.0 second. The capacitor reaches 63.2% of supply voltage in 1 second; it's effectively fully charged in 5 seconds. Discharge follows the same exponential curve.
LED current limiting resistors: R = (V_supply − V_forward) / I_LED. A standard red LED has V_forward ≈ 2.0V, rated current 20mA. On a 5V supply: R = (5 − 2.0) / 0.020 = 150Ω. Use the nearest standard value (150Ω or 160Ω). Without a current-limiting resistor, LEDs draw excessive current limited only by supply impedance — they burn out within seconds. Always calculate and install the series resistor.