Electronics Calculators
5 free electronics calculators — standards-based, with formulas, worked examples, and no sign-up.
Bench electronics runs on a handful of closed-form circuit laws, and this category implements the ones a breadboard session actually reaches for. The LED resistor calculator applies R = (Vs − ΣVf)/If for one or several LEDs in series, then snaps up to the next standard E24 resistor value (the preferred-number series of IEC 60063) and recommends a wattage with a 2× dissipation margin. The resistor color code tool decodes and generates 4-, 5-, and 6-band markings from the IEC 60062 digit, multiplier, tolerance, and temperature-coefficient tables — the same standard every printed band chart is transcribing.
The other three tools cover the passive-network staples. The voltage divider calculator gives Vout = Vin·R₂/(R₁+R₂) and — the part hand calculations usually skip — the loaded output when the next stage's input resistance sits in parallel with R₂, plus the current and per-resistor power that decide whether a divider is wasting battery. The capacitor calculator turns Q = CV and E = ½CV² into charge and stored energy across the pF-to-farad range, and the RC time constant tool computes τ = RC, the exponential charge and discharge curves, and the time to reach any target percentage — with the 1τ-to-5τ ladder (63%, 86%, 95%, 98%, 99%) laid out as a chart.
Everything here is component-level and supply-agnostic: the tools care about volts, ohms, and farads, not about what is upstream of the bench supply. The moment a question involves mains wiring, conductor sizing, breaker coordination, or code compliance, it belongs to the Electrical & MEP category next door — voltage drop over branch-circuit runs, NEC ampacity, fault current, and panel loads live there. A useful rule of thumb: if the answer changes when you swap a 9 V battery for a lab supply, it is an electronics question; if it changes when you swap 14 AWG for 12 AWG, it is an electrical one.
All Electronics Tools
LED Resistor Calculator
Series resistor for an LED string: R = (Vs − ΣVf)/If rounded up to the next E24 standard value, with wattage rating at 2× dissipation and the actual current delivered.
Resistor Color Code Calculator
Decode 4-, 5-, and 6-band resistor colors to value, tolerance window, and tempco per IEC 60062:2016 — or generate the bands for a value, snapped to the nearest E24/E96 standard.
Capacitor Charge & Energy Calculator
Stored charge Q = C·V and energy E = ½·C·V² in coulombs, joules, and watt-hours, from picofarad ceramics to supercapacitors, with unit switches that convert instead of erroring.
Voltage Divider Calculator
Vout = Vin·R₂/(R₁+R₂) plus the loaded output with R₂∥RL and its droop percentage, and the standing current and per-resistor power that decide stiffness vs battery drain.
RC Time Constant Calculator
τ = R·C, the charge/discharge state after any elapsed time, and the exact time to any target percentage (t = −τ·ln(1−x)) with the canonical 1τ–5τ ladder.
Frequently Asked Questions
The calculated resistor value is not one I can buy — which way should I round?
Round up to the next standard value, which is exactly what the LED resistor tool does against the E24 series. A slightly larger resistor means slightly less current — a dimmer LED, a slower RC charge — which is safe. Rounding down raises the current above what you designed for, eating into the LED's rated maximum and the resistor's power margin. When the two neighbours are nearly equidistant and current precision matters, check the actual current at each value: the tools report it.
Why does my voltage divider read correctly with a multimeter but sag when I connect the actual circuit?
Because the load's input resistance sits in parallel with R₂ and pulls the output down — a multimeter's ~10 MΩ input barely loads the divider, but a 10 kΩ load on a 10k/5k divider drops the output from 4.0 V to 3.0 V. The voltage divider calculator computes the loaded case directly. The standard cure is to keep the divider impedance roughly a tenth of the load impedance, at the cost of higher standing current.
How long does a capacitor take to charge "fully"?
Strictly never — the exponential only approaches the supply asymptotically. The working convention is five time constants: at t = 5τ the capacitor is above 99% charged, and at 1τ it is already at 63%. The RC time constant calculator inverts the curve too, giving the exact time to any target percentage (t = −τ·ln(1 − x)), which is the number you actually need for a reset delay or a debounce circuit.