Electronics Calculators
Practical tools for circuit design, component selection and bench work, with clear explanations alongside every result.
Electronics basics: the three numbers you always need
Every electronics calculation comes back to three quantities: voltage (V, in volts), current (I, in amperes), and resistance (R, in ohms). Ohm's Law connects them: V = I × R. From that single equation you can find any third value when two are known, size a current-limiting resistor, predict component heating, or explain why a fuse blows.
Power adds a fourth variable: P = V × I (watts). This determines heat, battery drain, and fuse ratings. A 5 V LED at 20 mA dissipates 0.1 W in the circuit; that same 20 mA through a 220 Ω resistor dissipates P = I² × R = 0.088 W in the resistor. When dissipation exceeds what a component is rated for, it runs hot, degrades, and eventually fails.
Series vs parallel: the practical difference
In a series circuit, all components share the same current; voltage divides between them proportionally. Resistors in series simply add: 100 Ω + 220 Ω = 320 Ω total. One break in the chain stops everything. Use for: limiting current to an LED, building a voltage divider.
In a parallel circuit, all branches share the same voltage; current splits between them. Total resistance is always lower than the smallest individual resistor. One branch failing does not affect the others. Use for: multiple LEDs from one supply, adding battery capacity.
Quick test: if removing one component would stop everything else working, they are in series. If everything else continues, it was in parallel.
Power dissipation: why component ratings matter
A resistor rated at 0.25 W in a circuit demanding 0.4 W will run warm and fail early. For continuous operation, use a component rated for at least double the calculated dissipation. The Ohm's Law calculator on this hub calculates power dissipation as part of every result, so you can cross-check before ordering parts.
Core calculators
AC & frequency calculators
Printable cheat sheets
A one-page reference for the bench. Open it, then print or save as PDF.
Electronics glossary
27 terms with plain-English definitions: Ohm's Law, impedance, reactance, forward voltage, hFE, AWG, E-series, SMD and more. Each term links directly to the relevant calculator on this hub.
What each tool does
| Calculator | What you solve |
|---|---|
| Ohm's Law | Any two of V, I, R, P known → calculates the remaining two. All six formula paths. |
| Watts / Volts / Amps | DC, single-phase AC and three-phase AC power. Includes power factor. |
| Voltage Divider | Vin + R1 + R2 → Vout, divider current, R1 voltage and resistor power. |
| LED Resistor | Supply voltage + LED Vf + current → required resistor value (with E12 nearest standard) and power dissipation. |
| Resistor Color Code | 4-band and 5-band resistor decoder. Color bands → resistance value and tolerance. |
| Capacitor Code | 3-digit SMD/ceramic capacitor code → pF / nF / µF. Unit converter included. |
| Voltage Drop | Wire length + gauge (AWG or mm²) + current → voltage drop and power loss. |
| Battery Life | Capacity (mAh) + load current → runtime in hours and minutes with efficiency factor. |
| Battery Capacity | mAh + voltage → watt-hours for power banks, e-bike packs and the 100 Wh airline carry-on limit. |
| Battery Charging Time | Capacity (Ah) + charge current + efficiency → charge time, amp-hours added and C-rate for any battery. |
| Inductor Reactance | Any two of XL, frequency, inductance → solves the third. Formula XL = 2πfL. Supports Hz/kHz/MHz and H/mH/µH/nH. |
| Capacitor Reactance | Any two of XC, frequency, capacitance → solves the third. Formula XC = 1/(2πfC). Supports µF/nF/pF. |
| Impedance Calculator | Resistance R + reactance X → impedance Z, phase angle θ, and power factor. Positive X = inductive, negative X = capacitive. |
When to use these tools
- Before soldering, verify resistor value, power rating and LED current
- Bench setup, confirm supply voltage and current headroom for a load
- Cable sizing, check voltage drop on long DC runs (LED strips, solar, automotive)
- Battery projects, estimate runtime before committing to a pack size
- Component identification, decode resistor bands and capacitor codes from the part bin
Working with Arduino, ESP32, and LED projects
The calculators on this hub are built around the real constraints of common maker platforms. GPIO pins have strict current limits; exceeding them damages the chip permanently.
| Platform | GPIO voltage | Max current per pin | First calculation you need |
|---|---|---|---|
| Arduino Uno / Nano | 5 V | 40 mA (20 mA recommended continuous) | LED resistor: (5 V minus LED forward voltage) divided by 0.020 A. Red LED (Vf ≈ 2.1 V): 145 Ω, use 150 Ω. |
| ESP32 / ESP8266 | 3.3 V | 12 mA per pin, 40 mA total GPIO | LED resistor: (3.3 V minus Vf) divided by 0.010 A. Never connect 5 V logic directly to an ESP32 pin. |
| Raspberry Pi | 3.3 V | 16 mA per pin, 50 mA total GPIO | Drive any load above 10 mA through a transistor or MOSFET. The Pi GPIO is a signal line, not a power driver. |
LED strip voltage drop
LED strips on runs longer than about 2 metres can show visible dimming at the far end. At 3 A over a 3-metre run of 24 AWG wire, the voltage drop is roughly 0.5 V, enough to visibly shift the colour temperature on warm-white LEDs. Solutions: switch to 24 V strips (same wattage, half the current, one-quarter the drop), inject power at both ends, or use heavier cable. The DC voltage drop calculator gives the exact figure for any combination of gauge, length, and current. Our LED strip wiring guide walks through the full layout, including power injection points and PSU sizing for longer runs.
3D printer electronics
Most printer boards run at 12 V or 24 V. The heated bed draws the highest current: typically 8 to 15 A at 12 V, or 4 to 8 A at 24 V. To size the PSU: sum all peak loads (bed, hotend, stepper drivers, fans), add 20% headroom. A typical 24 V Ender 3 peaks around 10 A, making a 360 W (15 A) supply the minimum safe choice.
Step-by-step sizing workflows
LED circuit: size a resistor in 5 steps
- Identify supply voltage (Vsupply). Arduino Uno or Nano: 5 V. ESP32 and ESP8266: 3.3 V. USB port or power bank: 5 V. LED strip supply: 12 V or 24 V.
- Find the LED forward voltage (Vf) and rated current (If). No datasheet? Use: red or yellow 2.0 to 2.2 V at 20 mA; blue, white, or green 3.0 to 3.4 V at 20 mA.
- Calculate resistor value: R = (Vsupply minus Vf) divided by If. Red LED on 5 V at 20 mA: R = (5 minus 2.1) / 0.020 = 145 ohm. Round up to the nearest E12 value: 150 ohm.
- Check resistor power dissipation: P = If squared times R. For 20 mA through 150 ohm: 0.0004 times 150 = 0.06 W. A standard 0.25 W resistor handles this safely. For any resistor running above 0.1 W continuously, use a 0.5 W or 1 W component.
- Multiple LEDs: series wiring sums the forward voltages (total must stay at least 1 V below Vsupply); parallel wiring gives each LED its own resistor at the full supply voltage. Run the LED Resistor calculator to verify before soldering.
Battery runtime: size a pack in 5 steps
- List all loads and active current draw (mA). If you know wattage only, divide by supply voltage: a 1.5 W sensor at 3.3 V draws 1.5 / 3.3 = 455 mA active.
- Calculate average current for duty-cycle devices: (active current times active fraction) plus (sleep current times sleep fraction). A sensor at 400 mA active for 5% of the time and 0.5 mA in sleep: (400 times 0.05) plus (0.5 times 0.95) = 20.5 mA average.
- Calculate raw capacity needed: average current (mA) times required runtime (hours). 20.5 mA for 168 hours (one week) = 3444 mAh.
- Add efficiency margin: divide by 0.8 to cover battery internal resistance, regulator or boost-converter loss, and temperature derating. 3444 / 0.8 = 4305 mAh. Choose the next standard pack size above this figure.
- Verify with the Battery Life calculator using the chosen pack capacity and your total continuous load, then cross-check the result against your duty-cycle average.
Common mistakes and how to avoid them
- No resistor for an LED. Without current limiting, a red LED on a 5 V supply draws 300 mA or more, burns bright for a second, and fails. Always calculate the resistor value first using the LED Resistor calculator.
- Underrating the resistor's power. A 220 Ω resistor carrying 40 mA dissipates P = (0.04)² × 220 = 0.35 W, which exceeds the standard 0.25 W rating. For any resistor running above 0.1 W continuously, use a 0.5 W or 1 W component.
- Mixing up DC and AC voltage drop. The DC voltage drop tool uses purely resistive loss. Mains cable sizing also involves reactive impedance and IEC voltage-drop limits. Use the AC Voltage Drop calculator in the Electrical hub for any mains installation.
- Treating all capacitors as interchangeable. A 100 nF ceramic and a 100 µF electrolytic are both capacitors but serve completely different roles. The ceramic suppresses high-frequency noise at the IC supply pin; the electrolytic stores energy for slow transient demands. The wrong type degrades circuit stability without always causing immediate failure.
- Using calculated values without measuring. A 10 kΩ resistor with ±5% tolerance could measure anywhere from 9.5 kΩ to 10.5 kΩ. For bias networks, filter cutoffs, or timing circuits, measure the actual component before soldering.
The tools on this page are designed for low-voltage DC bench electronics. For mains wiring (230 V / 120 V), see the Electrical hub. Contact with mains voltage is potentially lethal.
Frequently Asked Questions
Do these calculators work for both DC and AC circuits?
Most core tools target DC or low-frequency electronics. The Ohm's Law and Voltage Divider calculators assume resistive (DC) behaviour. The Watts/Volts/Amps tool covers DC, single-phase AC and three-phase AC with power factor. For AC work involving reactance and impedance, use the Inductor Reactance, Capacitor Reactance, and Impedance calculators. These tools do not model parasitic effects or distributed inductance at RF frequencies.
Are the results accurate enough for real circuits?
Yes, for the intended purpose: planning and sanity-checking before measurement. Ohm's Law, power dissipation and voltage division are exact formulas with no approximation in these calculators. What varies in practice are component tolerances (resistors ±1-5%, capacitors ±10-20%), temperature drift, and parasitic effects in the physical layout. Use these tools to catch obvious errors, then measure the real circuit with a multimeter.
What is the E12 resistor series shown in the LED calculator?
E12 is a standard series of 12 preferred resistor values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 Ω (and ×10 multiples). These are the values you find on the shelf at component suppliers. The calculator rounds your exact result to the nearest E12 value, always check whether rounding up or down keeps current within safe limits.
How do I wire multiple LEDs from the same supply?
Series wiring uses one resistor for the whole string but requires a supply voltage high enough for all LED forward voltages plus the resistor drop. For three red LEDs in series (Vf ≈ 2.1 V each) on a 12 V supply: the resistor sees 12 − 6.3 = 5.7 V. At 20 mA, R = 285 Ω. If one LED fails, all go out. Parallel wiring gives each LED its own resistor, so each can fail independently, but uses more components and draws proportionally more current from the supply. Most projects use parallel for robustness. For parallel setups, run the LED Resistor calculator once per LED using the full supply voltage.
What does the impedance calculator give that Ohm's Law does not?
Ohm's Law (V = I × R) applies to purely resistive circuits. In AC circuits, capacitors and inductors also oppose current flow through reactance, which is frequency-dependent: capacitive reactance drops at higher frequencies, inductive reactance rises. Impedance (Z) is the combined effect: Z = √(R² + X²). It determines the actual current in an AC circuit: I = V / Z. If you use only resistance in an AC calculation with significant inductance, you will overestimate current. The impedance calculator also returns the phase angle between voltage and current, which is the key input for any power factor calculation.
My circuit works on the bench but behaves oddly when I add more components. What should I check?
The three most common causes are supply voltage sag, missing decoupling capacitors, and ground return impedance. First, measure the actual supply voltage under full load; cheap USB supplies and regulated bench supplies can drop 0.3 to 0.5 V under load, enough to cause microcontroller brownout resets. Second, place a 100 nF ceramic capacitor directly between the Vcc and GND pins of each IC, as close to the chip as physically possible; this suppresses the fast current spikes that cause logic errors. Third, check that high-current loads do not share a long ground wire with signal circuitry: ground-path resistance creates voltage offsets that shift logic levels unpredictably.