Inrush Calculator

Inrush Calculator: engineering calculator for inrush. Formula derivation, tolerances, and design tips.

Ohm's Law is the foundational relationship governing electrical circuits: voltage (V), current (I), and resistance (R) are always related by the equation V = IR. Named after German physicist Georg Ohm, who published his findings in 1827, this simple formula underpins the design of every electrical device from household wiring to microprocessors. The calculator solves for any one of the three variables given the other two, and extends to electrical power using the related formula P = IV. Speed Calculator and Force Calculator are useful companion tools for circuit design and power planning.

Ohm's Law applies strictly to ohmic (linear) resistors at a constant temperature. Non-ohmic components — diodes, transistors, and most semiconductor devices — do not follow a simple linear V-I relationship. Nonetheless, Ohm's Law remains the essential starting point for DC circuit analysis, fault diagnosis, cable sizing, and all basic electrical engineering calculations.

Identify which two of the three quantities you know: voltage (V), current (A), or resistance (Ω).
Enter the known values and select the variable to solve for.
Read the calculated result and the power (W) derived from P = V × I.
For a series circuit: add resistances together before entering; total resistance = R1 + R2 + R3.
For a parallel circuit: use 1/R_total = 1/R1 + 1/R2 + 1/R3 to find the equivalent resistance before entering.
Cross-check the power result — if the calculated wattage exceeds a component's rated power, the component will overheat.

Ohm's Law and power formulas explained

V = I × R (Voltage = Current × Resistance)

I = V ÷ R (Current = Voltage ÷ Resistance)

R = V ÷ I (Resistance = Voltage ÷ Current)

P = V × I = I² × R = V² ÷ R (Power in watts)

Worked example: a 12V car battery connected across a 4Ω resistor: I = 12 ÷ 4 = 3A. Power dissipated = 12 × 3 = 36W. The resistor must be rated at least 36W to avoid burning out. For household wiring: a 240V circuit with a 13A fuse has a maximum safe power load of 240 × 13 = 3,120W — enough for a kettle (2,400W) but not two kettles simultaneously.

Understanding your circuit result

Common voltage, current, and resistance reference values

Standard mains voltages: 120V (US/Canada), 230V (EU/UK), 100V (Japan). Standard household fuse ratings: 5A (lighting), 13A (sockets), 32A (shower), 40A+ (cooker). Common component resistances: LED series resistor 220–470Ω, household bulb filament ~200–1000Ω, heating element ~20–50Ω. Wire resistance: standard 1.5mm² copper cable has ~12.1mΩ/m — significant over long cable runs, which is why voltage drop calculations matter in building wiring design.

Engineering tips and best practices

Always verify the power rating of a component against the calculated P = IV — exceeding it causes thermal damage or fire risk.
Temperature affects resistance: copper resistance increases by ~0.4% per °C — long cable runs in hot environments carry measurably more resistance than in cold ones.
For LED circuits, always include a current-limiting resistor: R = (Supply V − LED forward V) ÷ desired current in amps. A typical red LED needs ~20mA at 2.0V forward voltage.
Kirchhoff's Voltage Law (KVL) extends Ohm's Law to multi-loop circuits: the sum of voltage drops around any closed loop equals the supply voltage.
Multimeters measure voltage on parallel mode, current in series (breaking the circuit), and resistance with power off — using the wrong mode can damage the meter or circuit.
Ground fault circuit interrupters (GFCIs/RCDs) detect imbalances of ~30mA between live and neutral — far below the lethal threshold of ~100–200mA through the heart.

Human skin resistance ranges from ~1,000Ω (dry hands) to ~100Ω (wet hands) — explaining why wet contact with mains voltage is far more dangerous.
A standard UK 13A socket at 230V can supply a maximum of 2,990W — enough for a kettle, iron, or microwave, but not two simultaneously.
Copper has a resistivity of 1.68 × 10⁻⁸ Ω·m — one of the lowest of any common metal, which is why it dominates electrical wiring globally.
The world's power grids operate at 50Hz (UK/EU) or 60Hz (US) — AC rather than DC, which Ohm's original work didn't cover (AC uses impedance, not pure resistance).

Common mistakes to avoid

Applying Ohm's Law to non-ohmic devices like diodes or LEDs without accounting for their non-linear V-I characteristic curves.
Forgetting to convert units — entering milliamps (mA) as amps, or kilohms (kΩ) as ohms, produces answers that are wrong by a factor of 1,000.
Ignoring internal resistance of batteries — a 9V battery with 1Ω internal resistance only delivers 9V at zero current draw; under load the terminal voltage drops.
Treating resistors in parallel as simply additive — the parallel combination is always less than the smallest individual resistor.
Not derate for temperature — a resistor rated 0.25W at 25°C may only be safe at 0.15W at 70°C due to thermal derating curves.

This calculator is for educational and planning purposes. Electrical installations in dwellings must comply with BS 7671 (UK), NEC NFPA 70 (US), or equivalent national wiring regulations, and must be designed and certified by a qualified electrician. Working on live electrical systems without appropriate training and safety equipment is dangerous and may be illegal. In the UK, notifiable electrical work must be carried out or approved by a Part P registered contractor. This calculator does not constitute engineering design or certified electrical advice.