Ohm's Law Calculator
Solve Ohm's Law for any unknown — voltage (V), current (I), resistance (R), or power (P). Enter any two known values and get all four electrical quantities instantly. Furthermore, power dissipation and energy consumption per hour are calculated automatically for every result.
How to use the Ohm's Law Calculator
Choose the unknown from the dropdown: Voltage, Current, Resistance, or Power. Furthermore, the target field highlights green and clears, while the required input fields remain enabled for your known values.
Type your known electrical quantities. For voltage, use volts (V). For current, use amperes (A). Moreover, for resistance, use ohms (Ω). Decimal values, milliamps, and kΩ values can all be entered as decimals (e.g. 0.002 A for 2 mA).
The calculator applies the appropriate Ohm's Law formula and outputs all four quantities — V, I, R, and P. Furthermore, power is shown in watts, and energy consumption per hour is shown in watt-hours for practical circuit analysis.
Power P = V × I = I²R = V²/R tells you how much energy the component dissipates. Moreover, for resistors, this power appears as heat — comparing to the component's rated wattage ensures safe operation.
The insight line summarises the result and adds context — for example, the watt-hours consumed per hour and per day. Additionally, this is useful for battery life estimation and electricity cost calculations.
Variants, options and when to use each
| Solve for | Formula | Known inputs |
|---|---|---|
| Voltage V | V = I × R | Current and resistance |
| Current I | I = V / R | Voltage and resistance |
| Resistance R | R = V / I | Voltage and current |
| Power P | P = V×I = I²R = V²/R | Any two of V, I, R |
The formula explained
I = current in amperes (A) — the rate of charge flow
R = resistance in ohms (Ω) — opposition to current flow
P = power in watts (W) = energy dissipated per second
Energy = P × t (in watt-hours when t is in hours)
Ohm's Law states that voltage equals current multiplied by resistance for a linear, resistive component at constant temperature. Furthermore, Georg Simon Ohm published this relationship in 1827, establishing that current is directly proportional to voltage and inversely proportional to resistance. Moreover, the power equations P = V×I = I²R = V²/R are derived by substituting Ohm's Law into the definition of power (P = V×I), giving three equivalent forms depending on which two quantities are known.
Worked example — a 60Ω resistor powered by a 12V supply
A 60 Ω resistor is connected to a 12 V battery. Furthermore, what current flows and how much power is dissipated? Use the "Find Current" mode: V = 12 V, R = 60 Ω.
| Variable | Formula | Result |
|---|---|---|
| Current (I) | V / R = 12 / 60 | 0.2 A (200 mA) |
| Power (P) | V² / R = 144 / 60 | 2.4 W |
| Energy per hour | P × 1 h | 2.4 Wh |
| Safety check | Rated 3 W resistor? | 2.4 W < 3 W — safe |
What is Ohm's Law in electronics and physics?
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, provided temperature remains constant. Furthermore, it is the foundational relationship of circuit analysis, applicable to resistors, wires, simple sensors, and linear circuit elements. It was formulated by German physicist Georg Simon Ohm in 1827.The law applies to ohmic materials — conductors whose resistance is constant regardless of voltage or current. Moreover, many real components are non-ohmic: diodes, transistors, and electrolytic solutions do not follow Ohm's Law, though the concepts of voltage, current, and resistance still apply. For linear AC circuits, the law generalises to V = IZ where Z is the complex impedance.
Power dissipation is directly related to Ohm's Law through P = V×I. Additionally, the three power formulas — P = VI, P = I²R, and P = V²/R — are all equivalent and derived by combining Ohm's Law with the definition of power. Choosing which form to use depends on which quantities are known: I²R when current and resistance are known, V²/R when voltage and resistance are known.
Who uses this calculator?
Electronics engineers use Ohm's Law in every circuit design task — calculating resistor values, current limits, voltage dividers, and power ratings. Furthermore, students use it in physics and electronics courses as the entry point to circuit analysis. Electricians apply it when sizing wires and circuit breakers. Moreover, hobbyists building Arduino or Raspberry Pi projects use it to calculate current-limiting resistors for LEDs. Additionally, battery designers use it to estimate discharge currents and expected run times.
Historical context and related concepts
Georg Simon Ohm published his findings in 'Die galvanische Kette, mathematisch bearbeitet' (The Galvanic Circuit Investigated Mathematically) in 1827, based on experiments with wire samples of different lengths and cross-sections. Furthermore, his work was initially poorly received by the scientific establishment, and Ohm resigned from his teaching post in frustration. By the 1840s, his law had been widely validated and he was awarded the Copley Medal by the Royal Society in 1841. Moreover, the SI unit of resistance — the ohm (Ω) — was named in his honour at the 1881 International Electrical Congress.
Why Ohm's Law is the foundation of all circuit analysis
Every circuit analysis technique — Kirchhoff's laws, superposition, Thevenin's theorem, Norton's theorem — builds on Ohm's Law. Furthermore, before any component is placed in a circuit, an engineer uses Ohm's Law to check that current and voltage are within safe operating ranges. In power electronics, incorrect resistance or current calculations cause overheating, component failure, and potentially fire.Ohm's Law in safety-critical electrical systems
In automotive and aerospace electrical systems, every wire is sized to carry its expected current within a safe temperature rise using Ohm's Law and resistivity calculations. Furthermore, circuit breakers and fuses are rated based on the maximum current calculated from the load resistance and supply voltage. Moreover, in power distribution networks, line resistance (I²R losses) causes voltage drops and energy waste that Ohm's Law quantifies — minimising these losses is a major driver of transmission line design.
Frequently asked questions
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