Understanding the four quantities
Each quantity in Ohm's law has an SI unit and can be found from the other two basic quantities.
| Quantity | Symbol | SI unit | Formula |
|---|---|---|---|
| Voltage | V | volt (V) | V = I × R |
| Current | I | ampere (A) | I = V ÷ R |
| Resistance | R | ohm (Ω) | R = V ÷ I |
| Power | P | watt (W) | P = V × I |
- Ohm's law is exact only for ohmic (linear) components at constant temperature; filament lamps, diodes and semiconductors deviate from it.
- Conductor resistance rises with temperature for metals, so a value measured cold will differ from the value under load.
- This calculator is for education and design estimation. Any work on electrical installations must comply with local electrical codes and be carried out or verified by a qualified electrician.
What is Ohm's law?
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance between them: I = V ÷ R, usually written V = I × R. It was published by the German physicist Georg Simon Ohm in 1827 and is the foundational relationship of circuit analysis. The SI units are the volt (V) for voltage, the ampere (A) for current and the ohm (Ω) for resistance.
Electrical power follows directly from the same quantities: P = V × I, measured in watts (W). Substituting Ohm's law gives the two equivalent forms P = I² × R and P = V² ÷ R, which let you compute power from any two of the three basic quantities. Together the four quantities are related by twelve rearranged formulas, often drawn as the 'Ohm's law wheel'.
Ohm's law applies exactly to ohmic components — those whose resistance is constant over the operating range, such as metallic conductors at a stable temperature. Many real devices are non-ohmic: the resistance of a filament lamp rises as it heats, and diodes and transistors do not follow a linear V–I relationship at all. Conductor resistance also changes with temperature, so calculated values are exact only at the stated conditions.
How to use this Ohm's law calculator
- Select the quantity you want to solve for: voltage, current, resistance or power.
- Enter the two known values — for example voltage and resistance to find current.
- Read all four quantities: the calculator completes the set using V = I × R and P = V × I.
- Check units: results are in volts, amperes, ohms and watts; convert milliamps or kilohms to base units before entering them.
The formulas behind Ohm's law
The two governing relationships are V = I × R and P = V × I. Every other form is an algebraic rearrangement: I = V ÷ R, R = V ÷ I, P = I² × R, P = V² ÷ R, and so on.
Worked example: a 12 V supply across a 6 Ω resistance. Current I = 12 ÷ 6 = 2 A, and power P = 12 × 2 = 24 W. The same circuit checked with the alternative forms: P = I² × R = 4 × 6 = 24 W, and P = V² ÷ R = 144 ÷ 6 = 24 W — all three forms agree.
Common mistakes
- Entering mixed units — milliamps, kilohms or millivolts must be converted to amperes, ohms and volts first (500 mA = 0.5 A; 2.2 kΩ = 2,200 Ω).
- Applying Ohm's law to non-ohmic devices such as diodes, LEDs or lamps, whose resistance changes with operating point.
- Confusing power with energy: power (W) is a rate; energy consumption is power multiplied by time (Wh or kWh).
- Forgetting that resistance measured with the circuit unpowered can differ substantially from the effective resistance at operating temperature.
Pertanyaan yang sering diajukan
What is the Ohm's law formula?
V = I × R: voltage (volts) equals current (amperes) multiplied by resistance (ohms). Rearranged, I = V ÷ R and R = V ÷ I. Adding the power relation P = V × I lets you compute any of the four quantities — voltage, current, resistance, power — from two known ones.
How do I calculate power from voltage and current?
Multiply them: P = V × I. A device drawing 2 A from a 12 V supply dissipates 24 W. If you know current and resistance instead, use P = I² × R; from voltage and resistance, use P = V² ÷ R. All three forms are equivalent under Ohm's law.
Does Ohm's law work for AC circuits?
The same form applies to AC, but resistance generalizes to impedance (Z), which combines resistance with the frequency-dependent reactance of inductors and capacitors: V = I × Z. For purely resistive AC loads the DC formula applies directly using RMS values of voltage and current. This calculator computes the resistive (DC) case.
What is the difference between voltage, current and resistance?
Voltage is the electrical potential difference that drives charge through a circuit, measured in volts. Current is the rate of charge flow, measured in amperes. Resistance is the opposition a material presents to that flow, measured in ohms. Ohm's law ties them together: one volt pushes one ampere through one ohm.
Why does my measured value differ from the calculated one?
Real components have tolerances (resistors commonly ±1–5%), conductor resistance changes with temperature, meters have their own accuracy limits, and supply voltages sag under load. Ohm's law itself is exact for ohmic components; discrepancies usually come from these practical factors rather than the formula.
Is it safe to use this calculator for mains wiring?
The calculator is an educational and estimation tool only. Mains and fixed-wiring work is governed by local electrical codes and in most jurisdictions must be performed or certified by a qualified electrician. Use the results to understand a circuit, not as authorization to modify an installation.
Referensi
- Ohm GS. Die galvanische Kette, mathematisch bearbeitet. Berlin, 1827 — original statement of Ohm's law.
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019 — definitions of the volt, ampere, ohm and watt.
- Horowitz P, Hill W. The Art of Electronics, 3rd edition, Cambridge University Press, 2015 — Ohm's law and non-ohmic devices.