Ohm’s Law Calculator
Enter any two of voltage, current, resistance and power — the calculator solves the other two using Ohm’s law and the power equation.
Enter exactly two values.
How this conversion works
V = I × R, P = V × I, P = I² × R, V = √(P × R).Give any two quantities and the other two are fixed. Voltage in volts, current in amps, resistance in ohms, power in watts.
Ohm’s law, the backbone of DC design
Ohm’s law says voltage equals current times resistance; the power equation says power equals voltage times current. Together they let you find any two unknowns from any two knowns — the everyday math behind wire sizing, fuse selection, LED resistors and battery draw. It applies exactly to DC and to resistive AC loads.
Worked example
A 12 V circuit drawing 5 A has a resistance of 12 ÷ 5 = 2.4 Ω and dissipates 12 × 5 = 60 W. Knowing the power and voltage instead, current = P ÷ V, and resistance follows.
Quick reference: watts to amps at common DC voltages
| Load | @12 V | @24 V | @48 V |
|---|---|---|---|
| 60 W (lighting) | 5 A | 2.5 A | 1.25 A |
| 300 W (fridge + router) | 25 A | 12.5 A | 6.3 A |
| 1,000 W (small inverter) | 83 A | 42 A | 21 A |
| 3,000 W (large inverter) | 250 A | 125 A | 63 A |
This table is the practical argument for 24 V and 48 V systems: the same power at higher voltage means a fraction of the current, thinner cables, smaller fuses and lower resistive losses — all direct consequences of Ohm’s law.
Frequently asked questions
Ohm’s law states that voltage equals current times resistance (V = I x R). Combined with power (P = V x I), it links voltage, current, resistance and power.
Multiply them: power in watts equals voltage times current. For example, 12 V x 5 A = 60 W.
Divide voltage by current (R = V / I), or use R = V² / P, or R = P / I², depending on which values you know.
It applies directly to DC and to purely resistive AC loads. For reactive AC loads (motors, capacitors) you must also account for power factor and impedance.
Sources & standards
Ohm’s law and the power equation are foundational physics, not policy — the references below define the units and vocabulary this calculator uses.
- SI units — electric current — National Institute of Standards and Technology (NIST)
Defines the ampere and the electrical base units this calculator’s volts, amps, ohms and watts are built on. - NIST Special Publication 330 — The International System of Units (SI) — NIST
The authoritative US reference for the volt, ohm and watt used throughout the results. - IEC 60050 — International Electrotechnical Vocabulary — International Electrotechnical Commission (IEC)
Standard definitions for voltage, current, resistance and power that keep this tool’s terminology consistent worldwide. - NFPA 70 — National Electrical Code — National Fire Protection Association (NFPA)
Applies Ohm’s law in practice for conductor sizing and voltage-drop limits, the real-world use case this calculator feeds into.