Free Engineering Tool
Ohm’s Law Calculator
Enter any two known electrical parameters — Voltage, Current, Resistance, or Power — and instantly calculate all four, plus conductance and energy per hour.
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Ohm’s Law
Ohm’s Law defines the fundamental relationship between voltage, current, and resistance in an electrical circuit:
Power Relationships
Electrical power can be calculated from any combination of two known values:
The Ohm’s Law Wheel
All twelve formulas derived from V = IR and P = VI:
| Find | From V & I | From V & R | From I & R | From P |
|---|---|---|---|---|
| V | — | — | V = I × R | V = P / I or √(P × R) |
| I | — | I = V / R | — | I = P / V or √(P / R) |
| R | R = V / I | — | — | R = V² / P or P / I² |
| P | P = V × I | P = V² / R | P = I² × R | — |
Practical Example
Given: Supply = 12 V, LED forward voltage = 2 V, desired current = 20 mA
Voltage across resistor: 12 − 2 = 10 V
R = V / I = 10 / 0.020 = 500 Ω
P = V × I = 10 × 0.020 = 0.2 W (use ¼W resistor minimum)
Tip: Conductance (G) is the reciprocal of resistance: G = 1/R, measured in siemens (S). It’s useful in parallel circuit analysis where conductances add directly: Gtotal = G₁ + G₂ + G₃.
Ohm’s Law states that the current (I) flowing through a conductor is directly proportional to the voltage (V) across it and inversely proportional to the resistance (R): V = I × R.
Discovered by Georg Simon Ohm in 1827, it is one of the most fundamental laws in electrical engineering and applies to any resistive circuit element.
The four electrical parameters are related by Ohm’s Law and the power equation:
- V = I × R — voltage equals current times resistance
- P = V × I — power equals voltage times current
- P = I² × R — power from current and resistance
- P = V² / R — power from voltage and resistance
Knowing any two of the four values (V, I, R, P) allows you to calculate the other two.
Series connection: Resistances add directly.
- Rtotal = R₁ + R₂ + R₃ + …
- Same current flows through all resistors
- Voltage divides across resistors
Parallel connection: Reciprocals add.
- 1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + …
- Same voltage across all resistors
- Current divides among branches
For two parallel resistors: Rtotal = (R₁ × R₂) / (R₁ + R₂)
Conductance (G) is the reciprocal of resistance: G = 1/R. It is measured in siemens (S), formerly called mhos (℧).
Conductance is particularly useful when analyzing parallel circuits, because conductances in parallel simply add: Gtotal = G₁ + G₂ + G₃.
A 100 Ω resistor has a conductance of 0.01 S (10 mS).
Ohm’s Law applies to AC circuits when resistance (R) is replaced by impedance (Z):
- V = I × Z (AC version of Ohm’s Law)
- Z includes resistance (R), inductive reactance (XL = 2πfL), and capacitive reactance (XC = 1/(2πfC))
- Z = √(R² + (XL − XC)²)
For purely resistive AC loads (heaters, incandescent bulbs), V = IR still applies directly. For inductive or capacitive loads, you must use impedance and consider phase angle.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments and software. Used in 50+ countries for industrial maintenance.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
