Ohm's Law Formula (V = IR): Free Calculator & Worked Examples
Use V = IR (the IR formula) for voltage, current, or resistance. mA/kΩ traps, worked examples, and a free Ohm's law calculator.
How to Use Ohm's Law (Quick Answer)
Ohm's law is V = I R (often written V = IR or v = ir). Voltage equals current times resistance. If you know any two of V, I, and R, solve for the third.
Need a checked number now? Open the free Ohm's law calculator (V = IR) — then come back for unit traps and worked examples.
What Is V = IR (the IR Formula)?
V is voltage in volts, I is current in amperes, and R is resistance in ohms. For an ohmic resistor they stay in proportion: raise V at fixed R and I rises; raise R at fixed V and I falls.
Students often search “ir formula” or “v = ir” meaning the same relationship. Once two values are known, the third is fixed — that is why a rearranging calculator covers most homework checks.
Rearrangements
Solve any form on the free Ohm's law calculator.
When Ohm's Law Applies
Use it when the problem treats the component as an ohmic resistor — a single R that does not change with voltage in the range you care about. Metal-film resistors in lab kits are the usual case.
Do not force a single R onto diodes, LEDs, or other non-linear devices unless the question says to.
Unit Traps
Milliamps vs amperes
Convert before dividing: 30 mA = 0.030 A. Using 9 / 30 instead of 9 / 0.030 makes R wrong by 1000×.
Kilo-ohms vs ohms
2.2 kΩ = 2200 Ω. Leaving kΩ as 2.2 in V = IR breaks the answer.
Power vs Ohm's law
Watts use P = V I (or I²R, V²/R). When the unknown is power, use the electric power calculator — not Ohm's law alone.
Worked Examples
Example 1: I from 12 V and 4 Ω
I = V / R = 12 / 4.0 = 3.0 A.
Example 2: R from 9 V and 30 mA
30 mA = 0.030 A. R = 9 / 0.030 = 300 Ω.
Example 3: V from 0.025 A and 1.2 kΩ
R = 1200 Ω. V = I R = (0.025)(1200) = 30 V.
Check Examples 1–3 on the Ohm's law calculator.
Ohm's Law vs Dividers and Networks
Ohm's law handles one resistance relationship. For two resistors sharing a supply, use a voltage divider. For series/parallel pairs, find equivalent R first, then apply V = IR.
Series–parallel resistance calculator.
Common Mistakes
Free Ohm's Law Calculator
Use the free Ohm's law calculator to solve V = IR for voltage, current, or resistance with step-by-step working.
Master V = IR with clean amperes and ohms — then verify every homework answer.
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