Ohm's Law Calculator

V = IR

Ohm's Law calculator — find voltage, current, or resistance. V = IR with input validation.

Calculate

Voltage V (V)

Current I (A)

Enter values above to calculate

How it calculates

Given V (volts) and I (amperes)

Apply Ohm's Law: R = V / I

Result in ohms (Ω)

Formula

V = IR

Variable Table

Symbol	Quantity	SI Unit
V	Voltage (potential difference)	V
I	Current	A
R	Resistance	Ω

→ Ohm's Law article

How to Use This Calculator

This ohm's law calculator is built for quick physics checks and worked-problem review. Enter values in the units shown beside each input, then compare the result with the formula and variable table before using it in a longer solution. The calculator does the arithmetic, but the physics still depends on choosing a model that matches the situation.

Start by identifying the system, the known quantities, and the quantity you want to find. If a value is given in a non-SI unit, convert it before substitution. A correct numerical answer with mixed units can still be physically wrong, especially when squared units, inverse seconds, charges, temperatures, or distances are involved.

Assumptions and Limits

The formula V = IR is a model, not a universal description of every possible case. It assumes the quantities in the variable table are the relevant quantities for the problem and that hidden effects are either negligible or already included in the inputs. If friction, drag, relativistic speeds, changing fields, non-constant temperature, or geometry-specific effects matter, check whether a more complete model is needed.

Use the result as a magnitude and units check. Ask whether the answer has the right sign, whether it grows or shrinks when an input changes, and whether the limiting cases make sense. Setting an input to zero, doubling a quantity, or using a very large value is often enough to catch a formula choice or unit mistake before it reaches a final answer.

Worked Example

A 9 V battery drives a 3 A current. Find resistance.

Step 1: R = V / I

Step 2: R = 9 / 3

Answer: R = 3 Ω

Common Mistakes

Ohm's Law V = IR assumes constant temperature — resistance changes with heat
Current must be non-zero to compute resistance from V and I
Power P = IV = I²R = V²/R — don't confuse P with R

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Frequently Asked Questions

What are the three forms of Ohm's Law?

V = IR (find voltage), I = V/R (find current), R = V/I (find resistance). Use whichever suits what is known.

Does Ohm's Law always apply?

No — diodes, transistors, and most semiconductors are non-ohmic. Ohm's Law applies to metallic conductors at constant temperature.