Compute Ohm’s Law for Voltage, Current or Resistance

Identify the two quantities you know

Ohm's law connects voltage across an ohmic component, current through it and resistance: V = IR. Voltage is measured in volts, current in amperes and resistance in ohms. Choose the missing quantity before entering numbers. Use V = IR for voltage, I = V/R for current and R = V/I for resistance. The relationship assumes a suitable resistive model and consistent operating conditions. A battery voltage and a current measured in some unrelated circuit branch do not form a valid pair merely because their units fit the equation.

Solve for voltage from current and resistance

If current is 0.020 A through a 330 Ω resistor, voltage is 0.020 × 330 = 6.6 V. In the local Ohm's Law Calculator, choose Voltage, then enter current 0.020 and resistance 330 in the labeled fields. If the current was given as 20 mA, divide by 1000 first. Entering 20 as amperes would produce 6600 V, a thousand times the intended value. The calculation itself would be internally consistent with that wrong input, which is why the unit conversion belongs in your written setup.

Solve for current from voltage and resistance

For 9 V across 1500 Ω, current is 9/1500 = 0.006 A, or 6 mA. Choose Current and enter voltage 9 and resistance 1500. A resistance given as 1.5 kΩ must be converted to 1500 Ω before entry. The divisor must be positive in this passive-resistor mode. A zero-voltage input with a positive resistance correctly gives zero current. A zero resistance does not support the same finite division, so the calculator rejects it instead of reporting an ordinary current for an ideal short-circuit assumption.

Solve for resistance and verify the substitution

If 3.3 V produces 0.011 A through the component under the stated conditions, resistance is 3.3/0.011 = 300 Ω. Select Resistance and enter voltage 3.3 and current 0.011. Substitute the answer back into V = IR: 0.011 × 300 = 3.3 V. This checks the arithmetic, but one measurement pair does not establish that the component remains ohmic at other currents or temperatures. If current is zero, the ratio is undefined; zero volts and zero amperes do not determine a unique resistance.

Keep signed readings consistent

Signed voltage and current depend on the chosen reference directions. For a passive 4 Ω resistor, voltage −12 V and current −3 A satisfy V = IR and give a positive resistance of 4 Ω. Reversing both reference signs leaves resistance unchanged. The local tool supports this consistent convention: it permits signed current when solving voltage and signed voltage when solving current. When solving resistance, nonzero voltage and current must have matching signs. It rejects a negative resistance result because active negative-resistance behavior is outside this simple passive model.

Calculate power as a separate step

After finding current, you can calculate resistor power using P = VI with a consistent passive sign convention. For the 9 V, 1500 Ω example, power is 9 × 0.006 = 0.054 W, or 54 mW. The equivalent check V²/R gives 81/1500 = 0.054 W. The local Ohm's Law form returns the selected voltage, current or resistance; it does not automatically choose a power rating or solve for power. Component selection also depends on operating limits and conditions that are not supplied by these three numbers alone.

Check the model and the displayed precision

An ohmic resistor has a proportional voltage–current relationship over the conditions being considered. A filament lamp, diode or temperature-changing component may require a different model or measured characteristic. The local calculator evaluates the selected algebraic relation and does not infer those characteristics. Very small nonzero values appear in scientific notation, while unsupported arithmetic underflow or overflow produces an error. Keep the original measurement precision, units and operating conditions with the answer. More displayed digits are useful for checking arithmetic, but they do not increase the certainty of the measurements you entered.

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