Ohm's Law Calculator

Calculate voltage (V), current (I), or resistance (R) when two values are known.

📖 What is Ohm's Law?

Ohm's Law is the most fundamental relationship in electrical engineering and electronics. It states that the electric current flowing through a conductor between two points is directly proportional to the voltage across those two points, assuming physical conditions such as temperature remain constant. The law is expressed by the equation V = I × R, where V is the voltage in volts, I is the current in amperes, and R is the resistance in ohms.

Formulated by German physicist Georg Simon Ohm in 1827 and published in his landmark paper "Die galvanische Kette, mathematisch bearbeitet" (The Galvanic Circuit Investigated Mathematically), Ohm's Law laid the groundwork for all circuit analysis that followed. Every electrical engineer, electronics hobbyist, and physics student needs to understand this relationship.

The power formula extends Ohm's Law to energy transfer: P = V × I. Because V = IR, you can substitute to get P = I²R or P = V²/R - three equivalent ways to calculate the power dissipated by any resistive element. This matters practically because resistors have wattage ratings, and exceeding them causes failure or fire.

Ohm's Law applies to resistors, wires, and any ohmic conductor - materials where resistance is constant regardless of applied voltage. Non-linear components like diodes and transistors do not obey this simple law; they require more advanced circuit models.

Understanding Ohm's Law lets you design voltage dividers, calculate current limiting resistors for LEDs, determine wire gauges for safe current carrying capacity, and troubleshoot circuit faults. It is the starting point for all of Kirchhoff's circuit laws, Thevenin's theorem, and every other circuit analysis technique.

📐 Formula

Ohm's Law (solving for each variable):

- Voltage: V = I × R

- Current: I = V / R

- Resistance: R = V / I

Power formulas derived from Ohm's Law:

- P = V × I (power = voltage × current)

- P = I² × R (when V is unknown)

- P = V² / R (when I is unknown)

Variables:

- V = Voltage in volts (V)

- I = Current in amperes (A)

- R = Resistance in ohms (Ω)

- P = Power in watts (W)

Ohm's Law (solving for each variable):

- Voltage: V = I × R - Current: I = V / R - Resistance: R = V / I

Power formulas derived from Ohm's Law:

- P = V × I (power = voltage × current) - P = I² × R (when V is unknown) - P = V² / R (when I is unknown)

Variables: - V = Voltage in volts (V) - I = Current in amperes (A) - R = Resistance in ohms (Ω) - P = Power in watts (W)

📖 How to Use This Calculator

Identify which two of the three values (V, I, R) you know from your circuit or problem.

Enter those two values in the corresponding fields. Leave the unknown field blank.

Click Calculate to instantly see voltage, current, resistance, and power.

Use the Copy button to save your result for documentation or sharing.

If you enter all three values, the calculator will use your V and I to compute power and display all values as entered.

💡 Example Calculations

Example 1 - Find Current from Voltage and Resistance

A 12 V car battery powers a 60 Ω resistor. What current flows through the circuit?

- Known: V = 12 V, R = 60 Ω

- Solve: I = V / R = 12 / 60 = 0.2 A (200 mA)

- Power: P = V × I = 12 × 0.2 = 2.4 W

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Example 2 - Find Resistance from Voltage and Current

An LED circuit runs at 5 V and should draw 20 mA (0.02 A). The LED forward voltage is 2.1 V, so the resistor must drop 2.9 V.

- Known: V = 2.9 V (drop across resistor), I = 0.02 A

- Solve: R = V / I = 2.9 / 0.02 = 145 Ω (use nearest standard value: 150 Ω)

- Power dissipated in resistor: P = I² × R = 0.02² × 145 = 0.058 W (a standard 1/8 W resistor is fine)

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Example 3 - Find Voltage from Current and Resistance

A 100 Ω resistor carries 0.5 A of current. What voltage is across it?

- Known: I = 0.5 A, R = 100 Ω

- Solve: V = I × R = 0.5 × 100 = 50 V

- Power: P = I² × R = 0.25 × 100 = 25 W - this resistor needs to be rated at least 25 W, which is very large; practical circuits would reduce this current significantly.

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Frequently Asked Questions

What is Ohm's Law?+

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant. Mathematically, V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It was formulated by German physicist Georg Simon Ohm in 1827 and is the foundational relationship in electrical engineering.

What is the Ohm's Law triangle and how do I use it?+

The Ohm's Law triangle is a memory aid. Draw a triangle and place V at the top, I at the bottom-left, and R at the bottom-right. To find any value, cover it with your finger: if you cover V, you see I × R; if you cover I, you see V / R; if you cover R, you see V / I. The same principle applies to the power triangle with P, V, and I.

What is the relationship between Ohm's Law and power?+

Power (P) in watts is the rate of energy transfer in a circuit. It combines with Ohm's Law to give three equivalent power formulas: P = V × I (power equals voltage times current), P = I² × R (power equals current squared times resistance), and P = V² / R (power equals voltage squared divided by resistance). All three are derived from substituting V = IR into P = VI.

Does Ohm's Law apply to all materials?+

No. Ohm's Law applies to ohmic conductors - materials where resistance stays constant regardless of voltage and current. These include most metals at constant temperature. Non-ohmic components like diodes, LEDs, transistors, and thermistors do not follow the linear V-I relationship and cannot be analysed with the simple V = IR formula.

What units are used in Ohm's Law calculations?+

Voltage is measured in volts (V), current in amperes or amps (A), and resistance in ohms (Ω). In practical electronics, you will often encounter milliamps (mA, 1/1000 of an amp), kilohms (kΩ, 1000 ohms), and megaohms (MΩ, 1,000,000 ohms). Always convert to base SI units before calculating to avoid errors.

What is Ohm's Law and when does it apply?+

Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) through it: V = I x R, where R is resistance (Ohms). It applies to ohmic materials at constant temperature - most metals and resistors. Non-ohmic devices like diodes, transistors, and LEDs do not follow Ohm's Law because their resistance changes with voltage. For AC circuits, impedance (Z) replaces resistance but the same relationship holds: V = I x Z.

What is the relationship between power, voltage, and current?+

Electrical power (P) in watts is calculated as: P = V x I (power = voltage x current). Using Ohm's Law to substitute: P = I^2 x R = V^2 / R. These three power formulas allow calculation of power knowing any two of V, I, R. Example: a 60W light bulb on a 120V supply draws: I = P/V = 60/120 = 0.5 amps. Its resistance: R = V/I = 120/0.5 = 240 ohms. In a 230V supply: current = 60/230 = 0.26 amps.

Does Ohm's Law apply to AC circuits?+

Ohm's Law (V = IR) applies to DC circuits and to resistors in AC circuits. In AC circuits with reactive components (inductors and capacitors), the relationship becomes V = IZ, where Z is impedance (measured in ohms). Impedance combines resistance (R) and reactance (X): Z = sqrt(R^2 + X^2). Capacitive reactance is Xc = 1/(2*pi*f*C) and inductive reactance is XL = 2*pi*f*L. The phase relationship between voltage and current also changes - current leads voltage in capacitive circuits and lags in inductive circuits.

How does Ohm's Law apply to AC circuits?+

In DC circuits, V = IR with pure resistances. In AC circuits, Ohm's Law extends to impedance: V = IZ, where Z is impedance (complex number combining resistance R and reactance X). For a resistor, Z = R. For an inductor, Z = j times omega times L. For a capacitor, Z = 1/(j times omega times C). The magnitude of Z determines current amplitude; the phase angle determines the lead or lag between V and I.

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📌 Quick Tips

💡Always double-check your units before calculating. Voltage in volts, current in amperes, and resistance in ohms. Mixing milliamps with volts without converting will give wrong results.

💡In a series circuit, total resistance is the sum of all resistors. In a parallel circuit, total resistance is always less than the smallest individual resistor.

💡Power dissipated as heat in a resistor is P = I²R. Even a small current through a high resistance can generate significant heat - this is why resistor wattage ratings matter.