Engineering

Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values to find the remaining parameters instantly.

Did this calculator help you?

What is Ohm's Law Calculator?

Ohm's Law is a fundamental principle in electrical engineering and physics that describes the relationship between voltage, current, resistance, and power in an electrical circuit. Named after German physicist Georg Ohm, this law is the foundation for understanding how electrical circuits work and is essential for anyone working with electronics. Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance. This simple relationship allows you to calculate any one of these values if you know the other two, making it an indispensable tool for circuit design, troubleshooting, and analysis.

When to Use This Calculator

  • Circuit design — sizing resistors, loads, and supplies from known voltage and current.
  • Troubleshooting — finding a missing voltage, current, or resistance that explains a fault.
  • Electrical safety — estimating body current to understand shock risk in real situations.
  • Wire sizing — calculating voltage drop and cable heating for an installation.
  • Component selection — checking a resistor or device is rated for the power it will dissipate.
  • Physics and electronics education — solving Ohm's Law problems with V = IR and P = VI.

Steps:

  1. Select which value you want to calculate (voltage, current, resistance, or power).
  2. Enter the two known values in the corresponding fields.
  3. The calculator computes the unknown value instantly.
  4. View the power dissipation as a bonus calculation.
  5. Use the results for circuit design or troubleshooting.

Formula

V = I × R (Voltage = Current × Resistance) I = V / R (Current = Voltage ÷ Resistance) R = V / I (Resistance = Voltage ÷ Current) P = V × I (Power = Voltage × Current) Where: V = Volts, I = Amperes, R = Ohms, P = Watts

Use Cases

  • Designing electrical circuits for electronics projects
  • Troubleshooting household electrical problems
  • Sizing resistors and components for LED circuits
  • Understanding power consumption of electrical devices

Key Benefits

  • Calculate voltage current resistance power instantly
  • Solve unknown electrical values from two known
  • Verify circuit designs for safety compliance
  • Multiple power formulas complete analysis

Pro Tips

  • Ohm Law triangle V over I times R
  • Check power against component rated wattage
  • Series current constant voltage divides

Common Mistakes to Avoid

  • AC DC confusion Ohm Law direct DC not AC
  • Forgetting unit conversions milliamp to amp
  • Wrong power formula incorrect wattage

Key Terms Explained

Voltage V: Electrical potential volts
Current I: Charge flow amperes
Resistance R: Opposition ohms
Power P: Energy transfer rate watts

Related Concepts

  • Voltage Drop: Long cables lose voltage proportional to current times resistance. Our voltage drop calculator sizes conductors to keep that loss within limits.
  • Electrical Power: Power P = VI is the natural companion to Ohm's Law. Our power calculator computes electrical and mechanical power side by side.
  • Resistor Values: Real resistors use standard colour bands rather than printed numbers. Our resistor color code calculator decodes the bands to an ohm value and tolerance.
  • Energy and Work: Electricity consumed is power multiplied by time. Our work calculator relates force, distance, and energy, the mechanical counterpart of electrical energy.
  • Unit Conversion: Voltage, current, and resistance appear in many units and prefixes. Our unit converter converts between electrical and other unit families.

Example

A circuit has a 12V battery connected to a 4Ω resistor. To find the current: I = V/R = 12/4 = 3 amperes. The power dissipated by the resistor: P = V × I = 12 × 3 = 36 watts. This means the resistor must be rated for at least 36 watts to avoid overheating.

Interpreting Your Results

The calculator solves any unknown in the circuit relationship from the two values you enter: voltage (V), current (I), and resistance (R) follow V = IR, and power is computed alongside as P = VI (equivalently I²R or V²/R). The result shown is the quantity you asked for, with the power figure always available as a bonus. Keep inputs in base SI units — volts, amperes, and ohms — and convert first if your readings are in milliamps (÷1,000) or kilohms (×1,000). A larger resistance for the same voltage means a smaller current and lower power; a larger current for the same resistance means proportionally more heat, since heating grows with the square of the current.

Frequently Asked Questions

What does Ohm's Law tell us?
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It states that current is directly proportional to voltage and inversely proportional to resistance.
When doesn't Ohm's Law apply?
Ohm's Law applies to ohmic materials (like most metals) at constant temperature. It doesn't apply to semiconductors, diodes, transistors, or materials where resistance changes with voltage or temperature.
How is power related to Ohm's Law?
Electrical power (P = V × I) can be combined with Ohm's Law to give P = I²R or P = V²/R. These formulas help calculate how much energy a component dissipates as heat.
How do I choose the right resistor for an LED circuit?
An LED is not a resistor — it holds a roughly fixed forward voltage drop (about 2 V for red, 3 V for blue/white) and must be current-limited or it will burn out. The series resistor value comes from R = (Vsupply − Vled) / I. With a 12 V supply, a 2 V LED, and a target current of 20 mA (0.02 A): R = (12 − 2) / 0.02 = 500 Ω. Use the nearest standard value, commonly 470 Ω or 560 Ω. Check the resistor's power too: it must dissipate I²R = 0.02² × 470 ≈ 0.19 W, so a quarter-watt resistor works, but if you drive the LED harder or from a higher voltage, upgrade to a 0.5 W or 1 W part.
What is voltage drop, and why does it matter on long cables?
Every conductor has resistance, and by Ohm's Law that resistance turns current into lost voltage: V = I × R. The wire's resistance is small (a typical metre of house wire is under a hundredth of an ohm), but over long runs it adds up. A run with 0.5 Ω of total resistance carrying 10 A drops 5 V, so a 12 V supply arrives at the load as only 7 V. Wiring standards usually allow at most 3–5% voltage drop. Undersized cables cause dim lights, weak motors, and waste energy as heat, which is why the voltage drop calculator and proper wire-sizing tables are standard tools for every installation.
How do I analyse resistors in series and in parallel?
In series, resistances add: two 10 Ω resistors give 20 Ω, so a 12 V supply drives I = V/R = 12/20 = 0.6 A, with the voltage split evenly (6 V across each, since the current is identical through both). In parallel, the combined resistance is lower than either alone: two 10 Ω resistors give R = 10/2 = 5 Ω, so the same 12 V drives 12/5 = 2.4 A, with each resistor carrying 1.2 A. The general rules are R(total) = R₁ + R₂ for series, and 1/R(total) = 1/R₁ + 1/R₂ for parallel. Series divides voltage, parallel divides current — exactly what this calculator's series and parallel sections compute.
Why is a 230 V or 120 V shock dangerous, and how does body resistance matter?
Danger comes from current, not voltage, and Ohm's Law sets the current: I = V/R, where R is the human body's resistance. A dry body is roughly 100,000 Ω, so a 230 V contact delivers about 2.3 mA — a mild tingle that many people have felt. Wet skin drops resistance to about 1,000 Ω, and the same 230 V then pushes 230 mA, far above the roughly 30 mA that can cause ventricular fibrillation and the 50 mA that can stop breathing. This is why bathrooms and kitchens use residual current devices (RCDs) that trip at 30 mA or less, and why electrical work is done with dry hands and insulated tools.
How do I use a multimeter to measure voltage, current, and resistance?
A multimeter applies Ohm's Law in three different modes. For voltage, connect the meter across the component in parallel while the circuit is live — voltage is always measured across two points. For current, the meter must be wired in series so the current actually flows through it, and you start on the highest range, since a meter across a live circuit in the wrong mode acts like a short. For resistance, the component must be disconnected from the power and ideally removed from the circuit, because the meter applies its own test voltage to measure R = V/I. Never measure resistance on a live circuit, and always return the leads to the voltage position after current measurements.
What determines how much resistance a wire or material has?
Resistance is set by the material and the geometry: R = ρL/A, where ρ is the material's resistivity, L the length, and A the cross-sectional area. Copper has a resistivity of about 1.7 × 10⁻⁸ Ω·m, making it an excellent conductor, while nichrome (used in heaters) is roughly 60 times higher, and rubber or glass are insulators with resistivities billions of times greater. Doubling a wire's length doubles its resistance; doubling its cross-sectional area halves it — which is why thick cables (large A) carry high currents with low loss. Temperature also matters: most metals increase in resistance when hot, which is exactly why Ohm's Law holds strictly only at constant temperature.
Why do cables get warm, and how can I tell if a wire is undersized?
Current through resistance produces heat because the power dissipated is P = I²R. A cable with 0.5 Ω of resistance carrying 20 A generates 0.5 × 20² = 200 watts of heat, which is a serious fire risk if the wire can't shed it. Undersized cables get warm, hot, or even glowing under load. The checks follow Ohm's Law: measure the voltage at the supply and at the load — a large difference means excessive wire resistance; feel the cable after 30 minutes of full load; and size conductors so the running current stays below the wire's ampacity rating. Heating is exactly why fuses and breakers trip: they are sized so the current can never drive I²R heat past what the wiring tolerates.
Does Ohm's Law still work with alternating current (AC)?
Ohm's Law in its simple form V = IR applies directly to direct current (DC) and to AC circuits with purely resistive loads like heaters and incandescent bulbs. But AC circuits with motors, transformers, and capacitors contain reactance — energy stored and released as the voltage and current alternate — so the ratio of voltage to current becomes impedance Z, not plain resistance, and is written V = IZ. Reactance depends on frequency, which is why Ohm's Law alone can't predict the current in an inductive or capacitive circuit. For those, engineers use impedance with phase angles and power factors, while DC electronics, batteries, and simple heating circuits remain straight Ohm's Law territory.

Discover More Tools

Fresh picks from across our tool library.

SaaS Burn Rate Calculator

Letter Grade Calculator

Gravitational Force Calculator

Drywall Calculator

Roth IRA Calculator

Quadratic Formula Calculator