Engineering

Ohms Law Calculator

Solve for voltage, current, resistance, and power in a DC resistive circuit. Enter any two known values below — the calculator finds the rest and flags resistor power derating.

ohms-law

Fill in exactly two of the three fields (Voltage, Current, Resistance) and leave the rest blank. This tool assumes an ideal, purely resistive DC circuit — see design limitations below before applying results to safety-critical or AC work.

Known values (enter any two)

Optional: resistor rating check

Circuit values
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Voltage—
Current—
Resistance—
Power—
Suggested resistor rating—
Rating check—

How this calculator solves the circuit

Ohm's Law relates three quantities in a simple resistive DC circuit: voltage (V, in volts), current (I, in amps), and resistance (R, in ohms). Because any one of these can be derived from the other two, entering two known values is enough to fully solve the circuit. Once voltage and current are known (directly or derived), power in watts follows automatically.

Internally, every input is first converted to base SI units (volts, amps, ohms) using the unit selectors, so you can mix mV, kΩ, or µA without doing manual conversions. Results are then converted back into a readable, appropriately-scaled unit for display.

Formula steps

Step 1 — Identify the two known values (V, I, or R). Step 2 — Solve for the missing value: If V and I are known: R = V ÷ I If V and R are known: I = V ÷ R If I and R are known: V = I × R Step 3 — Solve for power (once V and I are both known): P = V × I P = I² × R (equivalent form) P = V² ÷ R (equivalent form) Step 4 — Compare calculated power to the resistor's rated power. A common design guideline is to select a component rated for at least 2× the calculated dissipation (i.e., keep actual load under ~50% of the rated power) to leave thermal headroom.

Resistor power derating guidance

A resistor's printed power rating (e.g., 0.25W, 1W, 5W) is typically a maximum figure measured under specific test conditions, usually with generous airflow at a moderate ambient temperature. In an enclosed project box, a hot environment, or with poor airflow, a resistor can run hotter than expected at the same calculated wattage.

Because of this, many engineers apply a derating margin rather than running a part at its rated limit. A widely used rule of thumb is to keep continuous dissipation at or below 50% of the component's rated power — in other words, choose a part rated for roughly double the calculated wattage. For components operating at elevated ambient temperatures, consult the manufacturer's derating curve, since power handling capacity typically decreases as ambient temperature rises above the component's rated reference temperature.

Electrical code cautions

This calculator is intended for basic component-level estimation, such as sizing a current-limiting resistor or sanity-checking a simple circuit. It is not a substitute for applicable electrical code requirements when working with building wiring, branch circuits, or any installation subject to inspection.

In particular: conductor ampacity and overcurrent protection sizing (breakers/fuses) should follow the relevant code for your jurisdiction — for example, NEC Article 310 and Table 310.16 in the United States — rather than a simplified resistive calculation. Continuous loads are commonly required to be sized at no more than 80% of a circuit's rating. Voltage drop over real wire runs, connector and contact resistance, and derating for conduit fill or ambient temperature are not accounted for here. For any circuit involving mains voltage, fixed wiring, or life-safety systems, consult a licensed electrician or engineer.

Design limitations of this calculator

  • Ideal resistive assumption: results assume a linear, purely resistive load. Diodes, LEDs, motors, and other non-linear or reactive components do not follow a constant V/I ratio.
  • DC only: this tool does not model AC impedance, reactance, phase angle, or frequency-dependent behavior.
  • No temperature coefficient: real resistors and conductors change resistance with temperature; this calculator uses the single resistance value you enter.
  • No source impedance: real batteries and supplies have internal resistance that can reduce the voltage actually delivered to a load under current draw.
  • No wire or contact resistance: voltage drop across real-world wiring, connectors, and switches is not included in the circuit model.
  • Component tolerance ignored: resistors typically carry ±1% to ±10% tolerance, and actual measured values will vary from nominal.

Frequently asked questions

  • Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R. It states that the voltage across a conductor is directly proportional to the current flowing through it, provided resistance stays constant. Combined with the power formula P = V × I, these four equations let you solve for any one quantity if you know two others.
  • Resistance equals voltage divided by current: R = V ÷ I. For example, a 12V supply driving 2A through a load means the load has a resistance of 12 ÷ 2 = 6 ohms. This calculator does this automatically when you enter voltage and current and leave resistance blank.
  • Current equals voltage divided by resistance: I = V ÷ R. For example, a 9V battery connected across a 470 ohm resistor produces 9 ÷ 470 ≈ 0.019A, or about 19mA. This is one of the most common calculations for sizing current-limiting resistors on LEDs and sensors.
  • Power equals voltage multiplied by current: P = V × I. If resistance and current are known instead, use P = I² × R. If voltage and resistance are known, use P = V² ÷ R. All three forms are mathematically equivalent and this calculator uses whichever pair of values you provide.
  • A common engineering guideline is to choose a resistor rated for at least double the calculated power dissipation, sometimes called 50% derating. For example, if a calculation shows 0.4W dissipation, a 1W resistor is a safer choice than a 0.5W resistor tightly at the edge of its rating, especially in enclosed spaces with limited airflow. Manufacturer derating curves for high ambient temperatures should also be consulted for critical designs.
  • Ohm's Law in its basic V = I × R form applies directly to purely resistive DC circuits. AC circuits with inductors or capacitors introduce reactance and phase shift, so the equivalent relationship becomes V = I × Z, where Z is impedance (a complex quantity combining resistance and reactance). This calculator assumes a simple resistive relationship and is not intended for reactive AC circuit analysis.
  • Ohm's Law gives the theoretical current for an ideal resistive load, but real wiring has its own resistance, connectors introduce voltage drop, and ambient temperature affects conductor performance. Wire ampacity and overcurrent protection sizing should always follow the applicable electrical code (such as NEC Article 310 in the US) rather than a simplified calculator, particularly for fixed building wiring or safety-critical circuits.
  • Frequent errors include mixing up units (milliamps vs. amps, kilohms vs. ohms), forgetting that resistance changes with temperature in real components, ignoring the internal resistance of a power source, and applying the formula to non-linear devices like diodes or LEDs, which do not have a constant resistance across their operating range.
  • Yes, this is one of its most common uses. Subtract the LED's forward voltage from the supply voltage to get the voltage that must be dropped across the resistor, then divide by the desired LED current: R = (Vsupply − Vf) ÷ I. This calculator can be used for that step by entering the voltage to be dropped and the desired current to solve for resistance.