Ohm's Law Calculator

What Is Ohm's Law?

Ohm's law describes how voltage, current and resistance relate in an electric circuit. The current through a conductor is proportional to the voltage across it, and the constant that links them is the resistance: V = I × R. The German physicist Georg Simon Ohm (1787 to 1854) was the first to demonstrate the relationship experimentally. OpenStax College Physics describes it as an empirical law, like the law of friction, that holds for many materials but not all of them.

Think of voltage as the push, current as the flow and resistance as the restriction. Doubling the voltage across a fixed resistor doubles the current, and doubling the resistance at a fixed voltage halves it. Materials that keep a constant resistance over a wide range of voltage are called ohmic. Copper wire and ordinary resistors are ohmic, while diodes, LEDs and superconductors are not.

This Ohm's law calculator solves the equation in any direction. Enter any two of voltage, current, resistance and power, and it returns the other two along with the exact formulas it used, so you can repeat the working by hand or check homework answers.

Units: Volt, Ampere, Ohm and Watt

All four quantities use SI units, and every formula on this page assumes base units. The table lists each unit as NIST defines it in its Guide to the SI.

Prefixes scale a unit by powers of 1,000: milli (m) is 0.001, micro (µ) is 0.000001, kilo (k) is 1,000 and mega (M) is 1,000,000. To convert mA to A, divide by 1,000, so 250 mA = 0.25 A. To convert kΩ to Ω, multiply by 1,000, so 4.7 kΩ = 4,700 Ω and 2.2 MΩ = 2,200,000 Ω.

The calculator converts every input to base units before it applies a formula, shows each conversion in the steps, and writes results back with the clearest prefix, such as 250 mA instead of 0.25 A or 4.7 kΩ instead of 4,700 Ω.

The Ohm's Law Triangle, Wheel and All 12 Formulas

The Ohm's law triangle puts V on top with I and R side by side underneath. Cover the quantity you want and the other two show the operation: covering V leaves I × R, covering I leaves V over R, and covering R leaves V over I. The Ohm's law wheel extends the triangle with power, giving three formulas for each of the four quantities.

Power comes from P = V × I. Substituting V = I × R gives P = I² × R, and substituting I = V ÷ R gives P = V² ÷ R. Solving those for the other quantities fills out the wheel. Every row below uses the same circuit, 12 V across 6 Ω carrying 2 A and dissipating 24 W, so each formula returns the same answer.

How to Calculate Resistance, Current and Voltage Manually

Every manual Ohm's law calculation follows three steps: convert both known values to base units, pick the formula from the wheel that uses exactly those two quantities, and substitute. Keep at least four significant figures until the final answer.

How to calculate resistance

Divide voltage by current. A 120 V incandescent bulb rated at 60 W draws 60 ÷ 120 = 0.5 A, so its hot resistance is 120 ÷ 0.5 = 240 Ω. The same answer comes from R = V² ÷ P = 14,400 ÷ 60 = 240 Ω.

How to calculate current

Divide voltage by resistance. A 9 V battery across a 1 kΩ resistor drives 9 ÷ 1,000 = 0.009 A, which is 9 mA. The resistor dissipates 9 × 0.009 = 0.081 W, or 81 mW, well inside a 1/4 W rating.

How to calculate voltage

Multiply current by resistance. A 20 mA current through a 220 Ω resistor drops 0.02 × 220 = 4.4 V across it. OpenStax calls this voltage the IR drop, and it is how a series resistor takes up the difference between a supply and a load.

Electric Power Formulas: P = VI, P = I²R and P = V²/R

Power is the rate at which a circuit converts electrical energy into heat, light or motion. One watt is one joule per second, and one volt times one ampere equals one watt. P = V × I applies to any DC load. The resistance forms, P = I² × R and P = V² ÷ R, apply to resistors and other resistive loads, and to AC resistive loads when you use rms values.

Because power rises with the square of current, a small rise in current produces a much larger rise in heat. Doubling the current through a 6 Ω resistor from 2 A to 4 A raises the power from 24 W to 96 W.

Resistance also changes with temperature. OpenStax College Physics works through a car headlight that draws 2.50 A at 12.0 V when hot, which is 4.80 Ω and 30.0 W. Its cold resistance is only 0.350 Ω, so at switch on it briefly draws about 12 ÷ 0.35 = 34.3 A and 411 W. Ohm's law holds at each instant, but the resistance value is not fixed.

The results also convert power into heat output with the NIST factor 1 BTU per hour = 0.2930711 W, which makes 1 W = 3.41214 BTU/h, and into energy per day in watt hours for a load left on for 24 hours. To turn that energy into a running cost, use the Electricity Calculator.

LED Resistor Calculation

An LED needs a resistor in series because its current climbs steeply once the voltage passes its forward voltage. The resistor takes up the difference between the supply and the LED, and Ohm's law sizes it from that difference.

Open the LED option under the inputs and enter the forward voltage. The calculator subtracts it from the supply before it applies Ohm's law, and the LED quick example loads the red LED case. When the exact result is not a standard value, round up to the next higher one so the current stays at or below the target. The results card lists that value as the next higher E24 resistor.

Resistor tolerance also shifts the current. A 150 Ω resistor with 5% tolerance can measure from 142.5 to 157.5 Ω, which gives between 21.05 mA and 19.05 mA with 3 V across it.

Standard Resistor Values (E12 and E24) and Wattage Ratings

Resistors are made in preferred values defined by IEC 60063. Each E series divides a decade into a fixed number of steps spaced on a roughly logarithmic scale: E12 has 12 values per decade and E24 has 24. The standard pairs E12 with a 10% tolerance and E24 with 5%, and E12 is every second value of E24.

The calculator finds the nearest value in each series by ratio rather than plain difference, because the series are spaced logarithmically. For 6 Ω, the nearest E24 value is 6.2 Ω, 3.3% above, and the nearest E12 value is 5.6 Ω, 6.7% below. To read a value from its color bands, use the Resistor Calculator.

Common resistor power ratings include 1/8, 1/4, 1/2, 1, 2 and 5 W. A common design practice is to choose a rating at least twice the calculated dissipation, which keeps the part cooler and allows for supply variation. The calculator applies that 2× margin: 60 mW needs 120 mW, so a 1/8 W (125 mW) resistor qualifies. Check the datasheet for the rated temperature and any derating.

Series and Parallel Resistance

When resistors share one path, the same current flows through each and their resistances add. When they sit side by side across the same voltage, each carries its own current and the combination has less resistance than the smallest branch. OpenStax College Physics derives both rules from Ohm's law and conservation of charge.

Two equal resistors in parallel give half the value, so two 1 kΩ resistors make 500 Ω. Find the combined value first, then enter it as the resistance in this calculator to get the total current and power drawn from the source. Wire is a series resistance too, and the Voltage Drop Calculator applies V = I × R to long cable runs.

Limitations: Non Ohmic Devices, AC Circuits and Short Circuits

Ohm's law holds only where resistance stays constant. Diodes and LEDs conduct very little below their forward voltage and far more above it, so a single resistance value cannot describe them. Lamp filaments and heating elements follow Ohm's law at any instant, but their resistance rises as they heat, so a cold reading with a meter does not predict the running current.

In AC circuits the formulas work for resistive loads such as heaters, kettles and incandescent lamps when you use rms voltage and current. Motors, transformers and electronic power supplies add inductance or capacitance, so the ratio of voltage to current becomes an impedance. HyperPhysics notes that the resistor power formulas hold for AC with rms values, while the general case adds a power factor term.

A short circuit is a path with almost no resistance. Because I = V ÷ R, the current is limited only by the wiring and the source. If a 12 V battery and its leads total 0.05 Ω, the current is 12 ÷ 0.05 = 240 A and the power is 12 × 240 = 2,880 W, which can overheat wiring and start a fire. That is why the calculator refuses R = 0 and explains the short instead of showing an infinite current.

Frequently Asked Questions

Sources & References

• National Institute of Standards and Technology. NIST Guide to the SI (SP 811), Chapter 4: The Two Classes of SI Units and the SI Prefixes (watt, volt, ohm and siemens definitions; SI prefixes). nist.gov
• National Institute of Standards and Technology. SI Units: Electric Current (ampere defined by the elementary charge; 1 V = 1 W/A; 1 Ω = 1 V/A). nist.gov
• National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.8: Factors for Units Listed by Kind of Quantity (1 Btu per hour = 0.2930711 W). nist.gov
• OpenStax. College Physics 2e, Section 20.2: Ohm's Law, Resistance and Simple Circuits (I = V/R, ohmic materials, IR drop). openstax.org
• OpenStax. College Physics 2e, Section 20.4: Electric Power and Energy (P = IV, P = V²/R, P = I²R; hot and cold headlight example). openstax.org
• OpenStax. College Physics 2e, Section 21.1: Resistors in Series and Parallel. openstax.org
• HyperPhysics, Georgia State University. Ohm's Law and DC Electric Power (resistor power with rms values in AC). gsu.edu, gsu.edu
• International Electrotechnical Commission. IEC 60063:2015 Preferred number series for resistors and capacitors, publication page, and the Slovenian Institute for Standardization preview of Table 1 (E24 and E12 values) and the tolerance table. iec.ch, sist.si

Results are estimates for ideal resistive circuits and are meant for education and low voltage hobby work. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.