Ohm's Law Calculator

How to use the Ohm's law calculator

Every simple resistive circuit is described by four linked quantities: voltage (U or V, in volts), current (I, in amperes), resistance (R, in ohms) and power (P, in watts). Knowing any two fixes the other two, so enter two of them and press Calculate.

• Each box has its own unit menu: millivolts to megavolts, milliamperes and amperes, ohms to megaohms, and milliwatts to gigawatts. The results are always printed in volts, amperes, ohms and watts.
• A box that is empty, or that holds 0, counts as not entered.
• If you fill in more than two, the first matching pair in this order is used: voltage and current, voltage and resistance, current and resistance, voltage and power, current and power, resistance and power.
• Negative values are accepted and give negative results, which can be useful for the direction of a current. Entering a negative resistance together with a power makes the square root undefined, and the calculator then prints nan.

Ohm's law and the power formulas

Georg Ohm found that in many conductors the current is directly proportional to the voltage across them. The constant of proportionality is the resistance:

V = I × Rvoltage = current × resistance

Electric power is the rate at which a circuit converts energy, and it is the product of voltage and current:

P = V × I

Substituting Ohm's law into the power equation gives two more forms that need only one of the electrical quantities plus the resistance:

P = I² × R = V² ÷ R

Combining these in every possible way produces the familiar twelve-formula wheel. The table below lists what the calculator uses for each pair of known values.

Worked examples

Voltage and current known. A 12 V supply drives 2 A through a load. R = 12 ÷ 2 = 6 Ω and P = 12 × 2 = 24 W.

Sizing an LED resistor. A 5 V supply feeds an LED that drops 2 V and should carry 20 mA. The resistor must absorb the remaining 3 V, so enter 3 V and 20 mA: R = 3 ÷ 0.02 = 150 Ω and P = 3 × 0.02 = 0.06 W. A common ¼ W resistor handles that easily.

Mains appliance. A 1,200 W kettle element runs from 230 V. I = 1,200 ÷ 230 ≈ 5.217391 A and R = 230² ÷ 1,200 ≈ 44.083333 Ω.

Voltage across a known resistor. A current of 250 mA passes through a 4.7 kΩ resistor. V = 0.25 × 4,700 = 1,175 V (a large figure, which shows why the milliampere and kilo-ohm menus matter) and P = 0.25² × 4,700 = 293.75 W.

Checking a power rating. A 470 Ω resistor rated ¼ W can carry at most √(0.25 ÷ 470) ≈ 23.06 mA, which corresponds to √(0.25 × 470) ≈ 10.84 V across it. Enter 470 Ω and 0.25 W to see the same pair of limits.

Limits of Ohm's law

Ohm's law holds for ohmic materials, where resistance stays constant while the voltage changes. Metals at a steady temperature are the classic case. It describes other parts only approximately:

• Temperature. A filament lamp's resistance rises sharply as it heats, so its current is not proportional to voltage.
• Semiconductors. Diodes, LEDs and transistors follow curved current-voltage relationships, not a straight line.
• Alternating current. Coils and capacitors add reactance, so the opposition to current is the impedance rather than a plain resistance. For a purely resistive load the formulas here still apply with RMS values.

Charging and discharging through a resistor is another common use: the time constant is R × C, and you can explore it with the capacitor charge calculator. Other physics tools are listed on the physics calculators page.

Common mistakes

• Mixing milliamperes and amperes. 20 mA is 0.02 A. Choose the unit in the menu instead of converting in your head.
• Forgetting the squares. Power grows with the square of current or voltage, so doubling the current through a fixed resistor quadruples the heat produced.
• Using the supply voltage for a single component. In a series circuit, only the voltage that actually falls across the part counts, as in the LED example.
• Ignoring the power rating. A resistor can have the right resistance and still burn out if it has to dissipate more than its rating.