Ohm's Law Calculator

An Ohm's Law calculator finds voltage, current, or resistance in an electrical circuit when the other two values are known, using the fundamental relationship V = I x R. It is used by electronics students, hobbyists, electricians, and engineers to design and troubleshoot circuits.

How to Use the Ohm's Law Calculator

1. Select which quantity to calculate: voltage (V), current (I), or resistance (R).
2. Enter the values for the other two quantities, selecting units from the dropdowns (volts, milliamps, kilohms, etc.).
3. Click "Calculate" to see the result with automatic unit scaling.
4. Optionally enable the power section to also calculate power (P = V x I) from the same inputs.
5. Use the circuit diagram display to confirm the relationship between the values visually.

The Formula

Ohm's Law, formulated by Georg Simon Ohm in 1827:

V = I x R

Where:

• V = Voltage (volts, V): the electrical potential difference driving current through the circuit.
• I = Current (amperes, A): the flow of electric charge past a point per unit time.
• R = Resistance (ohms, Ω): the opposition to current flow.

Rearranged forms:

• To find current: I = V / R
• To find resistance: R = V / I

Combined with the power formula (P = V x I), the full Ohm's Law wheel gives four equivalent power expressions:

• P = V x I
• P = I² x R
• P = V² / R
• V = sqrt(P x R)

This allows any two of the four quantities (V, I, R, P) to determine the other two.

Common unit prefixes in electronics:

• Milliamp (mA): 1 mA = 0.001 A
• Millivolt (mV): 1 mV = 0.001 V
• Kilohm (kΩ): 1 kΩ = 1,000 Ω
• Megohm (MΩ): 1 MΩ = 1,000,000 Ω

Real-World Example

You are building an LED circuit. The LED requires a forward voltage of 2.0 V and a current of 20 mA (0.020 A). Your supply voltage is 5 V. What resistor value do you need to protect the LED?

Step 1: Voltage drop across resistor. V_resistor = Supply voltage - LED forward voltage = 5.0 - 2.0 = 3.0 V

Step 2: Required resistance. R = V / I = 3.0 / 0.020 = 150 Ω

Step 3: Check power dissipated in the resistor. P = V x I = 3.0 x 0.020 = 0.06 W = 60 mW

A standard 150 Ω resistor rated at 1/4 W (250 mW) is more than sufficient. Choose the nearest standard resistor value from the E12 or E24 series (150 Ω is a standard E12 value, so this is a clean result).

Understanding Resistance in Series and Parallel

Real circuits often have multiple components. Ohm's Law applies to each component and the overall circuit, but resistances combine differently depending on the circuit topology.

Series circuit: resistors in series add directly. R_total = R1 + R2 + R3

The same current flows through each resistor; voltages divide proportionally.

Parallel circuit: the reciprocal of total resistance equals the sum of reciprocals. 1/R_total = 1/R1 + 1/R2 + 1/R3

The same voltage appears across each resistor; currents divide proportionally. For two resistors in parallel: R_total = (R1 x R2) / (R1 + R2).

Understanding these combinations is essential for calculating current draw, voltage drops, and power dissipation in any multi-component circuit.

Frequently Asked Questions

Does Ohm's Law apply to all materials? No. Ohm's Law applies to ohmic materials, where resistance is constant regardless of voltage and current. Most resistors are ohmic under normal conditions. Non-ohmic components, including diodes, LEDs, transistors, and thermistors, have resistance that varies with voltage, current, or temperature. For these components, Ohm's Law can still be applied at a specific operating point, but the relationship is not linear across all conditions.

What is the difference between AC and DC resistance? In direct current (DC) circuits, resistance is a straightforward opposition to current flow. In alternating current (AC) circuits, components also exhibit reactance due to inductance (coils) and capacitance (capacitors), which combine with resistance to form impedance (Z). Ohm's Law extends to AC as V = I x Z, where Z is impedance in ohms. For purely resistive AC circuits, impedance equals resistance and Ohm's Law applies identically to the DC case.

How do I use Ohm's Law to check if a fuse is correctly rated? Calculate the maximum expected current in the circuit: I = V / R (total circuit resistance). Select a fuse rated at approximately 125-150% of this maximum current to handle brief startup surges without nuisance blowing, while still protecting against sustained overcurrent. For example, a 12 V circuit with a total resistance of 2 Ω draws 6 A; a 7.5 A or 10 A fuse is appropriate.

What units does resistance come in besides ohms? Resistance is measured exclusively in ohms (Ω). Larger resistances use kilohms (kΩ, 10³ Ω) and megohms (MΩ, 10⁶ Ω). The reciprocal of resistance is conductance, measured in siemens (S), where 1 S = 1/Ω. Conductance was historically measured in mhos (ohm spelled backwards). In semiconductor physics, sheet resistance is expressed in ohms per square (Ω/sq), describing the surface resistance of thin films.

The resistor ladder around 150 ohms

The LED example above lands on exactly 150 ohms, which is a standard E12 value and therefore a clean result. Real builds often sit between two stock values instead. Leaving the 3.0 V drop fixed and moving through the nearby E12 values shows how much the LED current moves with each step.

Two readings follow. Stepping down one value to 120 ohms raises the LED current by a quarter, and many indicator LEDs are rated for a continuous 20 mA with an absolute maximum near 30 mA, so that step eats most of the margin. Stepping up to 180 ohms drops the current by a sixth and dims the LED without harming it. When the resistor value sits between two stock sizes, the usual choice is the larger one, because a slightly under-driven LED lasts longer than an over-driven one.

The power column also sets the resistor's physical size. A quarter-watt part at 60 mW runs at under a quarter of its rating, which is comfortable.

The three-resistor chain in numbers

Series and parallel combinations are easier to trust once the currents and drops are worked through. Take three resistors of 100, 220 and 470 ohms and a 5 V supply.

In series the resistances add: 100 + 220 + 470 = 790 ohms. The current is the same through all three at 5 divided by 790, which gives 6.329 mA. Each resistor then drops a share of the 5 V in proportion to its resistance.

The three drops add to 5.0000 V and the three powers add to 31.646 mW, which is the 5 V multiplied by 6.329 mA. Both totals reconcile, and that reconciliation is the check worth running when a series circuit returns an odd figure.

In parallel the reciprocals add. The total for these three is 59.98 ohms, and the two-resistor shortcut of the product over the sum agrees: 100 in parallel with 220 gives 68.75 ohms, and that pair in parallel with 470 gives 59.98 ohms. Any two of the three on their own give the same answer by either route.

A divider is the same arithmetic read from the other end. Across a 100 and a 220 ohm pair on 5 V, the share across the 220 ohm resistor is 5 x 220 / 320, which is 3.4375 V, at a current of 15.625 mA. A designer drawing current from the midpoint changes both figures, because the load sits in parallel with the lower leg. A 100 ohm resistor alone across 5 V takes 50 mA and burns 0.25 W, which is the full rating of a quarter-watt part and too close to it for a permanent build.

Stepping to the nearest preferred value

Stock resistors are not made in every value. IEC 60063 defines the preferred series, and the two most common have fixed base values per decade.

E12 holds twelve values per decade: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8 and 8.2. E24 holds twenty-four, adding 1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5 and 9.1 to the E12 set. Multiply any base value by a power of ten to reach the rest of the range.

Between 60 and 300 ohms, the E12 values are 68, 82, 100, 120, 150, 180, 220 and 270. E24 adds 75, 91, 110, 130, 160, 200 and 240 to that list. A calculation that returns 150 ohms is exact in both series. A calculation that returns 137 ohms has no exact part: the nearest E24 value is 130 ohms, which is 5.11% low, and 150 ohms is 9.49% high. At 137 ohms the LED current would be 21.90 mA, and at 130 ohms it is 23.08 mA, both above the 20 mA target and both workable for an indicator LED. A figure of 173 ohms steps to 180 ohms, which is 4.05% high and gives 16.67 mA.

Tolerance decides how much of this matters. An E12 part is usually sold at 5% and an E24 part at 1% or 2%, so a 5% resistor marked 150 ohms can measure anywhere from 142.5 to 157.5 ohms. Picking between two neighbours that sit within a few percent of each other is less precise than the tolerance of the part itself.

Power rating and the headroom rule

A resistor's wattage rating is a maximum, not a target. Dissipating 60 mW in a quarter-watt part uses 24% of the rating and leaves a 4.2 times margin. In a half-watt part the figure falls to 12%, and in a one-watt part to 6%.

Derating matters where a resistor sits near something hot. Running at half the rated power is a common industrial rule, and it costs a larger body rather than a different resistance. Read in the other direction, a 150 ohm quarter-watt resistor carries up to the square root of 0.25 divided by 150, which is 40.82 mA, at 6.12 V across it. Derated to half, that ceiling falls to 28.87 mA. Both numbers sit above the 20 mA a typical indicator LED wants, which is why a quarter-watt part is the usual choice for a single-LED build and a half-watt part appears once several LEDs share one resistor.

Sources: IEC 60063 for the E12 and E24 preferred value series. The Ohm's Law and power identities are the ones already stated on this page.

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