Voltage Current Resistance Calculator and Ohm’s Law

A voltage, current, and resistance calculator uses Ohm’s law to find one electrical quantity when you know the other two. Enter any two values in volts, amperes, or ohms; the calculator returns the missing quantity, with the equations and an example below.

Formula

Ohm’s law relates voltage, current, and resistance:

• Voltage: V = I × R
• Current: I = V ÷ R
• Resistance: R = V ÷ I

Here, V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). The calculator’s first result is resistance, R = V ÷ I. It also reports electrical power using P = V × I, where P is power in watts (W). The entered values must describe the same circuit point and operating condition.

Worked Example

With the default inputs, voltage is 12 V and current is 2 A.

1. Calculate resistance: R = V ÷ I = 12 ÷ 2 = 6 Ω.
2. Calculate power: P = V × I = 12 × 2 = 24 W.

The calculator therefore returns a resistance of 6.00 Ω and power of 24.00 W.

How Ohm’s Law Works

Voltage is the electrical potential difference that drives charge through a circuit. Current describes the rate of charge flow, while resistance describes how strongly a component opposes that flow. For a component that follows Ohm’s law, increasing voltage increases current if resistance stays the same; increasing resistance reduces current if voltage stays the same.

The law is commonly written as V = IR, but rearranging it gives the three calculation forms. To find a missing value, identify the two known quantities and use the matching equation. This calculator accepts voltage and current and derives resistance, so it is useful when you have measured those values or have them specified for a circuit.

Resistance is measured in ohms. Current is measured in amperes; small currents are often stated in milliamperes (mA), where 1 A = 1,000 mA. Voltage may be given in millivolts (mV), where 1 V = 1,000 mV. Convert values to consistent base units before calculating. For example, 500 mA is 0.5 A, not 500 A.

When to Use the Calculator

Use the calculation to estimate resistance from a known voltage and current, check a simple circuit calculation, or relate a component’s operating values. For instance, if a circuit has 9 V across a component and draws 0.03 A, its calculated resistance is 9 ÷ 0.03 = 300 Ω. Its power at that operating point is 9 × 0.03 = 0.27 W.

For a different unknown, use the corresponding rearrangement of Ohm’s law:

• Known current and resistance: multiply them to find voltage, V = I × R.
• Known voltage and resistance: divide voltage by resistance to find current, I = V ÷ R.
• Known voltage and current: divide voltage by current to find resistance, R = V ÷ I.

The equations also help check whether an answer is plausible. If a stated voltage is unchanged but resistance increases, the predicted current should decrease. If it does not, check the inputs, units, and whether the component behaves as an ohmic resistor.

Reading Results and Understanding Power

The calculator’s resistance result is a ratio of the entered voltage to current. A result of 6 Ω means that, at the stated operating point, the voltage is 6 volts per ampere of current. It does not automatically mean the component will have exactly that resistance under every condition.

The additional power result indicates the rate at which electrical energy is transferred or converted at those values. In the default example, 24 W is the calculated power at 12 V and 2 A. Power matters when selecting or evaluating components: a part must be suitable for the power it will dissipate, with appropriate design margin and attention to its specifications and cooling. A simple calculation does not establish that a circuit is safe or that a component is correctly rated.

Assumptions, Limits, and Common Mistakes

Ohm’s law in this form describes a linear, ohmic relationship: voltage and current are proportional while the relevant physical conditions remain sufficiently stable. Many resistors approximate this behavior over their specified operating range. Other devices, including diodes and lamps, can have a nonlinear relationship; their effective resistance may change with voltage, current, or temperature. For such devices, V ÷ I gives an operating-point ratio, not necessarily a fixed resistance.

• Mixing units: Convert milliamperes to amperes and millivolts to volts before using the equations.
• Dividing by zero: Resistance cannot be calculated as V ÷ I when current is zero. The result is undefined; an open circuit is not a finite resistance found by ordinary division.
• Confusing resistance and resistivity: Resistance, in ohms, applies to a particular component. Resistivity is a material property and requires additional information such as dimensions.
• Rounding too early: Keep useful precision through intermediate calculations, then round the displayed result. A calculated value may not match a standard component value exactly.
• Assuming ideal conditions: Real supplies, wires, and components have tolerances and losses. Measurements may differ from a simple prediction.

For alternating current (AC), the relationship can involve impedance rather than resistance alone. In circuits with capacitors or inductors, phase differences between voltage and current matter; multiplying RMS voltage by RMS current gives apparent power in volt-amperes, and it is not always the real power in watts. The calculator’s basic equations are most directly applicable to direct-current (DC) resistive circuits or suitable in-phase AC conditions.

Frequently Asked Questions

How do I calculate resistance from voltage and current?
Divide voltage in volts by current in amperes: R = V ÷ I. For example, 12 V ÷ 2 A = 6 Ω.

What is the formula for current?
For a resistive circuit, divide voltage by resistance: I = V ÷ R. Use volts and ohms to obtain current in amperes.

What is the formula for voltage?
Multiply current by resistance: V = I × R. With current in amperes and resistance in ohms, the result is in volts.

Can I calculate resistance if the current is zero?
No. V ÷ 0 is undefined, so the calculator cannot give a finite resistance from zero current. Check whether the circuit is open or whether the measurement is below the instrument’s resolution.

Does Ohm’s law apply to every electrical component?
No. It is a good model for ohmic components within their operating conditions, but devices such as diodes can be nonlinear. For those, voltage divided by current describes a particular operating point rather than a constant resistance.

Are volts, amps, and ohms the same in US and metric systems?
They are standard electrical units used internationally, not separate US customary and metric versions. Convert prefixes such as milli- or kilo- as needed; for example, 1 kΩ = 1,000 Ω.

Related

• Sanitize an .env File Before Sharing It
• Estimate Data Center Power Demand and Electrical Capacity
• Upper Circuit Meaning & How It Works in Stocks

in v | Voltage | V | 12 | 0
in i | Current | A | 2 | 0
out r | Resistance | ohm | v / i | 2
out p | Power | W | v * i | 2