Voltage Calculator

Q: What is the difference between voltage and current?

Voltage is the push . Current is the flow . Voltage is the energy pressure between two points, measured in volts (V). Current is how much charge actually moves each second, measured in amps (A). 4 In a water pipe, voltage is the pressure and current is the water moving through. You can have pressure with no flow, but you cannot have flow without pressure.

Q: How many volts are dangerous to a person?

OSHA requires exposed live parts at 50 volts or more to be guarded so people cannot touch them by accident. 9 But volts alone do not hurt you, current does. 8 For a 1-second shock from hand to foot, OSHA lists painful shock and loss of muscle control at about 6 to 30 mA, and 50 to 150 mA can stop your breathing and may kill. 9 Skin resistance decides how much current flows. Dry skin of about 200 kΩ lets only 0.6 mA through from 120 V. 8 Soaking-wet skin can drop to about 10 kΩ, and the same 120 V then pushes 12 mA, enough that you may not be able to let go. 8

Q: How do you measure voltage with a multimeter?

Follow these steps: Put the black probe in the COM jack and the red probe in the V jack. Turn the dial to DC volts (V⎓) for batteries, or AC volts (V∼) for outlets. Touch the probes across the part, not through it. The meter goes in parallel . 1 Never measure volts with the dial set to amps. That shorts the circuit and can blow the fuse or the meter.

Q: What is the difference between AC and DC voltage?

DC (direct current) voltage stays steady and pushes in one direction. 5 Batteries, USB ports, and solar panels give DC. AC (alternating current) voltage flips direction many times a second. It runs through 60 full cycles a second in the US and 50 in Europe. 5 Wall outlets give AC. The 120 V on a US outlet is not its peak value, and the real peak is about 170 V. 5 The outlet figure is an RMS value, which heats a resistor exactly as much as the same number of DC volts would. 10

Voltage Calculator

Introduction

This voltage calculator finds voltage (V) from any two of these three values: current (I), resistance (R), or power (P).

It uses three simple rules from Ohm's law and the power formula:

V = I × R, when you know current and resistance
V = P ÷ I, when you know power and current
V = √(P × R), when you know power and resistance

You can pick the units you want, like milliamps, amps, ohms, kilohms, watts, or kilowatts. The calculator changes them to base units before it solves, so you don't have to.

Along with the answer, you get a step-by-step solution, a table with all the other values (current, resistance, and power), and a chart that shows how voltage changes when one value goes up or down. That covers homework, lab work, or checking a circuit at home.

How to use our Voltage Calculator

Type any two of these three values (current, resistance, or power) and the calculator gives you the voltage, plus a step-by-step solution and a chart.

Current (I): Enter the current flowing through the part, then pick the unit you want: µA, mA, A, or kA.

Resistance (R): Enter how much the part resists the current, then choose mΩ, Ω, kΩ, or MΩ.

Power (P): Enter the power used by the part, then choose mW, W, kW, or MW. Leave one of the three boxes blank.

Voltage (V): This box is the answer, so you cannot type in it. Switch its unit to mV, V, or kV to see the result change right away.

Calculate: Click this button to solve, or just press Enter. The steps, the full table of values, and the graph update at the same time.

Reset: Click this to clear all boxes and start a new problem. Use Copy Result to copy the voltage answer.

What Is Voltage?

Voltage is the push that moves electric charge through a wire. It is also called potential difference.³ Think of a water pipe: voltage is the water pressure, current is how much water flows, and resistance is how narrow the pipe is. Voltage is measured in volts (V), named after Alessandro Volta.³

How Voltage Is Calculated

Voltage links to current, resistance, and power. If you know any two of these, you can find the voltage with one of these formulas:

V = I × R, current times resistance (Ohm's law)¹
V = P ÷ I, power divided by current²
V = √(P × R), the square root of power times resistance²

Example: a bulb pulls 2.5 amps through 5 ohms of resistance. Voltage = 2.5 × 5 = 12.5 volts.

The Key Terms

Current (I) is how much charge flows each second, measured in amperes (A).⁴
Resistance (R) is how hard it is for charge to flow, measured in ohms (Ω).
Power (P) is how fast energy is used, measured in watts (W).²
Voltage (V) is the energy given to each unit of charge, measured in volts (V).³

Common Units

Voltage values can be very small or very big, so we use prefixes. 1 millivolt (mV) = 0.001 V. 1 kilovolt (kV) = 1,000 V. The same idea works for current (µA, mA, A, kA), resistance (mΩ, Ω, kΩ, MΩ), and power (mW, W, kW, MW). Always change values to base units before you do the math, then convert the answer back.

Everyday Voltage Values

AA battery: 1.5 V
Car battery: 12 V³
USB charger: 5 V
Home outlet: 120 V (US) or 230 V (Europe)
Power lines: 100,000 V or more

Why It Matters

Knowing voltage helps you pick the right battery, resistor, or power supply. Too little voltage and a device will not run. Too much voltage can burn parts or start a fire. Students use these formulas in physics class, and electricians and engineers use them every day when they design and fix circuits.

Things to Remember

Ohm's law works best for simple direct current (DC) circuits with steady resistance. In alternating current (AC) circuits, parts like coils and capacitors add impedance, so the math gets harder. Real parts also heat up, and heat can change resistance.¹ Use these results as a close guide, not an exact promise.

Formulas used

Voltage from current and resistance (Ohm's law) ¹

V = I \times R

Voltage from power and current ²

V = \frac{P}{I}

Voltage from power and resistance ²

V = \sqrt{P \times R}

Power derived from voltage and current ²

P = V \times I

Resistance derived from voltage and current ¹

R = \frac{V}{I}

Current derived from voltage and resistance ¹

I = \frac{V}{R}

Unit conversion to and from SI base units

X_{\mathrm{SI}} = X_{\mathrm{input}} \times k_{\mathrm{unit}}, \qquad X_{\mathrm{display}} = \frac{X_{\mathrm{SI}}}{k_{\mathrm{unit}}}

Frequently asked questions

What is the difference between voltage and current?

Voltage is the push. Current is the flow.

Voltage is the energy pressure between two points, measured in volts (V). Current is how much charge actually moves each second, measured in amps (A).⁴

In a water pipe, voltage is the pressure and current is the water moving through. You can have pressure with no flow, but you cannot have flow without pressure.

Can you have voltage without current?

Yes. A fresh AA battery sitting on a table still has 1.5 volts across its ends, but no current flows.

Current only flows when there is a closed path (a circuit).⁴ Break the path with an open switch and the voltage is still there, waiting, while the current drops to zero.

This is why a wall outlet is still dangerous even with nothing plugged in.

How many volts are dangerous to a person?

OSHA requires exposed live parts at 50 volts or more to be guarded so people cannot touch them by accident.⁹

But volts alone do not hurt you, current does.⁸ For a 1-second shock from hand to foot, OSHA lists painful shock and loss of muscle control at about 6 to 30 mA, and 50 to 150 mA can stop your breathing and may kill.⁹

Skin resistance decides how much current flows. Dry skin of about 200 kΩ lets only 0.6 mA through from 120 V.⁸ Soaking-wet skin can drop to about 10 kΩ, and the same 120 V then pushes 12 mA, enough that you may not be able to let go.⁸

What is voltage drop and how do you find it?

Voltage drop is the voltage lost in a wire or part as current moves through it.

Use Ohm's law: V_drop = I × R.¹

Example: 10 amps flowing through a wire with 0.2 ohms of resistance loses 10 × 0.2 = 2 volts. If you started at 12 V, only 10 V reaches the device.

Long or thin wires have more resistance, so they drop more voltage. That is why lights dim at the far end of a long run.

Do batteries connected in series add up their voltage?

Yes. In series (end to end), the voltages add.

Two 1.5 V AA batteries in series = 3 V
Four 1.5 V AA batteries in series = 6 V

In parallel (all plus ends together), the voltage stays at 1.5 V, but the batteries last longer because they share the current.

Why is voltage the same across parts in a parallel circuit?

In a parallel circuit, every branch connects to the same two points. Those two points have one voltage between them, so every branch gets that same voltage.⁷

That is why home outlets are wired in parallel.⁷ Each one gets the full 120 V (or 230 V), no matter how many things you plug in.

In a series circuit it is different. There the voltage splits between the parts, and the biggest resistor takes the biggest share.⁷

How do you measure voltage with a multimeter?

Follow these steps:

Put the black probe in the COM jack and the red probe in the V jack.
Turn the dial to DC volts (V⎓) for batteries, or AC volts (V∼) for outlets.
Touch the probes across the part, not through it. The meter goes in parallel.¹

Never measure volts with the dial set to amps. That shorts the circuit and can blow the fuse or the meter.

What is the difference between AC and DC voltage?

DC (direct current) voltage stays steady and pushes in one direction.⁵ Batteries, USB ports, and solar panels give DC.

AC (alternating current) voltage flips direction many times a second. It runs through 60 full cycles a second in the US and 50 in Europe.⁵ Wall outlets give AC.

The 120 V on a US outlet is not its peak value, and the real peak is about 170 V.⁵ The outlet figure is an RMS value, which heats a resistor exactly as much as the same number of DC volts would.¹⁰

Why does a battery's voltage drop when it powers something?

Every battery has a small resistance inside it, called internal resistance.⁶ When current flows, some voltage is used up inside the battery itself.

The math is V = EMF − (I × r).⁶

Example: a 12 V car battery with 0.02 ohms inside, pushing 200 amps to a starter, drops 200 × 0.02 = 4 V. The terminals then read only about 8 V while cranking.

Run-down batteries, and rechargeable batteries that have been recharged many times, have higher internal resistance, so they sag more.⁶

What is a voltage divider used for?

A voltage divider uses two resistors in series to turn a big voltage into a smaller one.

The formula is V_out = V_in × R2 ÷ (R1 + R2).

Example: 9 V in, with R1 = 1 kΩ and R2 = 2 kΩ, gives 9 × 2000 ÷ 3000 = 6 V.

Dividers are common in sensors and volume knobs. They are not good for powering things, because the output sags as soon as you draw current.

How do you convert watts to volts?

You cannot go straight from watts to volts. You need one more value.

If you know current: V = P ÷ I
If you know resistance: V = √(P × R)²

Example: a 60 W bulb drawing 0.5 A runs on 60 ÷ 0.5 = 120 V.

Watts measure how fast energy is used. Volts measure pressure. They are different things, so one number alone is not enough.

What happens if you give a device more voltage than it needs?

Higher voltage pushes more current through the device. More current means more heat, and heat kills parts.

A small jump, like 5 V instead of 4.5 V, is often fine. Double the voltage and you get four times the power, which can burn out the device fast or start a fire.²

Always match the voltage on the label. Many devices do have built-in regulators, but those have limits too.

Sources

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