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Voltage Calculator
Use this free voltage calculator to solve any Ohm's law problem: enter any two of voltage, current, resistance and power, and it finds the other two instantly. It works in every direction — voltage from current and resistance, current from voltage and power, resistance from voltage and current — with millivolt, kilovolt, milliamp, kilohm, megohm, milliwatt and kilowatt units, so it covers everything from a small sensor circuit to a household appliance.
Enter values of 0 or more. Resistance and power cannot both be zero, and a zero in the wrong place (for example, dividing by zero current) will show a friendly message instead of a number.
How the voltage calculator works
Everything in this calculator comes from two relationships. The first is Ohm's law: voltage equals current times resistance, written as V = I × R. Voltage is the electrical pressure pushing charge around a circuit, current is the flow of that charge, and resistance is whatever opposes the flow. Double the resistance with the same current and the voltage doubles; double the current through the same resistor and the voltage doubles too.
The second relationship is the power law: power equals voltage times current, or P = V × I. Power, measured in watts, is the rate at which the circuit turns electrical energy into something else — heat in a heater, light in a bulb, motion in a motor. A 12-volt circuit carrying 3 amps uses 36 watts, and that 36 watts is what you pay for on your electricity bill and what sizes your wires, fuses and power supply.
Because there are four quantities and two equations, knowing any two is enough to find the other two. The calculator rearranges the formulas for you: from current and resistance it finds voltage with V = I × R and power with P = I² × R; from voltage and resistance it finds current with I = V ÷ R and power with P = V² ÷ R; from voltage and current it finds resistance with R = V ÷ I and power with P = V × I; from voltage and power it finds current with I = P ÷ V and resistance with R = V² ÷ P; from current and power it finds voltage with V = P ÷ I and resistance with R = P ÷ I²; and from resistance and power it finds voltage with V = √(P × R) and current with I = √(P ÷ R). The square-root cases are the only ones where the formula is not a plain multiply or divide, and they are the reason a calculator is worth having.
Units are converted to base units before any math happens. A milliamp is one thousandth of an amp, a kilovolt is a thousand volts, a kilohm is a thousand ohms, a megohm is a million ohms, a milliwatt is one thousandth of a watt, and a kilowatt is a thousand watts. That matters in real circuits: a sensor that draws 20 milliamps through a 220-ohm resistor drops 0.02 × 220 = 4.4 volts, and if you forget the milli- and multiply 20 × 220 you get 4,400 volts, which is wrong by a factor of a thousand. The calculator keeps the prefixes straight so you do not have to.
Worked example
A 12-volt battery powers a small heater. The heater draws 3 amps. What is the resistance of the heater, and how much power does it use?
Step 1: Resistance from voltage and current — R = V ÷ I = 12 ÷ 3 = 4 ohms.
Step 2: Power from voltage and current — P = V × I = 12 × 3 = 36 watts.
Step 3: Check with the other formula — P = I² × R = 3² × 4 = 9 × 4 = 36 watts, which matches.
Choose "Voltage (V) and current (I)" in the calculator, enter 12 volts and 3 amps, and it gives exactly these answers: resistance 4 Ω and power 36 W. The same circuit in milliamps would be 3,000 mA through 4 Ω, still 12 V — the units change, the physics does not.
Voltage drop and wire size
One common surprise is that the voltage at the load is lower than the voltage at the source. Wires have resistance, and current flowing through that resistance drops voltage along the way — V = I × R again, with R now being the resistance of the wire itself. A long, thin wire carrying a heavy current can drop several volts, which is why a 12-volt light at the end of a long garden cable looks dimmer than the same light next to the battery.
For a quick check, a 10-metre run of ordinary 1.5 mm² copper wire has about 0.12 ohms of resistance per conductor, and a 5-amp load drops about 5 × 0.24 = 1.2 volts over the out-and-back run — 10 percent of a 12-volt supply, which is usually too much. The fix is a thicker wire (lower resistance) or a shorter run, and our voltage drop calculator works out the exact loss for your wire length and size before you buy the cable. The same thinking applies to fuses: a fuse is sized from the current, and the current comes from the power and voltage — a 1,200-watt heater on a 120-volt supply draws 1,200 ÷ 120 = 10 amps, so it needs a fuse above 10 amps with a safe margin.
FAQ
How do you calculate voltage using Ohm's law?
Voltage equals current times resistance: V = I × R. If a circuit carries 3 amps through a 4-ohm resistor, the voltage across it is 3 × 4 = 12 volts. You can also find voltage from power and current with V = P ÷ I, or from power and resistance with V = √(P × R).
What is the difference between voltage, current, resistance and power?
Voltage is the electrical pressure that pushes charge around a circuit, measured in volts. Current is the flow of charge, measured in amps. Resistance opposes that flow, measured in ohms. Power is the rate at which energy is used, measured in watts. Ohm's law ties the first three together (V = I × R) and power ties in through P = V × I.
Can I find voltage if I only know power and resistance?
Yes. Since power equals voltage squared divided by resistance (P = V² ÷ R), rearranging gives V = √(P × R). A 36-watt load across a 4-ohm resistor has a voltage of √(36 × 4) = √144 = 12 volts. Choose the power and resistance mode in this calculator and it does the square root for you.
Why is my calculated voltage different from the battery voltage?
A battery's printed voltage is its open-circuit voltage with nothing connected. Once a load is connected, the battery's own internal resistance drops some voltage, and long wires add more drop, so the voltage at the load is lower. Measure the voltage at the load itself to check, and use a voltage drop calculator for long wire runs.
What units can I use in this voltage calculator?
Voltage in millivolts, volts or kilovolts; current in milliamps or amps; resistance in ohms, kilohms or megohms; and power in milliwatts, watts or kilowatts. The calculator converts everything to base units before applying Ohm's law, then shows all four answers in your chosen units.