Current Calculator
Calculate current step by step: I = V ÷ R = P ÷ V = √(P ÷ R).
How to calculate current
This current calculator calculates electrical current in amps. It is an electric current calculator, also called an electrical current calculator. Enter voltage and resistance, or power and voltage, and it gives you the current in amps, with the formula shown so you can check the answer. Current is the flow of electric charge in a circuit. Use this calculator to calculate current for a circuit, a device, or an electrical load, and to check the current rating you need. A worked example is below: 12 V across 4 ohms gives 3 A.
Formula: I = V ÷ R = P ÷ V = √(P ÷ R).
Current
3 A
- Formula
- I = V ÷ R
Electrical current calculator: amps, voltage, and resistance
Ohm's law links the three values. Current in amps equals voltage in volts divided by resistance in ohms, I = V / R. When you know power instead, current equals power divided by voltage, I = P / V, or the square root of power divided by resistance.
Which formula to use
| What you know | Formula | Example |
|---|---|---|
| Voltage and resistance | I = V / R | 12 V / 4 ohms = 3 A |
| Power and voltage | I = P / V | 60 W / 12 V = 5 A |
| Power and resistance | I = square root of P / R | 50 W / 8 ohms = 2.5 A |
Current rating calculation
The result is the steady current the load draws. Compare it with the rated ampacity of the cable, and with the rating of the fuse or breaker, before you choose them. This calculator does not size cables or breakers, and it does not include start-up current.
Current calculator for AC circuits
For AC, this calculator works with RMS voltage and a purely resistive load, such as a heater. It does not include power factor, so it does not calculate single-phase or three-phase motor and reactive loads.
What this calculator does
Calculate current step by step: I = V ÷ R = P ÷ V = √(P ÷ R). Worked example, questions and limitations included.
You enter known values, voltage, resistance and power, and the current calculator shows the answer along with the working, so you can check it or try other numbers.
The page shows the formula, a numeric worked example, and the assumptions that affect this electrical calculation.
Inputs and what they mean
- Known values
- — choice value.
- Voltage
- — number value.
- Resistance
- — number value.
- Power
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Derived from Ohm's law and P = V × I.
Division by zero and negative power under the square root are rejected.
For AC, use RMS values and a resistive load.
I = V ÷ R = P ÷ V = √(P ÷ R).
Inputs used: Known values, Voltage, Resistance, Power.
Worked examples
Worked example: 12 V across 4 ohms
- I = V / R
- I = 12 / 4
- I = 3 A
Worked example: 60 W device on a 12 V supply
- I = P / V
- I = 60 / 12
- I = 5 A
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Steady-state only; inrush current is not modelled.
- Resistive loads; reactive AC loads need power factor.
- Ignores supply and wire resistance.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this current calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Reference: OpenStax (Rice University)
Last reviewed:
Common questions
How do I calculate electrical current?
Current in amps equals voltage divided by resistance (I = V ÷ R), or power divided by voltage (I = P ÷ V), or the square root of power divided by resistance.
How do I find the current a device draws?
Divide its rated power by the supply voltage. A 60 W device on 12 V draws 5 A.
Why is this important for wiring?
Cable and fuse sizes are chosen from current. Too much current overheats conductors, so compare the result with the cable's rated ampacity.
Is start-up current included?
No. Motors and power supplies can draw several times their running current briefly at switch-on.
How do I calculate amps from watts and volts?
Divide watts by volts: I = P ÷ V. For example 60 W on a 12 V supply draws 60 ÷ 12 = 5 A.
What is Ohm's law for current?
I = V ÷ R: current in amps equals voltage in volts divided by resistance in ohms. 12 V across 4 Ω gives 3 A.
What is a current calculator?
A calculator that finds electrical current in amps, from voltage and resistance, or from power and voltage.
How do I calculate current?
Divide voltage by resistance, or divide power by voltage. For a simple DC circuit, I = V / R.
What is current capacity?
The maximum current a wire, circuit, or device can safely carry. Compare your result with the cable rated ampacity before you choose a cable or fuse.
Sources
Method based on OpenStax (Rice University).
Cite this page
Free Calculator. (2026, November 3). Current Calculator. https://freecalculator.tech/tools/current-calculator
"Current Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/current-calculator.
Current Calculator — Free Calculator, https://freecalculator.tech/tools/current-calculator
Related tools
Calculate electrical power step by step: P = V × I = I² × R = V² ÷ R (DC or resistive AC using RMS values).
Calculate voltage divider step by step: Vout = Vin × R2 ÷ (R1 + R2).
Calculate resistance step by step: R = V ÷ I, or for a conductor R = ρ × L ÷ A (A in m²).
Calculate series resistance step by step: R_total = R1 + R2 + … + Rn.