Current Calculator
Calculate current step by step: I = V ÷ R = P ÷ V = √(P ÷ R).
In short
Formula: I = V ÷ R = P ÷ V = √(P ÷ R).
Current
3 A
- Formula
- I = V ÷ R
What this calculator does
Calculate current step by step: I = V ÷ R = P ÷ V = √(P ÷ R). Worked example, questions and limitations included.
Use it to turn Known values, Voltage, Resistance, Power into a checked result you can compare, copy, or rerun with different assumptions.
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 example
12 V across 4 Ω
- I = 12 ÷ 4.
- I = 3 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.
Common questions
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 use the Current Calculator?
Enter the required values for Known values, Voltage, Resistance, Power. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Current Calculator use?
I = V ÷ R = P ÷ V = √(P ÷ R). The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Current Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
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.