Voltage · current · resistance · power

Ohm's Law
Calculator

Enter any two of voltage, current, resistance, and power — the calculator solves the other two. It's the one relationship behind every circuit: V = I × R, and P = V × I. V = I × R

The clearest Ohm's Law calculator — for students and pros alike.

The whole law, live live
FIG. 1 — V = I × R 4 Ω 12 V 3 A current P = 36 W two knowns — the other two follow · I = V ÷ R · P = V × I
Change the resistance →

12 volts across a resistor. Raise the resistance and the current falls — I = V ÷ R — and the power falls with it. Give any two of the four, and the other two are fixed.

V · I · R · P

the whole relationship

Any two → the other two

with the formula shown

Free & private

no signup, runs in your browser

Dr. Artie Vance

Written & reviewed by Dr. Artie Vance — Ph.D. in Physics, MIT · 14 years' experience

View articles

Artie has taught physics and electrical theory for over a decade and consulted on real-world electrical design — so every tool here is grounded in both the theory and the field.

Every calculator on this site is checked against the 2023 NEC before it ships — if the math doesn't match the code, it doesn't go live.

University physics lecturer·Consulted on commercial electrical systems·Last reviewed Jul 2026

Ohm's Law Calculator

Enter any two of voltage, current, resistance & power — get the other two.

—— V = I · R

Fill any two boxes — the other two solve live. Edit a solved box to make it a known and release the oldest.

Voltage
—
Current
—
Resistance
—
Power
—

Enter any two values to solve the rest.

All four values — two given, two solved

Voltage
—
Current
—
Resistance
—
Power
—

For DC and resistive (unity power-factor) circuits. In AC with motors or reactance, P = V×I is apparent power (VA) — real power needs the power factor, and resistance becomes impedance. Use the Watt-to-Amp and Power Factor calculators for AC power.

How to use it

How to use the Ohm's Law calculator

Type any two of voltage, current, resistance, and power — the calculator solves the other two and shows the exact formula it used. Set your units, read all four values, and apply it: size a resistor, find a current, or check a circuit.

  1. 01

    Enter any two

    Voltage, current, resistance, or power — any pair fixes the other two.

  2. 02

    Set the units

    Volts, amps, ohms, watts — plus milli and kilo. It converts before it calculates.

  3. 03

    Read all four & the formula

    Get the two you didn't enter — plus the exact Ohm's or Watt's law formula used.

  4. 04

    Apply it

    Size a resistor, find a current — or convert to amps and watts for a real load.

The core idea

Ohm's Law and Watt's Law

Voltage, current, resistance, power — all connected.

Ohm's Law says voltage pushes current through resistance: V = I × R. Watt's Law adds power — the rate energy is used: P = V × I. Together they link all four, so any two give the rest.

The two triangles — cover what you want to find

Ohm's Law V I R

V = I × R
I = V ÷ R
R = V ÷ I

Watt's Law P V I

P = V × I
V = P ÷ I
I = P ÷ V

Cover the value you want — the other two show the formula. The horizontal bar means divide; side-by-side means multiply.

At 12 V across 4 Ω I = 12 ÷ 4 = 3 A P = 12 × 3 = 36 W

Ohm's Law links voltage, current & resistance; Watt's Law brings in power. Between them, every pair of knowns fixes the other two.

The rule

V = I × R (Ohm's) · P = V × I (Watt's) — any two of the four give the other two

The formula

The power wheel

Twelve formulas, four quantities — pick what you want, pick what you know.

VIRP V=IR V=√(PR) V=P/I I=V/R I=P/V I=√(P/R) R=V/I R=P/I² R=V²/P P=VI P=I²R P=V²/R

How to read it

Pick the quadrant for the value you want (V, I, R or P), then pick the formula that uses the two you already know. Twelve formulas, one wheel.

Know 12 V and 4 Ω?

Want current → I = V ÷ R = 12 ÷ 4 = 3 A

Then power → P = V × I = 12 × 3 = 36 W

Voltage (V)V = I × RV = P ÷ IV = √(P × R)
Current (I)I = V ÷ RI = P ÷ VI = √(P ÷ R)
Resistance (R)R = V ÷ IR = V² ÷ PR = P ÷ I²
Power (P)P = V × IP = I² × RP = V² ÷ R
The core

V = I × R and P = V × I — everything on the wheel is these two, rearranged

Worked example

From two values to all four

A 12-volt supply across a 4-ohm resistor. We know voltage and resistance — the wheel gives the current and the power, one formula at a time.

Find the current & power

Given V = 12 V R = 4 Ω
1
Current — cover I on the wheel I = V ÷ R = 12 ÷ 4 = 3 A
2
Power — cover P on the wheel P = V × I = 12 × 3 = 36 W
Current3 A
Power36 W

Power checks three ways

P = V × I = 12 × 3 = 36 W

P = I² × R = 9 × 4 = 36 W

P = V² ÷ R = 144 ÷ 4 = 36 W

Same answer every route — that's the wheel being consistent.

Same two formulas, household scale

120 V across 10 Ω → I = 120 ÷ 10 = 12 A

then P = 120 × 12 = 1,440 W

Reference

Units, and the AC catch

Ohm's Law is exact — as long as your units line up and the circuit is DC or resistive. Here's the units reference, and the one place the wheel needs help: AC power.

The four quantities & their units

Ohm's Law quantities, symbols, base units, and common multiples
QuantitySymbolBase unitAlso seen
VoltageVvolt (V)mV · kV
CurrentIamp (A)mA
ResistanceRohm (Ω)kΩ · MΩ
PowerPwatt (W)mW · kW

m milli = ÷1,000 · k kilo = ×1,000 · M mega = ×1,000,000

The wheel works in base units. Convert kΩ → Ω, mA → A, kW → W before you calculate — mixing prefixes is the single most common Ohm's Law mistake.

The wheel is for DC & resistive circuits

In AC with motors, transformers, or any reactance, P = V × I is apparent power (VA) — not the real watts. Real power needs the power factor (P = V × I × PF), and resistance becomes impedance (Z).

Power Factor → Watt to Amp →
01

Ohm's Law vs Watt's Law

Ohm's Law (V = IR) links voltage, current, and resistance. Watt's Law (P = VI) brings in power. The wheel fuses them, so any two of the four give the rest.

02

Mind the prefixes

The #1 error: 4.7 kΩ is 4,700 Ω, and 250 mA is 0.25 A. Convert everything to volts, amps, ohms, and watts before you calculate.

03

The square-root formulas

Know power and resistance? V = √(P × R) and I = √(P ÷ R). Know power and current? V = P ÷ I. The wheel covers every pair — even the awkward ones.

04

AC needs more

In AC circuits, use impedance (Z) in place of resistance, and treat P = VI as apparent power until you apply the power factor.

05

Where you use it

Sizing an LED's series resistor, checking a heater's draw, confirming a circuit isn't overloaded — Ohm's Law is the first tool for all of it.

How it behaves

How they connect — and what trips people up

Ohm's Law is one tight relationship: change one quantity and the others move in step. Most errors aren't the math — they're units, or forgetting the wheel assumes a DC, resistive circuit.

How the four move together

the relationships

  • Voltage up, current upAt a fixed resistance, current tracks voltage — double the volts, double the amps (I = V ÷ R).
  • Resistance up, current downAt a fixed voltage, more resistance chokes the current (I = V ÷ R).
  • Power climbs with voltage²Double the voltage and power quadruples (P = V² ÷ R) — it rises fast.
  • Current makes heatP = I² × R: high current in a small conductor means real heat — size for it.

Common mistakes

watch for these

  1. 01
    Prefix slips4.7 kΩ is 4,700 Ω; 250 mA is 0.25 A. Convert to base units before you calculate.
  2. 02
    P = V×I on ACThat's apparent power (VA). Real watts need the power factor: P = V × I × PF.
  3. 03
    Resistance vs impedanceIn AC, resistance becomes impedance (Z) — the DC wheel doesn't apply straight.
  4. 04
    Ignoring the heatForget that I²R is heat and you undersize the resistor or conductor.

The point

Two knowns is all it takes — the wheel gives the other two

Voltage, current, resistance, power — pin down any two and the other two are fixed. There's nothing else to look up; the whole circuit follows from a single pair.

The wheel is DC / unity power-factor. For AC power with motors or reactance, use impedance in place of resistance and apply the power factor — the Power Factor and Watt-to-Amp calculators handle that.

Quick chart

Ohms to amps, at a glance

Current is voltage over resistance — I = V ÷ R. Pick a resistance and a voltage to read the current and the power, or scan the table. At 12 V, a 4 Ω load draws 3 A.

Current = voltage ÷ resistance. Power = voltage × current.

Current

3 A

12 V ÷ 4 Ω · 36 W

Current (A) — resistance vs voltage

Current in amps by resistance in ohms and voltage, from I equals V divided by R
R5 V12 V24 V120 V
1 Ω51224120
2 Ω2.561260
4 Ω1.253630
6 Ω0.832420
8 Ω0.631.5315
10 Ω0.51.22.412
20 Ω0.250.61.26
50 Ω0.10.240.482.4
100 Ω0.050.120.241.2

Read your resistance row and voltage column for the current. Power = voltage × that current.

Ohms to amps table: current (A) by resistance and voltage, I = V ÷ R
Resistance5 V12 V24 V48 V120 V240 V
1 Ω5122448120240
2 Ω2.56122460120
4 Ω1.2536123060
6 Ω0.832482040
8 Ω0.631.5361530
10 Ω0.51.22.44.81224
15 Ω0.330.81.63.2816
20 Ω0.250.61.22.4612
50 Ω0.10.240.480.962.44.8
100 Ω0.050.120.240.481.22.4
What is Ohm's Law?

Ohm's Law states that voltage equals current times resistance: V = I × R. The current through a resistor is proportional to the voltage across it and inversely proportional to its resistance.

What is the Ohm's Law formula?

The core formula is V = I × R. Rearranged: I = V ÷ R and R = V ÷ I. Add Watt's Law (P = V × I) and you get the full power wheel — twelve formulas linking voltage, current, resistance, and power.

How do I calculate current with Ohm's Law?

Divide voltage by resistance: I = V ÷ R. A 12-volt supply across a 4-ohm resistor draws 12 ÷ 4 = 3 amps.

How do I calculate resistance?

Divide voltage by current: R = V ÷ I. If 3 amps flow under 12 volts, the resistance is 12 ÷ 3 = 4 ohms.

How do I calculate power?

Power is voltage times current: P = V × I. You can also use P = I² × R or P = V² ÷ R — all three give the same watts.

What does V = IR mean?

It means voltage (V) equals the current (I) multiplied by the resistance (R). Push more current, or add more resistance, and the voltage across the element rises.

What's the difference between Ohm's Law and Watt's Law?

Ohm's Law links voltage, current, and resistance (V = IR). Watt's Law brings in power (P = VI). Together they cover all four quantities — that's the power wheel.

Does Ohm's Law work for AC?

The formulas are exact for DC and resistive circuits. In AC with reactance, use impedance in place of resistance, and P = V × I becomes apparent power — real power needs the power factor.

Keep going

Related electrical calculators

Ohm's Law is the foundation. These build on it — turn watts into amps, convert between amps, kW and kVA, and correct the power factor for AC circuits.

01 Watt to Amp Calculator Turn watts or kW into amps for DC, single-, or three-phase circuits. Open 02 Ampere Calculator Convert between amps, kW, and kVA — the apparent-vs-real power hub. Open 03 Power Factor Calculator Find or correct power factor, and size the capacitor kVAR you need. Open
Browse all electrical calculators

How we keep this accurate

Calculations use the standard relationships — V = I × R and P = V × I, and the twelve-formula wheel derived from them. The wheel is exact for DC and resistive (unity power-factor) circuits; AC power needs impedance and the power factor. Results are for planning and learning — confirm designs with a licensed professional.