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Ohms Law Wheel

Tap two known quantities on the wheel to see the matching formulas and the other two values calculated.

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An ohms law wheel (Ohm’s law wheel) is a circular chart of the 12 formulas that connect voltage (V), current (I), resistance (R) and power (P), three for each quantity. On this page the wheel is live: click or tap two quantities on the inner ring to choose what you know, type the two values in the form, and the other two are calculated while the formulas that produced them are highlighted in the outer ring. With 12 V and 0.5 A, for example, the wheel marks R = V ÷ I and P = V × I and shows 24 Ω and 6 W. Each sector of the inner ring also displays its current value. The form works without the wheel, and the wheel can be used with the keyboard (Tab, then Enter or Space). A table under the result repeats the formulas used, with their values.
Ohms law wheel example

How to use the Ohm’s Law Wheel

You can set the known pair on the wheel or in the form; both stay in step.

  1. Click or tap one quantity on the inner ring of the wheel, then a second one. The You know menu switches to that pair and the cursor moves to its first box. Clicking the same quantity twice cancels the choice.
  2. Or open You know directly and pick a pair such as Voltage and current (V, I) or Resistance and power (R, P).
  3. Type the two values. Voltage (V) takes mV, V, kV or MV; Current (I) takes µA, mA, A or kA; Resistance (R) takes mΩ, Ω, kΩ or MΩ; Power (P) takes mW, W, kW or MW.
  4. Read Voltage (V), Current (I), Resistance (R) and Power (P) in the results. The same values appear in the inner ring of the wheel.
  5. Look at the outer ring: the two formulas used for your answer are drawn bold and shaded. The steps and the “Formulas used for your answer” table under the result show them with their numbers.

Four example buttons (“12 V and 0.5 A”, “5 V across 1 kΩ”, “2 A and 100 W”, “8 Ω and 50 W”) load a complete case. Copy result and Copy link share the answer, Print / save as PDF keeps a copy, and Reset returns to the defaults.

How to read the Ohm’s law wheel?

Find the quantity you want in the inner ring, then pick the one formula in its outer sector that uses only the two values you know. The inner ring holds P, V, I and R; each outer sector holds the three formulas that solve for the quantity beside it.

The outer ring prints only the right-hand side of each formula, because the left-hand side is the quantity in the inner ring next to it:

Inner ringFormulas in its outer sector
VI × R, P ÷ I, √(P × R)
IV ÷ R, P ÷ V, √(P ÷ R)
RV ÷ I, V² ÷ P, P ÷ I²
PV × I, I² × R, V² ÷ R

To read a paper wheel by hand:

  1. Note the two values you have, for example V = 120 V and P = 60 W.
  2. Go to the sector of the unknown, here I, and choose the formula that uses only V and P: P ÷ V.
  3. Work it out: 60 ÷ 120 = 0.5 A.
  4. Repeat for the last unknown. In the R sector, V² ÷ P gives 14,400 ÷ 60 = 240 Ω.

The live wheel does steps 2 to 4 for you and highlights the two formulas it chose.

What are the 3 Ohm’s law formulas?

The three Ohm’s law formulas are V = I × R, I = V ÷ R and R = V ÷ I. They are one relationship rearranged three ways, so any one of voltage, current and resistance can be found from the other two. The other nine formulas on the wheel bring in power.

Examples the tool returns for each of the three:

FormulaYou knowResult
V = I × R2.5 A and 8 Ω20 V
I = V ÷ R5 V and 1 kΩ5 mA
R = V ÷ I12 V and 0.5 A24 Ω

The power formulas follow from P = V × I. Replacing V with I × R gives P = I² × R, and replacing I with V ÷ R gives P = V² ÷ R. That is how a resistor of 8 Ω dissipating 50 W must carry 2.5 A at 20 V.

Worked examples

InputResult
V = 12 V, I = 0.5 AR = 24 Ω, P = 6 W
V = 5 V, R = 1 kΩI = 5 mA, P = 25 mW
I = 2 A, P = 100 WV = 50 V, R = 25 Ω
R = 8 Ω, P = 50 WV = 20 V, I = 2.5 A

Calculated by the tool above; open it with the same values to see each step.

Questions, briefly answered

What is the Ohm’s law wheel?

It is a chart that arranges the 12 formulas for voltage, current, resistance and power in a circle, three per quantity, so you can find the right equation from any two known values at a glance.

What is 1 Ω equal to?

One ohm is one volt per ampere: 1 V across 1 Ω drives 1 A. A kilohm is 1,000 Ω and a megohm is 1,000,000 Ω, and both are in the Resistance (R) unit list.

Can I use the page without the wheel?

Yes. The You know menu and the value boxes give the same results, and the wheel only changes which pair is selected.

What does a highlighted formula mean?

It is one of the two formulas the tool used for your answer. They are also listed with their values in the “Formulas used for your answer” table.

Does the wheel work for AC circuits?

Only for DC circuits and AC loads that are purely resistive, such as heaters. Motors and other reactive loads need impedance and power factor.

Why are there 12 formulas and not 3?

The three Ohm’s law forms cover V, I and R only. Adding power gives a fourth quantity, and each of the four can be solved from three different pairs.