Ohm's Law Triangle: Visual Guide to V, I, R

What Is the Ohm's Law Triangle?

The Ohm's Law triangle is a simple visual tool that eliminates the need to memorize three separate formulas. Instead of remembering V=IR, I=V/R, and R=V/I individually, the triangle encodes all three in a single diagram. It is the most popular study aid for electrical engineering students and one of the first tools HVAC apprentices learn.

The triangle works because Ohm's Law is a single equation (V = I × R) with three variables. The triangle simply shows which two variables multiply together and which sits alone on top. When you "cover" the variable you want to find, the remaining layout tells you whether to multiply or divide.

The VIR Triangle Explained

Here is the Ohm's Law triangle:

        ┌───────┐
        │   V   │
        │───────│
        │ I  ×  R│
        └───────┘
        

The layout has three positions:

• Top: V (Voltage, in Volts)
• Bottom-left: I (Current, in Amperes)
• Bottom-right: R (Resistance, in Ohms)

The horizontal line between V and I×R represents division. The space between I and R (no line) represents multiplication.

How to Use the Triangle — Step by Step

Follow this process for any Ohm's Law problem:

1. Identify which variable you need to find: V, I, or R.
2. Cover that variable with your finger (or mentally block it out).
3. Read what remains visible — that's your formula.

Finding Voltage (V)

        ┌───────┐
        │  [V]  │  ← covered
        │───────│
        │ I  ×  R│
        └───────┘
        

V is covered → you see I and R below the line with no line between them → V = I × R

Finding Current (I)

        ┌───────┐
        │   V   │
        │───────│
        │[I] ×  R│
        └───────┘
        

I is covered → you see V above the line and R below the line → the line means divide → I = V ÷ R

Finding Resistance (R)

        ┌───────┐
        │   V   │
        │───────│
        │ I  × [R]│
        └───────┘
        

R is covered → you see V above and I below → the line means divide → R = V ÷ I

The Power Triangle

A second triangle handles power calculations. It follows the same principle:

        ┌───────┐
        │   P   │
        │───────│
        │ V  ×  I│
        └───────┘
        

Cover the unknown to find:

• Cover P: P = V × I (power equals voltage times current)
• Cover V: V = P ÷ I (voltage equals power divided by current)
• Cover I: I = P ÷ V (current equals power divided by voltage)

Extended Power Formulas

By combining both triangles (substituting V=IR into P=VI), you get two additional power formulas:

• P = I²R — Use when you know current and resistance
• P = V²/R — Use when you know voltage and resistance

These six formulas (three from each triangle) handle virtually every basic circuit calculation you'll encounter.

Memory Tricks & Mnemonics

Trick 1: "Very Important Rabbits"

Very Important Rabbits → V = I × R

Picture a big rabbit on top (V) with two smaller rabbits underneath (I and R). To find the big rabbit, multiply the two small ones. To find a small rabbit, divide the big one by the other small one.

Trick 2: "PIV on the Pivot"

For the power triangle: Power Is on top with Voltage below → PIV, like a pivot point. P is the balanced top, V and I are the base.

Trick 3: "Over the Line Means Divide"

In both triangles, the horizontal line is a fraction bar. Anything above the line is the numerator. Anything below is the denominator. If the covered variable is on top, the answer is the bottom values multiplied. If a bottom variable is covered, divide the top by the other bottom value.

Trick 4: Draw It on Your Calculator

Many electricians and students write the triangle on a sticky note and tape it to their multimeter case. During an exam or on a job site, a quick glance is all you need.

Practice Problems

Test your understanding with these 8 problems. Use the triangle to find the formula, then calculate. Check your answers with our Ohm's Law Calculator.

When the Triangle Isn't Enough

The VIR and PIV triangles work perfectly for DC circuits and simple AC resistive loads. For more complex scenarios, you need additional tools:

• AC circuits with motors: Replace R with Z (impedance) and include power factor: P = VI × cos φ.
• Three-phase systems: Use P = √3 × V_LL × I × cos φ for balanced loads.
• Series-parallel circuits: Calculate equivalent resistance first (series: R_total = R1 + R2 + …; parallel: 1/R_total = 1/R1 + 1/R2 + …), then apply the triangle.
• Reactive circuits: Use impedance triangle: Z = √(R² + (X_L − X_C)²), then V = IZ.

Quick Reference Card

Standards Reference

• IEC 60071 — Insulation Co-ordination (voltage levels and clearances)
• IEEE 141 — Recommended Practice for Electric Power Distribution for Industrial Plants
• NEC (NFPA 70) — National Electrical Code (conductor sizing, overcurrent protection)
• NFPA 70E — Electrical Safety in the Workplace

Frequently Asked Questions

What is the Ohm's Law triangle?

The Ohm's Law triangle is a visual mnemonic that shows V (voltage) on top, with I (current) and R (resistance) on the bottom separated by a horizontal line (representing division). Cover the unknown variable to reveal the formula: cover V to see I×R, cover I to see V÷R, cover R to see V÷I.

How do you use the Ohm's Law triangle?

Step 1: Identify which variable you need to find (V, I, or R). Step 2: Mentally cover that variable in the triangle. Step 3: The remaining visible variables show the formula. If the horizontal line is visible, divide the top by the bottom. If no line is visible, multiply the two bottom values.

Is there a power triangle for Ohm's Law too?

Yes. The power triangle has P (power, watts) on top, with V (voltage) and I (current) on the bottom. Cover P to get P=V×I, cover V to get V=P÷I, cover I to get I=P÷V. Combined with the VIR triangle, you can solve any basic circuit problem.

What is the best mnemonic for Ohm's Law?

The most popular mnemonic is "Very Important Rabbits" — V=IR with V on top. Another is the "EIR" mnemonic (E for Electromotive force = voltage): "Eggs In Rows." For the power triangle, use "PIV" — Power on top, I and V below (think of a "pivot" point).

Can the Ohm's Law triangle be used for AC circuits?

The VIR triangle works for AC circuits if you replace R with Z (impedance) and use V=IZ instead of V=IR. The power triangle (P=VI) also works for AC, but for accurate real power you must include the power factor: P=VI×cos φ. The triangle is a quick starting point, but AC problems often require additional steps.