Ohm's Law Cheat Sheet
Fast reference for V = I × R and power: P = V × I, P = I² × R, P = V² / R.
Unit tip: Keep units consistent.
20 mA → 0.020 A, 1 kΩ → 1000 Ω.
How to Use
- Pick what you’re solving for: V, I, R, or P.
- Use the “Quick Solve” table to choose the right formula.
- Check “Worked Examples” for sanity checks and resistor wattage guidance.
- Use “Copy” or “Print” from the right rail when needed.
Core Relations
| Relation | Meaning |
|---|---|
| V = I × R | Ohm’s Law (voltage across a resistance) |
| P = V × I | Electrical power |
| P = I² × R | Power from current & resistance |
| P = V² / R | Power from voltage & resistance |
Quick Solve
| Find | Use |
|---|---|
| V | V = I × R |
| I | I = V / R |
| R | R = V / I |
| P | P = V × I = I² × R = V² / R |
Wattage margin: choose the next standard power rating above calculated P (common practice: 2× if heat/airflow is unknown).
Worked Examples
-
Given 5 V and 220 Ω, find I and P
I = V/R = 5/220 ≈ 0.0227 A (≈ 22.7 mA)
P = V²/R = 25/220 ≈ 0.114 W → pick ≥ 0.25 W resistor. -
LED @ 20 mA from 5 V, LED drop 2 V → find series R and P
R = (5 — 2)/0.020 = 150 Ω
P = I²×R = 0.020²×150 = 0.060 W → 0.25 W is plenty. -
12 V device draws 0.5 A → find R and P
R = V/I = 12/0.5 = 24 Ω
P = V×I = 12×0.5 = 6 W → pick ≥ 10 W in practice. -
24 V supply, 10 W load → find I and R
I = P/V = 10/24 ≈ 0.4167 A
R = V²/P = 576/10 = 57.6 Ω
Reality check: real sources, wiring, and temperature change results. Use fusing and derating.
Quick Table — Common Rails
Handy values for typical resistors on 5 V / 12 V / 24 V rails. (I in A and mA; P in W.)
| V (V) | R (Ω) | I (A) | I (mA) | P (W) |
|---|---|---|---|---|
| 5 | 10 | 0.5000 | 500 | 2.5000 |
| 5 | 100 | 0.0500 | 50 | 0.2500 |
| 5 | 1 k | 0.0050 | 5 | 0.0250 |
| 5 | 10 k | 0.0005 | 0.5 | 0.0025 |
| 12 | 10 | 1.2000 | 1200 | 14.4000 |
| 12 | 100 | 0.1200 | 120 | 1.4400 |
| 12 | 1 k | 0.0120 | 12 | 0.1440 |
| 12 | 10 k | 0.0012 | 1.2 | 0.0144 |
| 24 | 10 | 2.4000 | 2400 | 57.6000 |
| 24 | 100 | 0.2400 | 240 | 5.7600 |
| 24 | 1 k | 0.0240 | 24 | 0.5760 |
| 24 | 10 k | 0.0024 | 2.4 | 0.0576 |
Resistor power rating: pick the next standard wattage above P. In hot enclosures or poor airflow, derate aggressively.
Simple Cheat Sheet
| Need | Plug This | Units |
|---|---|---|
| Voltage | V = I × R | V in volts, I in amps, R in ohms |
| Current | I = V / R | 1 A = 1000 mA |
| Resistance | R = V / I | 1 kΩ = 1000 Ω |
| Power (basic) | P = V × I | P in watts (W) |
| Power (by I & R) | P = I² × R | Use when current & resistance are known |
| Power (by V & R) | P = V² / R | Use when voltage & resistance are known |
Always verify component wattage and temperature rise. Derate in hot enclosures or poor airflow.
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