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Resistor Calculator

Decode color bands or calculate from voltage and current

Resistance

1kΩ

Tolerance

±5%

Resistance Value

1kΩ

Min

950Ω

Max

1.1kΩ

Example Calculations

1Brown-Black-Red-Gold (1kΩ)

Inputs

Band 1Brown (1)
Band 2Black (0)
Band 3Red (×100)
ToleranceGold (±5%)

Result

Resistance1,000Ω (1kΩ)
Range950Ω – 1,050Ω

(1×10 + 0) × 100 = 1,000Ω. ±5% = 950–1,050Ω.

2LED Resistor: 5V supply, 20mA

Inputs

Voltage5V
Current0.02A (20mA)

Result

Resistance250Ω
Power100mW

R = 5 / 0.02 = 250Ω. P = 5 × 0.02 = 0.1W. Use 270Ω (next E24 value) rated 1/4W.

Frequently Asked Questions

Q

How do I read resistor color bands?

A 4-band resistor: Band 1 = first digit, Band 2 = second digit, Band 3 = multiplier (number of zeros), Band 4 = tolerance. Brown-Black-Red-Gold = 10 × 100 = 1kΩ ±5%.

  • Band 1: first significant digit (0–9)
  • Band 2: second significant digit (0–9)
  • Band 3: multiplier (1 to 1,000,000,000)
  • Band 4: tolerance (Gold = 5%, Silver = 10%)
ColorDigitMultiplierTolerance
Brown1×10±1%
Red2×100±2%
Orange3×1k—
Yellow4×10k—
Green5×100k±0.5%
Q

How do I calculate resistance from voltage and current?

Ohm's Law: R = V / I. If you need 20mA through an LED with 5V supply and 2V LED forward voltage: R = (5-2) / 0.02 = 150Ω.

  • R = V / I (voltage divided by current)
  • P = V × I (power dissipated in watts)
  • LED resistor: R = (Vsupply - Vled) / Iled
  • Always check power rating: P = I² × R
Q

What tolerance should I choose?

For most hobby projects, 5% (gold band) is fine. Precision circuits (audio, measurement) need 1% (brown band). 10% (silver) resistors are cheap but imprecise.

  • 20% (no band): very imprecise, avoid
  • 10% (silver): basic circuits, LED current limiting
  • 5% (gold): most common, good for general use
  • 1% (brown): precision analog circuits
Q

What happens if I use the wrong resistor?

Too low resistance = too much current, can burn components. Too high resistance = insufficient current, circuit may not work. Always calculate before installing.

  • Too low: excess current, overheating, component damage
  • Too high: LED too dim, circuit malfunction
  • Exceeded power rating: resistor overheats and fails
  • When in doubt, use the next higher standard value

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Last Updated: Mar 19, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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