Resistor Calculator
Decode resistor colour codes (4-band and 5-band) and calculate series or parallel resistance. Includes E12/E24 preferred value tables and tolerance guide.
This resistor calculator decodes colour codes (4-band and 5-band), calculates total resistance in series and parallel circuits, and helps you find the nearest standard preferred value for any calculated resistance.
Resistor Colour Code — 4-Band Resistors
For standard 4-band resistors:
- Bands 1–2: First and second significant digits
- Band 3: Multiplier
- Band 4: Tolerance
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 (1Ω) | — |
| Brown | 1 | ×10 (10Ω) | ±1% |
| Red | 2 | ×100 (100Ω) | ±2% |
| Orange | 3 | ×1,000 (1kΩ) | — |
| Yellow | 4 | ×10,000 (10kΩ) | — |
| Green | 5 | ×100,000 (100kΩ) | ±0.5% |
| Blue | 6 | ×1,000,000 (1MΩ) | ±0.25% |
| Violet | 7 | ×10,000,000 (10MΩ) | ±0.1% |
| Grey | 8 | — | ±0.05% |
| White | 9 | — | — |
| Gold | — | ×0.1 (0.1Ω) | ±5% |
| Silver | — | ×0.01 (0.01Ω) | ±10% |
| None | — | — | ±20% |
Example: Brown, Black, Red, Gold = 1, 0, ×100, ±5% = 1,000Ω (1kΩ) ±5%
Example: Yellow, Violet, Orange, Silver = 4, 7, ×1,000, ±10% = 47,000Ω (47kΩ) ±10%
5-Band Resistors (Precision)
Precision resistors (±1% and tighter) use 5 bands:
- Bands 1–3: First, second, and third significant digits
- Band 4: Multiplier
- Band 5: Tolerance
Example: Brown, Green, Black, Brown, Brown = 1, 5, 0, ×10, ±1% = 1,500Ω (1.5kΩ) ±1%
Tip: The tolerance band (last band) on a 5-band resistor is often further from the body edge — read from the end where the first band is closest to the edge.
Series and Parallel Resistance Formulas
Resistors in Series
In series, resistance values simply add:
R_total = R1 + R2 + R3 + ...
Example: 100Ω + 220Ω + 470Ω = 790Ω
Series resistors carry the same current. Total voltage splits between them proportionally to their resistance.
Resistors in Parallel
In parallel, the reciprocal of total resistance equals the sum of reciprocals:
1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
For two resistors only: R_total = (R1 × R2) ÷ (R1 + R2)
Example: 1kΩ in parallel with 1kΩ:
R_total = (1,000 × 1,000) ÷ (1,000 + 1,000) = 1,000,000 ÷ 2,000 = 500Ω
Two equal resistors in parallel always give half the resistance of one.
Example: 470Ω in parallel with 1kΩ:
R_total = (470 × 1,000) ÷ (470 + 1,000) = 470,000 ÷ 1,470 = 319.7Ω
Parallel resistors all see the same voltage. Total current splits between them proportionally to their conductance.
Standard Preferred Values — E12 and E24 Series
Resistors are manufactured in standardised preferred values. The E12 series (±10% tolerance) has 12 values per decade; the E24 series (±5%) has 24.
E12 series values (×1, ×10, ×100... up to ×1MΩ): 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
E24 series adds: 1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, 9.1
E96 series (±1%) has 96 values and is used for precision resistors.
When your calculated resistance doesn't match a preferred value, choose the nearest E12/E24 value. For LED current-limiting resistors, choose the next value up to reduce current slightly and protect the LED.
Tolerance and Worst-Case Analysis
A 1kΩ ±5% resistor can measure anywhere from 950Ω to 1,050Ω. In circuits where exact resistance matters (voltage dividers, filter networks, oscillators), the tolerance affects the output:
| Nominal | Tolerance | Min | Max |
|---|---|---|---|
| 1kΩ | ±20% | 800Ω | 1,200Ω |
| 1kΩ | ±10% | 900Ω | 1,100Ω |
| 1kΩ | ±5% | 950Ω | 1,050Ω |
| 1kΩ | ±1% | 990Ω | 1,010Ω |
For audio circuits, timing circuits, and sensor inputs, use ±1% (E96) resistors. For most general-purpose circuits, ±5% (E24) is sufficient.
Frequently Asked Questions
Which end do I start reading the colour bands from?
Start from the end where the bands are clustered closest together, or where the tolerance band (gold, silver, or brown for ±1%) is furthest away. If in doubt, measure the resistance with a multimeter — this is always faster and more reliable than reading the bands.
What does the tolerance band mean in practice?
It tells you the manufacturing precision. A 100Ω ±5% resistor guarantees the actual value is between 95Ω and 105Ω. In most circuit applications this doesn't matter. In precision voltage dividers or oscillator circuits, tighter tolerances matter — use ±1% resistors there.
Can I connect resistors in parallel to make a non-standard value?
Yes. This is a common technique. For example, if you need 150Ω and only have 100Ω and 300Ω available: in parallel, (100 × 300) ÷ (100 + 300) = 30,000 ÷ 400 = 75Ω. Series gives 400Ω. Neither gives 150Ω — but two 300Ω in parallel gives exactly 150Ω.
What are pull-up and pull-down resistors?
Pull-up resistors connect a digital input pin to the positive supply voltage (VCC), ensuring a defined HIGH state when the pin is otherwise unconnected. Pull-down resistors connect to ground, ensuring LOW. Without them, floating inputs produce random noise. Typical values are 4.7kΩ–10kΩ for digital circuits.
What wattage rating do I need?
Resistors dissipate power as heat. Ensure the resistor's wattage rating exceeds the actual dissipation: P = I² × R. Common ratings are 1/4W (0.25W), 1/2W (0.5W), 1W, and 2W. For a 1kΩ resistor carrying 10mA: P = (0.010)² × 1,000 = 0.1W — a 1/4W rating is fine.
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