Why Resistors Wear Stripes
A resistor is too small to print "4,700 ohms" on legibly, so the industry encodes the value in colored bands — a system dating to the 1920s that every electronics hobbyist eventually memorizes. Each color stands for a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9. Learn the mnemonic "BB ROY of Great Britain has a Very Good Wife" and the sequence locks in permanently.
The bands after the digits are a multiplier — a power of ten — and a tolerance. Brown-black-red-gold reads as digits 1 and 0, multiplier ×100, tolerance ±5%: 1,000 Ω ±5%. The multiplier is where gold and silver earn their keep beyond tolerance duty: gold as a multiplier means ×0.1 and silver means ×0.01, producing the sub-10-ohm values used in current sensing. A resistor marked green-blue-gold-gold is 56 × 0.1 = 5.6 Ω ±5%.
4-Band vs. 5-Band vs. 6-Band
Standard 4-band resistors carry two significant digits, a multiplier and a tolerance — good for the ±5% and ±10% parts that fill most hobbyist kits. 5-band resistors add a third significant digit for precision work: brown-black-black-red-brown is 100 × 1,000 = 100 kΩ ±1%. The extra digit matters when you are setting gain, timing or reference voltages. 6-band parts add a temperature coefficient band (ppm/K) after the tolerance, telling you how much the resistance drifts per degree of temperature change — critical in precision instruments and anything operating outdoors.
How do you tell them apart at a glance? Count the bands, and check the tolerance band: 5- and 6-band resistors use brown, red, green, blue or violet tolerance bands (±1% down to ±0.1%), while 4-band parts use gold or silver (±5%/±10%). When the count is ambiguous, the gap before the tolerance band is slightly wider on real components — the calculator above sidesteps all of this by letting you choose the band count explicitly.
Which End Do You Read From?
The classic beginner trap is reading the bands backwards. Two rules resolve it. Rule one: gold and silver are never first-digit bands, so the end with the gold or silver band is read last. Rule two: if there is no gold or silver band, the bands are grouped slightly closer together at the reading end. On 5-band resistors the tolerance band (brown, red, green...) sits at the far end with a visible gap. When all else fails, try both directions — one of them will produce a standard E-series value and the other usually will not, which is a strong hint.
Tolerance: What ±5% Really Means
Tolerance is a manufacturing guarantee, not a measurement of your specific part. A 1 kΩ ±5% resistor is guaranteed between 950 Ω and 1,050 Ω; your particular one might be 987 Ω. For LED current limiting, pull-ups and most digital work, ±5% is plenty. For voltage dividers feeding an ADC, filter corner frequencies or current sensing, step up to ±1% — the price difference is pennies.
Note the asymmetry of real distributions: manufacturers sort production into tolerance bins, so a batch of ±5% resistors is missing the ones that measured within ±1% — they were sold at the higher grade. Your ±5% part is therefore more likely to sit near the edges of its band than dead center. It rarely matters, but it is a fun piece of insider knowledge.
Temperature Coefficient: the 6th Band
Resistance drifts with temperature, and the 6th band quantifies it in parts per million per kelvin. Brown (100 ppm/K) means a 1 kΩ resistor changes 0.1 Ω per degree — 10 Ω across a 100°C swing, a full 1%. Violet (5 ppm/K) drifts twenty times less. For a thermostat or an outdoor sensor, TCR can dominate your error budget; for an LED resistor, it is irrelevant. The calculator shows the TCR whenever you select 6-band mode, with the standard values: brown 100, red 50, orange 15, yellow 25, blue 10, violet 5.
The E-Series: Values You Can Actually Buy
Resistors are manufactured in geometric series — the E12 series (12 values per decade) for ±10% parts and E24 (24 per decade) for ±5% — because geometric spacing keeps the tolerance bands of adjacent values just touching. E12 runs 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 per decade; E24 fills the gaps. There is no 350 Ω resistor on any shelf — the calculator rounds to the nearest standard value (360 Ω) and tells you the percentage error, which is exactly the decision you make at the parts drawer.
This is why the band colors and the E-series agree with each other: a 4-band resistor's value always lands on (or very near) an E24 value, because the part was selected from that series. If your band reading gives 3.7 kΩ, you misread a band — the real value is 3.6 kΩ or 3.9 kΩ.
Worked Examples
Example 1 (4-band): yellow-violet-red-gold → digits 4, 7 → 47 × 100 = 4,700 Ω, ±5%. Range: 4,465–4,935 Ω. Nearest E24: 4.7 kΩ exact.
Example 2 (5-band): brown-black-black-orange-brown → digits 1, 0, 0 → 100 × 1,000 = 100 kΩ, ±1%. Range: 99–101 kΩ.
Example 3 (6-band): orange-orange-black-black-brown-red → digits 3, 3, 0 → 330 × 1 = 330 Ω, ±1%, TCR 50 ppm/K.
Example 4 (sub-ohm): red-red-silver-gold → 22 × 0.01 = 0.22 Ω ±5%, a current-sense value.
Try each in the calculator: click the band buttons, set the colors, and watch the value, tolerance window and nearest standard values update. It is the fastest way to build the pattern recognition that makes the chart unnecessary.
Beyond Bands: SMD Codes
Surface-mount resistors usually skip colors for printed codes: "103" means 10 × 10³ = 10 kΩ (two digits plus a multiplier digit), and EIA-96 codes like "01C" cover the ±1% series. Zero-ohm links are marked "0" or "000". The band system survives mainly in through-hole parts and education — but the underlying ideas (significant digits, multipliers, tolerance) are identical, so the fluency transfers directly.
Power Ratings and Physical Size
Bands encode resistance, not power — yet power is what kills resistors. The common through-hole sizes are ⅛ W, ¼ W, ½ W, 1 W and 2 W, roughly scaling with physical size: a ¼ W resistor is about 6 mm long, a 1 W about 11 mm. The rating is the maximum the part can dissipate continuously in free air at room temperature; exceed it and the resistance drifts, the paint discolors, and eventually the part fails open (or, excitingly, flames). Always derate: run a resistor at no more than half its rating in enclosed or warm spaces.
Size the wattage with P = V²/R across the resistor's actual voltage drop — not the supply voltage. A 1 kΩ resistor across 12 V dissipates 0.144 W: a ¼ W part is fine, a ⅛ W part is marginal. In the LED example from the companion calculator (350 Ω dropping 7 V at 20 mA), dissipation is 0.14 W — again a ¼ W resistor with comfortable margin. When a resistor runs too hot to touch, that is not normal operation; it is a design error asking for a bigger part.
Reading Resistors in Circuit
One caution for real-world troubleshooting: measuring a resistor in circuit rarely gives its marked value, because parallel paths through the rest of the circuit lower the reading. A 1 kΩ resistor reading 600 Ω in-circuit is usually fine — the meter sees the whole network. For a trustworthy reading, lift one leg of the resistor (power off, capacitors discharged). And remember the bands read the nominal value: a gold-band 4.7 kΩ resistor legitimately measures anywhere from 4,465 to 4,935 Ω. Only readings far outside the tolerance window indicate a failed part — resistors usually fail open (infinite) or drift high, almost never short.