Resistors — Types, Values, and Markings

Section Overview

The first three chapters taught you how components behave. This chapter turns to a different, equally essential skill: identifying the components you'll actually hold in your hand at a repair bench. We start with the most common component of all, the resistor. A resistor rarely prints its value in plain numbers — it uses a code, either colored bands on a through-hole part or a tiny number code on a surface-mount one. This section teaches you to read both: the resistor color code and the SMD resistor code. It also covers what tolerance means, why resistors come in standard preferred values, and the handful of resistor types and ratings that determine which part to fit as a replacement.

Why This Matters

You cannot repair what you cannot identify. When a resistor fails, you need to know its value to replace it — and since it doesn't say "1000 ohms" on the side, you must read its code. Reading resistors fluently is one of the first hands-on skills of the trade: it's how you confirm you're looking at the right part, find a replacement, and understand a circuit from its board. It also connects directly to what you already know: the value is the resistance from Chapter 1, and the power rating is the wattage limit from Section 1.6 — a rating you must respect, since fitting an under-rated resistor is a repeat failure waiting to happen. This section makes the codes second nature so the rest of your repair work can proceed.

Required Prerequisites

Resistance — Opposition to Flow — what resistance is and the ohm; this section is about reading that value off a real part.
Ohm's Law — The Foundation of Circuit Analysis — you'll use a resistor's value with Ohm's law once you've read it.
Power and Energy in DC Circuits — the power rating is one of a resistor's three key specs, and the reason a replacement must match it.

Recommended Consumables

No consumables required. A handful of assorted resistors — or a resistor kit — makes ideal, reusable practice material, but nothing is used up.

Recommended Practice Hardware

An assortment of through-hole resistors with colored bands (a cheap resistor kit is perfect) and, if you have them, a few surface-mount resistors on a scrap board
A magnifier and good light — resistor bands and especially SMD codes are small, and good lighting prevents color misreads
A multimeter (from Section 1.7) to measure a resistor and compare it to what you decoded — the best way to confirm you've read the code correctly

No powered circuit is needed; you are reading and measuring loose or de-energized components.

Real-World Applications

Every repair that involves a resistor starts with reading it. A technician replacing a failed part decodes the survivors around it, or an identical resistor on a matching board, to find the value to fit. Designers and hobbyists read codes constantly while building and checking circuits. And the codes are everywhere: the color bands have been standard for decades on through-hole parts, while the tiny SMD number codes cover the resistors on virtually every modern circuit board. Being able to glance at a resistor and know its value — or measure it and confirm — is a skill you'll use in nearly every session at the bench.

Common Challenges

Reading the bands in the wrong direction. A resistor has no arrow — you have to tell which end is the start. The tolerance band (often gold or silver) is usually set slightly apart, at the far end; start reading from the opposite end.
Confusing similar colors. Brown, red, and orange can look alike under poor light, and a faded or heat-darkened resistor is worse. Good lighting, a magnifier, and confirming with a meter guard against misreads.
Ignoring the power rating. The color code tells you resistance and tolerance but not the wattage. Power rating is judged mostly by physical size, and getting it wrong — fitting a tiny resistor where a big one belongs — causes overheating, as Section 1.6 warned.

Safety Notes

Risk Level: Low. This section is reading and measuring loose or de-energized resistors — no live circuits, no stored energy. The one thing to respect is the rating of a replacement.

Professional Tips Before Starting

When in doubt, measure. The color code is a label, not a guarantee — decode it to find the value, then confirm with a meter, especially on a faded, old, or heat-stressed part.
Learn the color-to-digit order once and it sticks for life; a mnemonic helps, but simply decoding a dozen real resistors cements it faster than memorizing a chart.
Keep resistance, tolerance, and power in mind as three separate specs. The code gives you two of them; the third — power — you read from the resistor's size and must match on any replacement.

Reading and Choosing Resistors

The Color Code

Through-hole resistors mark their value with colored bands, and the resistor color code is the key. The foundation is the color-to-digit mapping, which is worth memorizing:

Black 0 · Brown 1 · Red 2 · Orange 3 · Yellow 4 · Green 5 · Blue 6 · Violet 7 · Grey 8 · White 9

On the most common 4-band resistor, the bands mean, in order from the starting end: first digit, second digit, multiplier (a power of ten, using the same colors), and tolerance. You read the two digits as a two-digit number, then multiply by the multiplier. For the multiplier band the colors mean powers of ten — Black is ×1, Brown ×10, Red ×100, Orange ×1000, and so on — while Gold means ×0.1 and Silver ×0.01 (for values below ten ohms). The tolerance band is usually Gold (±5%) or Silver (±10%).

Two worked examples:

Brown, Black, Red, Gold. Digits 1 and 0 make 10; the Red multiplier is ×100; so 10 × 100 = 1000 Ω = 1 kΩ, and Gold means ±5%.
Yellow, Violet, Orange, Gold. Digits 4 and 7 make 47; the Orange multiplier is ×1000; so 47 × 1000 = 47,000 Ω = 47 kΩ, ±5%.

More precise resistors use a 5-band code: three significant digits, then the multiplier, then tolerance. For example Brown, Black, Black, Brown, Brown reads as digits 1-0-0 = 100, multiplier Brown = ×10, giving 100 × 10 = 1000 Ω = 1 kΩ, and the final Brown band means ±1% — a tighter-tolerance version of the same 1 kΩ value. The extra digit is what lets a 5-band part specify a value like 1.00 kΩ precisely.

SMD (Chip) Resistor Codes

Surface-mount resistors are too small for bands, so they use a printed number code instead. The common SMD resistor code is three digits: the first two are significant digits and the third is the number of zeros (the power-of-ten multiplier). So:

"472" = 47 followed by 2 zeros = 4700 Ω = 4.7 kΩ.
"103" = 10 followed by 3 zeros = 10,000 Ω = 10 kΩ.
"331" = 33 followed by 1 zero = 330 Ω.

The single most common beginner mistake here is reading the code as a literal number: "331" is 330 Ω, not 331 Ω, and "103" is 10 kΩ, not 103 Ω. Always treat the last digit as a zeros-count (a power-of-ten multiplier), never as part of the value.

A four-digit version (three significant digits plus a multiplier) works the same way for tighter tolerances — "1002" is 100 followed by 2 zeros = 10 kΩ. An R stands in for a decimal point in small values: "4R7" means 4.7 Ω. And a marking of "000" (or just "0") is not a resistor value at all — it's a zero-ohm jumper, a resistor-shaped link used as a wire, common on modern boards. (A denser three-character EIA-96 code also exists for precision parts, using a two-digit value plus a letter multiplier; you'll meet it occasionally, but the plain digit codes cover most cases.)

Preferred Values and Tolerance

You'll notice resistors don't come in every possible value — you see 4.7 kΩ, 10 kΩ, 47 kΩ, but not 43.8 kΩ. That's because manufacturers make a standard set of preferred values, spaced so that, allowing for tolerance, the ranges of adjacent values just cover the gaps between them. The common E12 series (twelve values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) suits ±10% parts; the finer E24 series suits ±5%. This is why the same handful of "round" values keeps appearing.

Which brings in tolerance: the permitted percentage by which a resistor's real resistance may differ from its marked value. A 1 kΩ ±5% resistor is guaranteed only to lie between 950 Ω and 1050 Ω, and measuring one at 1020 Ω is perfectly normal, not a fault. Tighter tolerance (±1%) parts hold closer to their mark, which is why precision circuits use them. When you measure a resistor and it reads a little off its decoded value, tolerance is usually why — the part is fine.

Types and the Three Ratings

A few resistor types cover most of what you'll meet:

Carbon film — inexpensive, general-purpose, the classic beige cylinder; fine for most non-critical uses.
Metal film — more precise and lower-noise, common where accuracy matters (often the blue-bodied ±1% parts).
Wirewound — a resistance wire wound on a core, used for higher power; physically large, sometimes in a ceramic case.

And the ratings that matter when choosing a replacement are three: the resistance value, the tolerance, and the power rating (the wattage it can safely dissipate, from Section 1.6). The color or number code gives you the first two; the third you judge from physical size — a bigger body dissipates more power — and it must meet or exceed the original's. A resistor of the right ohms but too low a wattage overheats and fails again, so all three ratings count.

Common Mistakes

Reading the bands backwards. Start from the end opposite the slightly-separated tolerance band. Reading a 4-band resistor from the wrong end can turn 1 kΩ into a wildly different value.
Mistaking a zero-ohm jumper for a resistor. "000" or "0" is a wire link, not a value — don't try to decode it as ohms.
Assuming the marked value is exact. Tolerance means a small difference between marked and measured is normal, not a fault.
Matching only the resistance on a replacement. Value, tolerance, and — critically — power rating all matter; an under-rated replacement runs hot and fails.

Troubleshooting Guidance

Reading resistors is central to diagnosis and repair. To find a replacement for a failed resistor, decode a good example of the same part — or an identical resistor on a matching board — since the failed one may be unreadable. And that's the catch that connects back to Section 1.4: a burnt or heat-discolored resistor often can't be read at all, its bands charred or its printing gone. When that happens, you get the value from the circuit's schematic, from an identical unit, or by measuring an equivalent resistor elsewhere — and, crucially, you ask why it burned, since a resistor that overheated usually did so because of a fault elsewhere (or because it was under-rated), not on its own. Reading the survivors, and matching all three ratings on the replacement, is how you turn a charred, unreadable part back into a known value.

Verification & Testing Methods

Check your understanding before moving on:

[ ] Decode a 4-band resistor (for example Brown-Black-Red-Gold) into its resistance and tolerance.
[ ] Decode a 3-digit SMD code (for example "472") and recognize a "000" zero-ohm jumper.
[ ] Explain what tolerance means and why a measured value can differ slightly from the marked one.
[ ] Name the three ratings that matter for choosing a replacement resistor and say which one the color code does not give.

Then try the practice exercises below — decoding and measuring loose resistors, no powered circuits.

Practice Exercises

Decode the bands (10 minutes, pen and paper or real parts). Work out the resistance and tolerance for: (a) Green-Blue-Brown-Gold; (b) Red-Red-Red-Silver; (c) the 5-band Brown-Green-Black-Red-Brown. Show the digits, multiplier, and tolerance for each.
Read the SMD codes (5 minutes). Give the resistance for each SMD marking: (a) "220"; (b) "104"; (c) "4R7"; and state what "000" means.
Decode-then-measure (10 minutes, with real resistors and a meter). Take three resistors, decode each from its bands, then measure with your multimeter. Confirm each measurement falls within the tolerance of your decoded value, and note that a small difference is normal.
Pick a replacement (5 minutes, reasoning). A failed resistor is decoded as 470 Ω ±5% and its body is large (a couple of watts). List the three ratings you'd match on a replacement and explain why fitting a physically tiny 470 Ω part would be a mistake.

These core ideas — the color code, SMD codes, tolerance and preferred values, and the three ratings — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

The resistor color code reads, from the starting end, as digit, digit, ×multiplier, tolerance (4-band) — with the color-to-digit order Black 0 through White 9, and Gold/Silver as small multipliers or as ±5%/±10% tolerance.
A 5-band code adds a third significant digit for more precise values (e.g. Brown-Black-Black-Brown-Brown = 1 kΩ ±1%).
The SMD resistor code is usually three digits — two significant digits and a zeros count (e.g. "472" = 4.7 kΩ) — with "R" as a decimal point and "000"/"0" meaning a zero-ohm jumper (a wire link, not a value).
Resistors are made in standard preferred values (E12/E24) spaced to suit their tolerance, which is why the same round values recur.
Tolerance is the allowed percentage difference between marked and actual resistance, so a small measured difference is normal, not a fault.
Choosing a replacement means matching three ratings — resistance, tolerance, and power — and the code does not give the power rating, which you judge from physical size.

Skills Learned

You can now decode a 4-band or 5-band resistor into its resistance and tolerance.
You can now read a 3-digit SMD resistor code and recognize a zero-ohm jumper.
You can now explain tolerance and preferred values and why a measured value can differ slightly from the marked one.
You can now identify common resistor types and state the three ratings needed to choose a replacement.
You can now find a failed resistor's value from a readable example and match all three ratings on the part you fit.

Glossary Additions

resistor color code — the system of colored bands that marks a through-hole resistor's value and tolerance: read from the starting end as significant digits, a power-of-ten multiplier, and a tolerance band, using the color-to-digit order Black 0 through White 9.
tolerance — the permitted percentage by which a resistor's actual resistance may differ from its marked value (for example ±5%), shown by a dedicated band or code; a measured value within tolerance is not a fault.
preferred values — the standard set of resistor values manufacturers produce (such as the E12 and E24 series), spaced so that, with tolerance, adjacent values cover the range between them, which is why resistors come in recurring "round" values.
SMD resistor code — the printed number code on a surface-mount resistor: commonly three digits (two significant digits plus a zeros-count multiplier, e.g. "472" = 4.7 kΩ), with "R" marking a decimal point and "000"/"0" indicating a zero-ohm jumper.

Suggested Next Sections

Must read next:

Capacitors — Types, Ratings, and Identification — the same identification skills applied to capacitors: their markings, the voltage and temperature ratings that matter, and how to tell the types apart.

Recommended:

Resistance — Opposition to Flow — the resistance you're now reading off the part, revisited.
Power and Energy in DC Circuits — the power rating that the code doesn't show but a replacement must meet.