バンドを順番に読む
カラーバンドが存在するのは、小さな抵抗器には数字を印刷する余地がないためです。それぞれの色は数字を表し、バンドは許容範囲から最も遠い端から始めて左から右に読まれます。許容範囲（通常は金または銀で、残りの部分から少し離れて配置されています）がどの端が最後であるかを示します。
| 色 | 桁 | 乗数 |
|---|---|---|
| 黒 | 0 | ×1 |
| 茶色 | 1 | ×10 |
| Red | 2 | ×100 |
| オレンジ | 3 | ×1,000 |
| 黄色 | 4 | ×10,000 |
| Green | 5 | ×100,000 |
| 青 | 6 | ×1,000,000 |
| バイオレット | 7 | ×10,000,000 |
| グレー | 8 | ×100,000,000 |
| 白 | 9 | ×1,000,000,000 |
On a 4-band resistor, the first two bands are significant digits, the third is the multiplier, and the fourth is tolerance. A brown-black-red-gold resistor reads: 1, 0, ×100, so 10 × 100 = 1,000 Ω (1 kΩ), ±5%.
Brown-Black-Red-Gold = 1, 0, ×100, ±5%
= 10 × 100 = 1,000 Ω ± 5% (950–1,050 Ω)
5 バンド抵抗器は乗数の前に 3 つの有効数字を使用するため、より細かい精度が得られ、公差が厳しい部品に役立ちます。 6 バンド抵抗器は、最終的な温度係数バンドを追加します。これは、温度が 1 度変化するごとに抵抗が百万分率で何パーセント変化するかを示します。
Combining resistors: series vs. parallel
直列の抵抗は同じ電流経路を次々と共有するため、抵抗は単純に加算されます。並列の抵抗器は同じ 2 つの接続点を共有し、電流が複数の経路を流れるようにします。そのため、合成抵抗はグループ内の単一の抵抗器よりも常に低くなります。
Series: R_total = R1 + R2 + R3 + …
Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + …
Two 100 Ω resistors in series total 200 Ω. The same two resistors in parallel work out very differently:
1/R_total = 1/100 + 1/100 = 2/100
R_total = 100/2 = 50 Ω
As a shortcut, two equal resistors in parallel always combine to exactly half the value of one of them — useful for a quick sanity check before trusting a more complex calculation with mismatched values.
Checking power dissipation against a resistor's rating
Every resistor has a maximum power rating — commonly 1/4 watt for small through-hole parts in low-voltage electronics, higher for parts built to handle more heat. Before wiring one into a circuit, it's worth checking that the power it will actually dissipate stays comfortably under that rating.
P = V² ÷ R
12V across a 100 Ω resistor: P = 144 ÷ 100 = 1.44 W
That example would burn out a typical 1/4-watt resistor almost immediately — it needs a part rated for at least 2 watts, with headroom left over as a safety margin. Running a resistor near its rated limit shortens its life and can shift its actual resistance value from the printed one as it heats up.
Why resistor values look so specific
Resistors aren't manufactured in every possible value — they follow preferred-number series (E-series) spaced so that, combined with a given tolerance, every practical resistance is covered without huge gaps or wasteful overlap. E12 (12 steps per decade) suits ±10% parts, E24 suits ±5%, and E96 packs in enough steps for ±1% precision resistors. That's why you'll see resistors labeled 4.7 kΩ or 220 Ω rather than a round 5,000 or 200 — those odd-looking numbers are the E-series values.
Related electrical calculations
抵抗の値がわかれば、オームの法則計算機 solves for voltage, current, or power in the circuit it's part of, and the 電気計算機 covers the broader set of power and cost formulas for a full circuit or appliance.