Check a 100 ohm resistor in the calculator
The calculator is set to 100 Ω with a gold tolerance band. Switch to 5 or 6 bands, change the tolerance, or choose “Read the bands” to decode a resistor you are holding.
100 ohm resistor colour bands
A 100 Ω resistor carries the same value in every marking system; only the number of bands changes. Read from the end with the bands bunched closest to it, with the tolerance band, usually gold or brown and set slightly apart, on the right.
| Marking | Colours | Drawing | Where you see it |
|---|---|---|---|
| 4 bands, ±5 % | brown, black, brown, gold | The most common through-hole marking | |
| 5 bands, ±1 % | brown, black, black, black, brown | Precision metal-film resistors | |
| 6 bands, ±1 %, 100 ppm/K | brown, black, black, black, brown, brown | Adds a temperature-coefficient band | |
| 3 bands, ±20 % | brown, black, brown | Old parts with no tolerance band |
How to read brown, black, brown, gold
- First band, brown: first digit 1.
- Second band, black: second digit 0, giving 10.
- Third band, brown: multiply by 10, giving 100 Ω.
- Fourth band, gold: tolerance ±5 %.
On the 5-band version the first three bands are digits (100) and the fourth is the multiplier (×1), which comes to the same 100 Ω. Check any other combination with the resistor color code calculator.
What a 100 ohm resistor should measure
Tolerance is how far the real resistance may sit from the marked value. Measured out of circuit with a meter, a good 100 Ω resistor reads within these limits:
| Tolerance | Band | Lowest | Highest |
|---|---|---|---|
| ±1 % | Brown | 99 Ω | 101 Ω |
| ±2 % | Red | 98 Ω | 102 Ω |
| ±5 % | Gold | 95 Ω | 105 Ω |
| ±10 % | Silver | 90 Ω | 110 Ω |
| ±20 % | None (no band) | 80 Ω | 120 Ω |
Measure it out of circuit, or with one leg lifted: anything connected in parallel in the circuit pulls the reading down. Set a manual meter to the first range above 100 Ω.
Is 100 ohm a standard resistor value?
Yes. 100 Ω belongs to the E3, E6, E12, E24, E48, E96, E192 series of IEC 60063, so it is made in every tolerance from ±20 % down to ±0.1 %. The neighbouring E12 values are 82 Ω below and 120 Ω above. All the series are listed on our E-series resistor values page.
SMD code for 100 Ω
- 3-digit code (usually ±5 %):
101. - 4-digit code (usually ±1 %):
1000. - EIA-96 code (±1 % chips in 0603 and smaller):
01A: code 1 for the digits, the letter for the multiplier.
Decode any chip marking with the SMD resistor code calculator.
Current, power and wattage
By Ohm’s law, I = V ÷ R and P = V² ÷ R. With the whole supply across a 100 Ω resistor:
| Across | Current | Power | Rating to use |
|---|---|---|---|
| 3.3 V | 33 mA | 109 mW | ¼ W |
| 5 V | 50 mA | 250 mW | ½ W |
| 12 V | 120 mA | 1.44 W | 3 W |
| 24 V | 240 mA | 5.76 W | a power resistor |
A ¼ W 100 Ω resistor reaches its full rating at 5 V (V = √(P × R)). In series with a red LED (about 2 V) on a 5 V supply it passes 30 mA, more than the 20 mA most indicator LEDs are rated for. The LED resistor calculator finds the right value for any LED and supply.
What a 100 ohm resistor is used for
100 Ω is the usual series damping resistor: placed in a fast signal line it softens ringing and limits the current into a protected pin, and gate resistors for MOSFETs commonly sit between 10 and 100 Ω. On its own across 5 V it lets through more than a standard indicator LED wants, so LED circuits normally use a higher value.
Mistakes when reading a 100 ohm resistor
The multiplier band moves the value by a factor of ten for every colour step, so it is the band to check twice, in good light. These are the misreadings that catch people out with 100 Ω:
- Brown read as red. Brown, black, red, gold would be 1 kΩ, 10 times too high.
- Brown read as orange. Brown, black, orange, gold would be 10 kΩ, 100 times too high.
- Reading the 4-band code backwards. It can’t be done: gold is not a digit colour, which is how you know which end comes first.
- Reading the 5-band code backwards. Brown, black, black, black, brown reads as 100 Ω ±1 %, a plausible value, so this mistake goes unnoticed. The tolerance band is usually a little apart from the others; if you can’t tell, measure it.
Making 100 Ω from other values
No 100 Ω in the drawer? Two common E12 values can stand in, within 1 %: in series, 18 Ω + 82 Ω = 100 Ω; 33 Ω + 68 Ω = 101 Ω; in parallel, 180 Ω ∥ 220 Ω = 99 Ω. Two 100 Ω resistors give 200 Ω in series and 50 Ω in parallel.
Colour codes for other common values
- 1 ohm resistor: brown, black, gold
- 220 ohm resistor: red, red, brown
- 330 ohm resistor: orange, orange, brown
- 470 ohm resistor: yellow, violet, brown
- 1k resistor: brown, black, red
- 4.7k resistor: yellow, violet, red
- 10k resistor: brown, black, orange
- 100k resistor: brown, black, yellow
- 1M resistor: brown, black, green
Colours follow IEC 60062:2016. Codes, ranges and currents on this page are computed by the same engine as the resistor colour code calculator; see our methodology.