100 Ω resistor color code
A 100 Ω resistor is Brown, Black, Brown, Gold: 10 × 10 = 100 Ω, ±5%, so anything from 95 to 105 Ω is in spec.
10 × 10 = 100 Ω
5-band version
Brown · Black · Black · Black · Brown = 100 Ω ±1%
100 × 1 = 100 Ω. Range 99–101 Ω (±1%).
Open the 5-band version
Where you'll meet it
A gentle LED on two AA cells
Two AA cells give 3 V. A 100 Ω resistor between the battery's plus and a red LED runs it at a gentle current: clearly lit, and easy on the batteries.
I = (3 V − 2.0 V) / 100 Ω = 10 mA
That is half the 20 mA most standard LEDs are rated for: still clearly lit, and the cells last longer than at full current.
A blue or white LED drops about 3.0 V, as much as the two cells give, so the formula leaves (3 V − 3.0 V) / 100 Ω = 0 mA. It may still glow on fresh cells, but for blue or white use a bigger battery.
The resistor turns (10 mA)² × 100 Ω = 0.01 W into heat, so any ¼ W (0.25 W) part stays cool.
2.0 V and 20 mA are typical figures for a standard 5 mm red LED. Check your LED's datasheet. Fresh cells give a little over 3 V and less as they empty, and with only 1 V across the resistor the current follows them closely, so the LED dims steadily as they run down. The longer leg goes toward the battery's plus; wired backwards the LED just stays dark, because 3 V is less than the 5 V most LEDs are rated to block.
On SMD parts
Look for 101, 1000 or 01A
- 101101 = 10 × 10¹ = 100 Ω (3-digit code).
- 10001000 = 100 × 10⁰ = 100 Ω (4-digit code, usually ±1% parts).
- 01A01A = 100 × 1 = 100 Ω (EIA-96: 01 means 100, A means ×1).
100 Ω has an EIA-96 code because 100 is the first value in the E96 series.
Don't mix it up
Only band 3 changes
These four share Brown, Black and Gold. Brown (×10) is 100 Ω; black is ten times less and red ten times more.
1 Ωband 3 gold
10 Ωband 3 black- 100 Ωband 3 brownThis page
1 kΩband 3 red
Check it with a meter
Read 95–105 Ω
- Take the resistor out of the circuit.
- Set the meter to Ω, range 200 Ω (or auto).
- Touch one probe to each lead. Keep your fingers off the metal: your body is a resistor too.
- Anything from 95 to 105 Ω is a good ±5% part. A 5-band ±1% part should read 99 to 101 Ω.
A meter has a small error of its own, so a reading a hair outside the range does not prove the part is bad. Measure again with firm contact before you swap it.
Spot it
Can you find 100 Ω in a pile?
Practise reading it, and its look-alikes, in the resistor challenge.
Try the resistor challenge100 Ω resistor FAQ
Is 100 Ω the right resistor for an LED?
On two AA cells (3 V), yes: a typical red LED (about 2.0 V) gets (3 V − 2.0 V) / 100 Ω = 10 mA, clearly lit and half the 20 mA most standard LEDs are rated for. On a 5 V Arduino pin it is too small for a red LED: (5 V − 2.0 V) / 100 Ω = 30 mA, more than the 20 mA a pin should give. Use 220 Ω there.
What is the SMD code for a 100 Ω resistor?
101 on 3-digit parts, 1000 on 4-digit parts, or 01A on EIA-96 parts (01 means 100, A means ×1).
What are the 5-band colors for 100 Ω?
Brown, Black, Black, Black, Brown: 100 × 1 = 100 Ω with ±1% tolerance, so 99 to 101 Ω.
Is an SMD resistor marked 100 a 100 Ω part?
No. On a 3-digit SMD part the last digit is the number of zeros after the first two, so 100 means 10 followed by no zeros: 10 Ω. A 100 Ω part is marked 101, or 1000 on a 4-digit part.
Bands, ranges and codes come from the same engine as the calculator. Last reviewed .