1M Ohm Resistor Color Code — Megohm Measurement & High-Z Design

1 MΩ (megohm): when color codes aren't enough

Brown · Black · Green · Gold → $10 × 10^5$ = 1 MΩ. The green multiplier band means ×100 k — the main failure mode is confusing 100 kΩ (yellow multiplier) with megohm parts.

4-band: Brown · Black · Green · Gold → 10 × 100,000 = 1,000,000 Ω = 1 MΩ (±5%) · 5-band (±1%): Brown · Black · Black · Yellow · Brown → 100 × 10,000 = 1 MΩ (±1%)

This guide covers measurement physics and high-impedance design — not LEDs, dividers, or I²C. Those topics live on 220 Ω, 1 kΩ, and 4.7 kΩ respectively.

Measuring 1 MΩ correctly (procedure)

Standard DMM tips fail at megohms:

1. Out of circuit — any parallel path dominates
2. Do not touch probe metal or the resistor body — skin adds 100 kΩ–1 MΩ in parallel
3. Hold one probe tip only or use insulated clips
4. Zero the meter in the same range before measuring
5. Expect 950 kΩ – 1.05 MΩ for ±5% carbon film

Humidity and dust on the PCB can shunt 1 MΩ networks — clean dry boards for repeatable readings.

1 MΩ × 1 µF = 1 second (timer you can build)

R	C	$\tau = R×C$	5τ to ~99%
1 MΩ	1 µF	1 s	~5 s
1 MΩ	100 nF	100 ms	0.5 s

Identify the 1 MΩ leg by brown-black-green before wiring — a 100 kΩ mistake changes timing by 10×.

Op-amp input bias error (order-of-magnitude)

If input bias current $I_b ≈ 10 nA$ flows through 1 MΩ feedback:

$V_{error} ≈ I_b × R = 10 nA × 1 MΩ = 10 mV

That's visible in millivolt sensor chains — choose lower R or a CMOS op-amp with pA bias. Color code identifies R, not op-amp suitability.

High voltage & bleeder safety (read before touching)

Megohm resistors in HV supplies can dissipate significant power if sized wrong: $P = V^2/R$. At 400 V into 1 MΩ, $P = 0.16 W$ — verify rating. Always discharge through designed bleeders and verify with a rated meter — not just color bands.

5-band 1 MΩ precision

Brown · Black · Black · Yellow · Brown → $100 × 10k$ = 1 MΩ ±1%. Yellow multiplier (×10⁴) vs green (×10⁵) is the critical distinction from 100 kΩ parts.

FAQ

1M vs 100k color code?

1 MΩ uses green multiplier (×10⁵). 100 kΩ uses yellow (×10⁴): Brown-Black-Yellow-Gold.

Can I use a megohm multimeter range on 1 kΩ parts?

Yes, but at 1 MΩ you need a meter with ≥1 GΩ range accuracy specified — cheap meters saturate early.

Related guides (different topics)

• 10 kΩ color code — button pull-ups & NTC dividers

• 100 Ω color code — RC τ table & wattage at 12 V

• 1 kΩ color code — divider math & strong pull-ups

• Values hub — pick by application

• How to read any resistor (method)

• Decode bands online