Reading a Resistor's Value and Tolerance

A resistor marking identifies a nominal value and often an initial tolerance; it does not promise that every measurement will equal the printed value. Leaded resistors may use colored bands, while surface-mount parts often use printed codes or no visible value at all. Correct interpretation combines the marking scheme, the exact product documentation and the conditions under which resistance is specified and measured.

A four-band code uses two digits, a multiplier and a tolerance

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Diagram description

An enlarged axial resistor shows brown, black, red and gold bands with their digit, multiplier and tolerance roles. A generic unbanded resistor below is contextual rather than the decoded specimen.

Four-band value

R_nom = (10d₁ + d₂) × 10ᵐ Ω

The first two bands supply significant digits and the third supplies the decimal multiplier. Gold and silver multiplier bands represent negative exponents. The fourth band gives tolerance in the common commercial scheme.

R_nom

nominal resistance, Ω

d₁

first significant digit

d₂

second significant digit

m

base-ten multiplier exponent represented by the third band

Example

Brown–black–red gives (10 × 1 + 0) × 10² Ω = 1000 Ω = 1.0 kΩ. Gold then adds a ±5% initial tolerance designation.

Determine orientation before decoding

A separated tolerance band often appears at the right, but spacing, body shape, ageing, lighting and manufacturing style can make orientation ambiguous. Check whether one direction produces a valid sequence and a plausible value for the circuit. Do not rely on color alone when the part is scorched, dirty, faded or safety-critical; use documentation, circuit records and an appropriate de-energized measurement.

How five- and six-band codes differ

A common five-band precision code uses three significant-digit bands, then a multiplier and tolerance. A common six-band code adds a temperature-coefficient band. Some military or manufacturer-specific schemes assign positions differently, so band count alone is not enough. Consult the exact component standard or manufacturer chart before treating the final band as TCR.

Common five-band value

R_nom = (100d₁ + 10d₂ + d₃) × 10ᵐ Ω

This common commercial pattern provides an additional significant digit. It must not be imposed on a part using a different marking convention.

d₁,d₂,d₃

three significant digits

m

base-ten multiplier exponent

Common resistor families and how the type changes the choice

Family	Typical reason to consider it	What must still be checked
Carbon or metal film, leaded	General through-hole use; metal film is commonly chosen when tighter tolerance, lower noise or stability is needed	Exact series, power, maximum working voltage, pulse load, TCR, size, coating and environment
Thick-film chip	Compact surface-mount general-purpose circuits with a very broad value range	Package power derating, working voltage, pulse capability, TCR, sulfur or moisture exposure and board temperature
Thin-film or foil precision	Low TCR, tighter tolerance or stability-sensitive measurement paths	Required accuracy over temperature and time, board stress, power, voltage, cost and calibration budget
Wirewound or power resistor	Higher dissipation, surge energy or braking and load duties	Surface temperature, enclosure heat, pulse energy, mounting, clearance and inductance where fast signals are involved
Current-sense or shunt	Very low resistance used to infer current from a measured voltage drop	Four-terminal connection where needed, allowable loss, TCR, overload, amplifier range and safe fault energy
Fusible or flame-resistant	A specified failure behaviour is required in addition to resistance	The exact safety approval and manufacturer conditions; an ordinary resistor must not be assumed to act as a fuse

Package appearance does not identify construction or ratings reliably. Start with the circuit function, resistance, tolerance and power, then add working voltage, pulse or surge energy, TCR, noise, frequency behaviour, environment, failure mode, mounting and service needs. Confirm the exact manufacturer part number before substitution.

Tolerance turns a nominal value into permitted limits

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Diagram description

A horizontal interval is centred at 1000 ohms and bounded by 950 and 1050 ohms. A note states that specification limits do not define a probability distribution.

Symmetric percentage tolerance limits

R_min = R_nom(1 − t); R_max = R_nom(1 + t)

Convert the percentage to a fraction before calculating: 5% = 0.05. The limits normally apply to initial resistance under datasheet test conditions. They do not automatically include long-term drift, soldering shift, moisture, temperature change or measurement uncertainty.

R_min

lower specified resistance limit, Ω

R_max

upper specified resistance limit, Ω

R_nom

nominal resistance, Ω

t

fractional tolerance, such as 0.05 for 5%

Example

For 1000 Ω ±5%, the deviation is 1000 × 0.05 = 50 Ω. Therefore R_min = 950 Ω and R_max = 1050 Ω.

Temperature coefficient estimates drift from a reference

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Diagram description

A graph of normalized resistance versus temperature shows rising and falling first-order examples around a reference point. A formula relates fractional resistance change to TCR and temperature change.

Local first-order TCR model

ΔR/R_ref ≈ αΔT; R(T) ≈ R_ref[1 + α(T − T_ref)]

When TCR is given in ppm/K, multiply by 10⁻⁶ to obtain per kelvin. A temperature difference of one kelvin equals one degree Celsius difference. The relation is a local linear estimate; actual specifications may use interval methods, curves or separate limits, and self-heating means component temperature can exceed ambient.

ΔR

change from reference resistance, Ω

R_ref

resistance at the stated reference temperature, Ω

α

temperature coefficient, 1/K

ΔT

component-temperature change from reference, K

T_ref

specified reference temperature

Example

For a 1000 Ω resistor that is exactly 1000 Ω at reference, a ±25 ppm/K limit over a 50 K change corresponds to an estimated magnitude up to 1000 × 25 × 10⁻⁶ × 50 = 1.25 Ω, subject to the datasheet's definition and interval.

A ± TCR specification usually bounds magnitude; it does not say every part has the same sign or a perfectly straight response. Nor should nominal tolerance and TCR simply be added as percentages without a defined worst-case model. Begin with a possible initial value, apply the temperature model to that value under the relevant reference and then include other stated shifts and uncertainty consistently.

A reliable identification workflow

Step	Action
1. Inspect	Count bands or read the printed code; note body style, spacing, damage and board reference designator
2. Decode	Apply the documented scheme and retain units, significant figures and tolerance
3. Cross-check	Compare with schematic, bill of materials, circuit role and plausible series value
4. Identify	Use the exact manufacturer part number where ratings, TCR or reliability matter
5. Measure safely	De-energize, isolate, discharge and account for parallel paths, leads and uncertainty
6. Assess conditions	Include temperature, self-heating, power, voltage, pulse, environment and long-term stability
7. Record	Document the decoded value, source, measured condition and acceptance basis

Tolerance and TCR describe value behaviour; they do not replace power, voltage and pulse checks. Use Resistance, Nonlinear Loads and Heat for those independent limits, and Using a Multimeter and a Diagnostic Sequence for the conceptual boundary around resistance measurements.

One-minute questions

Questions to answer before you apply this lesson

What should I select or check first for Reading a Resistor's Value and Tolerance?

Keep four ideas separate. Nominal resistance is the named design value. Tolerance defines permitted initial limits under specified conditions. Temperature coefficient estimates change with temperature near a reference. Measurement uncertainty describes how confidently a test result represents the measurand.

Which assumption can change the conclusion?

Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.

What can go wrong if I copy the example directly?

Do not measure resistance on an energized circuit. Resistance mode applies an internal test stimulus and is intended for an appropriately de-energized path.

How do I verify the result in the real system?

Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.

Sources and further reading

1. Resistor Color Code CalculatorVishay Intertechnology · 2026-10-13Manufacturer tool reviewed for common four-band and five-band digit, multiplier and tolerance positions.
2. Resistor Color Code ChartVishay Intertechnology · 2026-10-13Manufacturer chart reviewed for digit colors, multipliers, tolerance colors and common commercial four- and five-band arrangements.
3. Color Code and Standard Resistance SeriesVishay Intertechnology · 2026-10-13Manufacturer reference reviewed for four-, five- and six-band roles, TCR-band caveats and standard resistance series context.
4. TNPW e3 — High Stability Thin Film Flat Chip ResistorsVishay Intertechnology · 2026-10-13Manufacturer datasheet, document 28758 revision 10-Apr-2026, reviewed for TNPW0603 e3 tolerance and TCR options, resistance ranges and temperature-related conditions.
5. Resistors, Fixed — Product Families and Selection FieldsVishay Intertechnology · 2026-10-15Manufacturer portfolio reviewed for fixed-resistor technology families and the selection dimensions of resistance, tolerance, power, voltage, TCR, mounting style and operating temperature.
6. Digital Multimeter Safety and Resistance Measurement GuidanceFluke Corporation · 2026-10-15Manufacturer user guidance reviewed for the requirement to de-energize a circuit before resistance or continuity measurement and to avoid unsafe or misleading readings.
7. NIST/SEMATECH e-Handbook of Statistical Methods — Measurement Process CharacterizationNational Institute of Standards and Technology · 2026-10-13Metrology reference reviewed for keeping measurement-process uncertainty and variability distinct from a component's specification limits.