Candela to Candlepower Converter
Quickly convert from Candela to Candlepower.
How to convert
Formula:
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Luminous intensity describes how bright a light source appears in a specific direction — the directional power of light.
Where is it used?
• Automotive Lighting Regulations — Headlamp intensity is regulated in candelas (e.g., ECE R112, FMVSS 108). Minimum and maximum intensity values at specific horizontal and vertical angles are specified to ensure...
Examples:
• 1 candela (cd) = 1 lm/sr
• 1,000 millicandela (mcd) = 1 cd
Luminous intensity describes how bright a light source appears in a specific direction — the directional power of light. It is the SI base photometric unit and the foundation for all other photometric quantities, used in everything from lighthouse specifications to LED datasheets and automotive headlamp regulations.
Luminous intensity is the luminous flux emitted per unit solid angle in a particular direction. The SI base unit is the candela (cd), one of the seven SI base units. One candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation at 540 × 10¹² Hz (green light, 555 nm) with a radiant intensity of 1/683 W per steradian. Historically derived from a standard candle (~1 cd), the candela is now defined via a fixed numerical value of the luminous efficacy of that specific radiation (683 lm/W).
Where is it used?
- Automotive Lighting Regulations — Headlamp intensity is regulated in candelas (e.g., ECE R112, FMVSS 108). Minimum and maximum intensity values at specific horizontal and vertical angles are specified to ensure visibility without glare.
- LED & Lamp Datasheets — LEDs are characterised by luminous intensity (mcd or cd) at a specified current and measurement angle. A standard 5mm white LED may have 2,000–6,000 mcd (millicandela) at a 15° half-angle.
- Signal & Indicator Lighting — Traffic signals, aircraft obstruction lights, and marine navigation lights have legally specified minimum intensities in candelas to ensure visibility at required distances.
- Laser Safety — Laser pointers and equipment are characterised by radiant intensity (W/sr), the radiometric equivalent of luminous intensity, for safety classification.
- Photography & Cinematography — Studio light intensity in candelas (or its inverse-square-law derivative, lux at a distance) is used to calculate exposure settings.
Common Conversion Mistakes
Confusing candela with lumen
Candela (cd) is directional intensity — flux per steradian in one direction. Lumen (lm) is total flux emitted in all directions. A narrow-beam spotlight may have 10,000 cd but only 300 lm total; a bare bulb may have 1,000 lm but only 80 cd in any given direction. Choose candela when directionality matters, lumens when total output matters.
Ignoring the measurement half-angle for LEDs
LED datasheets specify intensity at a rated viewing half-angle (e.g., ±15°, ±60°). Comparing an LED rated at 1,000 mcd at ±15° with one at 1,000 mcd at ±60° is misleading — the narrower-angle LED appears much brighter in its beam but has far less total flux. Always check viewing angle alongside intensity.
Applying the inverse-square law to extended sources
The inverse-square law (E = I/d²) applies to point sources only. For luminaires and surfaces viewed at close range relative to their size, the formula breaks down. Extended-source luminance (cd/m²) must be used for surfaces, and the inverse-square law only applies when distance is much larger than the source dimensions.
Confusing the old candlepower with the modern candela
The old English unit 'candlepower' (cp) was originally based on a spermaceti wax candle. In modern usage, 1 candlepower = 1 candela (post-1948 redefinition). The terms are interchangeable today, but older photometric literature may use cp with slightly different calibration bases.
Quick Reference Table
| From | To |
|---|---|
| 1 candela (cd) | 1 lm/sr |
| 1,000 millicandela (mcd) | 1 cd |
| Typical wax candle | ~1 cd |
| Standard LED (5 mm, ±15°) | 2,000–6,000 mcd |
| Car headlamp (low beam, peak) | ~20,000 cd |
| High-power searchlight | ~10⁹ cd |
| Isotropic 1 cd source (all dirs) | 4π ≈ 12.566 lm total |
| 100 W incandescent (~1,600 lm) | ~127 cd average (spherical) |
Frequently Asked Questions
Why is the candela an SI base unit?
The candela is one of the seven SI base units because human vision is the primary use case for lighting standards, and the relationship between radiant power and perceived brightness requires a physiological weighting factor (the luminosity function). Defining candela as a base unit allows all photometric quantities to be derived consistently from it without needing to reference watts except through the fixed conversion constant 683 lm/W.
How is luminous intensity related to illuminance?
By the inverse-square law: E (lux) = I (cd) / d² (m²), valid for a point source. A 100 cd source illuminates a surface 2 m away with E = 100/4 = 25 lux. Double the distance to 4 m: E = 100/16 = 6.25 lux. The illuminance drops to one-quarter each time distance doubles — the fundamental reason lighting design places fixtures close to work surfaces.
What is the difference between luminous intensity and luminance?
Luminous intensity (cd) is the flux per steradian from a source — a single directional value for the whole source. Luminance (cd/m²) is intensity per unit projected area of the source surface — how bright the surface appears when you look at it. A large diffuse panel and a small bright spot can have the same luminous intensity in a given direction, but the spot has far higher luminance.
What does millicandela (mcd) mean on an LED datasheet?
1 mcd = 0.001 cd. LEDs are often rated in millicandela because their output, while intense in a narrow beam, is lower than traditional light sources in absolute terms. A 5,000 mcd LED at ±15° half-angle concentrates its flux into a small cone, making it appear very bright in that direction despite a modest total lumen output. Use mcd for comparing LEDs; use lumens for comparing with other light sources.
Sources & Standards
- Bureau International des Poids et Mesures (BIPM) — SI Brochure, 9th edition (candela definition)
- CIE 018.2:1983 — The Basis of Physical Photometry
- NIST Special Publication 330 — The International System of Units (SI)
- IEC 60050-845 — International Electrotechnical Vocabulary: Lighting
Reviewed by The Unit Hub Editorial Team · March 2026