Candela to Lumens Calculator
Convert luminous intensity (candela) to luminous flux (lumens) using beam angle.
Understanding Candela and Lumens
Lighting terminology can often be confusing. Two of the most frequently used and misunderstood terms are candela and lumens. Though related, they measure different aspects of light and are not interchangeable without additional context. This calculator helps you bridge that gap and convert candela (cd) into lumens (lm) using the beam angle of your light fixture.
What Is Candela (cd)?
Candela is a unit of luminous intensity. It measures how bright a light source is in a particular direction. A laser pointer, for example, has a high candela rating because it emits light in a narrow, concentrated beam. It doesn’t necessarily emit a lot of light overall, but it’s extremely intense in that one direction.
What Are Lumens (lm)?
Lumens are a unit of luminous flux — they measure the total quantity of visible light emitted in all directions from a source. A bulb with a higher lumen rating produces more overall light, regardless of how focused or diffuse it is.
The Relationship Between Candela and Lumens
The relationship between candela and lumens is determined by the beam angle. The wider the beam, the more directions the light spreads out, and therefore the greater the total lumens required to maintain the same intensity (candela).
(Beam Angle in degrees, convert to radians internally)
Example Calculation
Suppose you have a spotlight rated at 500 candela with a beam angle of 60°:
- Half-angle = 30° → convert to radians = π/6 ≈ 0.5236
- cos(30°) = ~0.866
- 1 - cos(30°) = 0.134
- Lumens = 2π × 500 × 0.134 ≈ 421.4 lm
Applications
- Spotlights: Often rated in candelas due to directional intensity.
- Flashlights: High cd = focused beam; high lm = total output.
- Automotive lighting: Headlights are often rated in cd, but lumens help understand total output.
Why This Calculator Is Useful
- Understand product specs when only candela is listed.
- Design lighting layouts based on total brightness (lumens).
- Compare LED lights based on total output and directional intensity.
Conversion Limitations
This calculator assumes a uniform circular beam and does not account for elliptical or asymmetric distributions. Also, real-world fixtures may lose light to reflection and diffusion, so actual lumens may vary.
Frequently Asked Questions
Can I convert lumens to candela?
Yes, but you’ll need to know the beam angle or coverage pattern. The formula is inverted: cd = lm ÷ (2π × [1 - cos(θ/2)]).
Is higher candela always brighter?
Not always. High candela means more directional brightness, but a light can have high lumens and low candela if it's diffused.
Why do manufacturers list candela?
It’s especially useful for applications where directional brightness matters — like stage lighting or spotlights.
Is this calculator accurate for all beam shapes?
No. This assumes a uniform circular beam. Elliptical or rectangular beams require different geometry and custom calculations.
Can I use this for outdoor lighting?
Yes. It’s particularly helpful for comparing landscape spotlights and floodlights.