Enter the bulb’s luminous efficacy (lumens per watt, lm/W) and the electrical power (watts) to estimate the average candlepower (mean spherical luminous intensity) assuming uniform emission into 4π steradians. For directional beams, candlepower depends on the beam’s solid angle.
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Watts to Candlepower Formula
The following equation estimates the average candlepower (mean spherical luminous intensity, in candela) from watts and luminous efficacy, assuming the light is emitted uniformly into a full sphere (4π steradians).
C = \frac{W \times PF}{4\pi}- Where C is the average candlepower (luminous intensity) in candela (cd)
- W is the electrical power (watts)
- PF is the luminous efficacy (lumens per watt, lm/W)
To estimate average candlepower for an isotropic source, multiply the wattage by the luminous efficacy to get lumens, then divide by 4π (≈ 12.566). For a directional beam, use cd = lm / Ω, where Ω is the beam solid angle in steradians.
| Watts (W) | Candlepower (cd) |
|---|---|
| 1 | 7.958 |
| 2 | 15.915 |
| 3 | 23.873 |
| 4 | 31.831 |
| 5 | 39.789 |
| 7 | 55.704 |
| 10 | 79.577 |
| 12 | 95.493 |
| 15 | 119.366 |
| 20 | 159.155 |
| 25 | 198.944 |
| 30 | 238.732 |
| 40 | 318.310 |
| 50 | 397.887 |
| 60 | 477.465 |
| 75 | 596.831 |
| 100 | 795.775 |
| 150 | 1193.662 |
| 200 | 1591.549 |
| 300 | 2387.324 |
| * Rounded to 3 decimals. Uses Φ = I × 4π and luminous efficacy = 100 lm/W. Formula: cd = (W × lm/W) / (4π). (Here, “cd” is the average/mean spherical value for an isotropic source.) | |
What is candlepower?
Definition:
Candlepower is an older term used for luminous intensity. In modern usage, luminous intensity is measured in candela (cd). In informal contexts, “candlepower” is often treated as the same as candela.
How to Calculate Candlepower from Watts?
Example Problem:
The following example outlines the steps and information needed to estimate Watts to Candlepower (average cd), assuming the lamp emits uniformly into 4π steradians.
First, determine the total electrical power supplied to the bulb. For this example, the power is 10 watts.
Next, determine the luminous efficacy of the bulb (how much light it produces per watt). In this case, it is 100 lumens per watt (100 lm/W).
Finally, calculate the (average) candlepower using the formula above:
C = (W × PF) / (4π)
C = (10 × 100) / (4π)
C ≈ 79.58 candlepower (cd)
