Candela to Lux Calculator

About the Candela to Lux Calculator

A candela to lux calculator applies the inverse square law to convert luminous intensity in candela to illuminance in lux at a given distance — one of the most practically useful calculations in lighting, photography, security lighting design, and sports facility planning. The inverse square law states that illuminance decreases with the square of the distance from the source: double the distance, quarter the illuminance. This means a 1,000 cd source illuminates at 1,000 lux at 1 m, but only 250 lux at 2 m, 111 lux at 3 m, and just 10 lux at 10 m. Our calculator makes this distance-dependent falloff immediately visible, showing illuminance at the entered distance and at half and double that distance for quick sensitivity analysis. This is used by photographers calculating exposure at different subject distances from a monolight, security engineers sizing floodlights for perimeter surveillance, sports lighting designers verifying adequate illuminance at field level, and theatrical lighting designers predicting fixture output at the stage. The result is shown in both lux and footcandles for international compatibility. In electrical design, circuit building, and engineering, adherence to physical laws like Ohm's Law or the National Electrical Code (NEC) is vital for system safety and efficiency. Calculating parameters like voltage drop, power factor, or wire gauge before installing hardware prevents equipment damage, reduces energy waste, and avoids potential safety hazards. This tool provides instant conversions and calculations based on established formulas, helping electricians, hobbyists, and engineers design and troubleshoot systems with confidence. Furthermore, individual circumstances and local regulations can significantly impact the practical application of these figures. Users in the USA, Canada, the United Kingdom, Australia, and New Zealand often face different regional guidelines, tax brackets, or baseline measurements (such as USDA zones, CRA guidelines, HMRC allowances, or ATO schedules) that should be factored into any serious planning. By entering your specific parameters into this calculator, you can model multiple scenarios side by side to see how minor changes in inputs affect the overall outcome. This makes the tool an indispensable asset for regular monitoring and long-term goal setting, helping you adjust your strategies as your needs evolve over time. In addition, when incorporating this calculator into your regular planning and routines, it is highly recommended to document your results over a period of weeks or months. Keeping a structured log or digital archive of your calculations allows you to trace trends, identify patterns, and detect any sudden anomalies that may require adjustments. Whether you are managing electrical circuit loads, tracking personal health and fitness parameters, analyzing educational grade distributions, or balancing a household budget, consistent record-keeping turns one-off calculations into a powerful long-term strategy. Always verify that your input data is sourced from reliable references before drawing major conclusions, and consult with qualified experts when making decisions that impact your physical health, safety, or financial security.

How It Works

Lux = cd / d², where d is the distance in metres. This applies to point sources — for large-area sources (LED panels, softboxes), the formula applies only at distances much greater than the source size. Example 1 (stage lighting): a 5,000 cd Fresnel theatrical fixture at 8m distance. Lux = 5,000 / 8² = 5,000 / 64 = 78.1 lux. Not enough for broadcast TV (which needs 800-1,000 lux). Move to 3m: 5,000 / 9 = 556 lux — better, but still below broadcast standard. Example 2 (photography): A 25,000 cd monolight strobe at 2m: 25,000 / 4 = 6,250 lux incident light. At ISO 100, 1/200s: EV = log₂(6,250 × 0.1 / 0.65) = log₂(961) ≈ 10 → approximately f/11. Example 3 (security): a 3,000 cd PIR floodlight at 20m perimeter: 3,000 / 400 = 7.5 lux — barely adequate for surveillance camera operation (cameras need minimum 3-10 lux for acceptable image). To compute this value manually, follow these standard steps: 1. Identify all the required input variables (such as base values, rates, dimensions, or constants) and convert them to matching units. 2. Apply the primary mathematical formula or conversion factor designated for this specific calculation. 3. Perform the arithmetic operations step by step, ensuring you strictly follow the standard order of operations (PEMDAS/BODMAS). 4. Verify the result by running the calculation in reverse or checking against known reference tables. By following this structured methodology, you can verify your results and gain a deeper understanding of the relationships between the different variables involved in the calculation.

Tips & Best Practices

• ✓Photographer guide numbers: flash guide numbers (GN) encode the candela-to-lux relationship. GN = √(Lux × d²) = √(cd). At GN 40 (metres, ISO 100): peak candela = 40² = 1,600 cd. At 2m subject distance: lux = 1,600 / 4 = 400 lux. f-number = GN / distance = 40/2 = f/20 at ISO 100.
• ✓Security camera minimum lux: IP cameras typically need: colour mode 0.1-3 lux minimum; low-light mode 0.01-0.1 lux; IR-assisted: can work at 0.001 lux. Calculate required floodlight candela for your perimeter distance: cd = Lux × d². For 3 lux at 30m: cd = 3 × 900 = 2,700 cd.
• ✓Sports lighting design: FIFA minimum for international matches: 1,400 lux horizontal, 1,000 lux vertical. For high-mast floodlights at 20m height: each fixture needs cd = lux × d²/count. Precise sports lighting design uses photometric software (DIALUX, AGI32) rather than point-source approximations.

Who Uses This Calculator

Photographers and cinematographers calculating expected lux at a subject distance from a light source with known candela. Security engineers sizing floodlights for perimeter surveillance at specified distances. Lighting designers verifying architectural accent light levels at target surfaces. Sports facility engineers performing preliminary floodlight specification calculations. Common practical scenarios for this tool include: - Professional scenarios: Engineers, financial analysts, accountants, health practitioners, and educators use this calculation to verify data, draft official reports, and double-check manual calculations quickly. - Consumer and everyday scenarios: Homeowners, students, fitness enthusiasts, and travelers use the tool to make quick estimates on the go, budget for upcoming projects, and track personal goals. - Educational learning: Students and teachers use this tool as a step-by-step visual aid to understand mathematical formulas and verify homework answers.