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Lighting Design / Foot-Candle Calculator

Size lighting layouts using the IES lumen method with room cavity ratios, fixture spacing, and IECC compliance checks

Source-aware lighting planning screen for electricians, engineers, and facility managers doing early indoor fixture-count work. Enter room dimensions to compute Room Cavity Ratio (RCR), choose local foot-candle planning presets or a custom target, enter fixture lumens and watts, and screen spacing-to-mounting-height (S/MH) ratio, connected load, and local lighting-power-density rows. The app labels IES, IECC, ASHRAE 90.1, manufacturer photometric files, exterior lighting, controls, emergency lighting, and AHJ review as source gaps instead of treating the output as a final lighting design.

Pro Tip: RCR is useful for early screening because a long narrow room and a square room with the same area can need different fixture counts. Treat the result as a planning estimate until the selected luminaire photometric file, CU table, LLF, spacing criterion, glare, controls, adopted code edition, and project conditions are checked.

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Lighting Design / Foot-Candle Calculator
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How It Works

  1. Enter Room Dimensions

    Input room length, width, work plane height, and mounting height of fixtures. The calculator computes the Room Cavity Ratio (RCR), which determines how efficiently light reaches the work surface from the fixture mounting position.

  2. Select Space Type

    Choose a local planning preset for common indoor spaces or enter a custom target. Verify the actual maintained illuminance target against current IES task guidance and project requirements.

  3. Choose Fixture Type

    Select a common LED row or enter custom fixture lumens and watts. The local CU screen is simplified and must be replaced with manufacturer photometric data before design use.

  4. Set Surface Reflectances

    Adjust ceiling, wall, and floor reflectance percentages. Standard values are 80/50/20 for light-colored commercial spaces. Darker finishes reduce the CU and require more fixtures. Warehouses with exposed dark ceilings may drop to 30/30/10.

  5. Review Layout and S/MH Ratio

    The app rounds the fixture count into a simple rectangular grid and applies a generic S/MH warning. The luminaire photometric report and point-by-point model control the real spacing and uniformity decision.

  6. Screen LPD and Retrofit Energy

    The app compares connected load against local LPD planning rows and calculates basic retrofit energy arithmetic. Adopted IECC/ASHRAE 90.1 paths, controls, schedules, utility rates, rebates, and AHJ review still control.

Built For

  • Electricians sizing lighting circuits and fixture counts for commercial tenant buildouts
  • Facility managers organizing LED retrofit wattage, schedule, and cost assumptions before financial review
  • Architects organizing early lighting assumptions before photometric modeling
  • Energy auditors doing a preliminary connected-load and retrofit arithmetic screen
  • Warehouse managers estimating high-bay fixture quantities before manufacturer layout review

Assumptions

  • Room geometry is rectangular with uniform ceiling height.
  • Fixtures are arranged in a symmetric grid pattern with equal spacing.
  • Light Loss Factor (LLF) accounts for lamp lumen depreciation and luminaire dirt depreciation over the maintenance cycle.
  • Coefficient of Utilization values are interpolated from local simplified rows, not manufacturer or licensed IES tables.

Limitations

  • Does not perform point-by-point calculations for non-uniform fixture layouts or task lighting.
  • Outdoor and area lighting applications require different calculation methods not supported here.
  • Dimming and sensor savings are average-use overlays and do not model code-required controls, commissioning, schedules, tariff, or demand charges.

References

  • IES Illuminance Selector and Lighting Library source pointers
  • IES TM-34-19 and LS-6-20 lighting calculation source pointers
  • IES/NALMCO RP-36-24 lighting maintenance source pointer
  • ICC 2021 IECC Chapter C405 and ASHRAE/IES Standard 90.1 source pointers

Frequently Asked Questions

The lumen method estimates average maintained illuminance on a horizontal work plane from room area, fixture lumens, coefficient of utilization, and light loss factor. It is useful for early indoor screening, but final lighting work should use manufacturer photometric files and point-by-point software where uniformity, glare, vertical illuminance, life safety, exterior lighting, or code compliance matter.
The app includes local office planning ranges, but the actual target should be selected from current IES guidance for the task, occupants, age, contrast, glare, visual display use, and owner requirements. Do not treat the local preset as a licensed IES table value.
The Room Cavity Ratio is a dimensionless number that describes the proportions of the room cavity between the fixture plane and the work plane. The formula is RCR = 5 x Cavity Height x (Room Length + Room Width) / (Room Length x Room Width). Higher RCR values indicate deeper or narrower rooms where light is less efficiently distributed. An RCR of 1-3 is typical for standard offices, while tall warehouses may have RCR values of 5-8.
The S/MH ratio compares fixture spacing to mounting height above the work plane. This app uses a generic local warning threshold only. The selected luminaire photometric report, layout model, task area, ceiling conditions, and required uniformity control the real spacing decision.
Lighting power density is connected lighting watts divided by floor area. This app uses local planning rows to flag connected-load risk, but adopted IECC or ASHRAE 90.1 edition, compliance method, exceptions, controls, daylight zones, allowances, and AHJ interpretation determine actual code compliance.
A retrofit can reduce watts while changing delivered light distribution, glare, color, controls, maintenance, and measured illuminance. The app compares entered lumens and watts only; verify existing conditions, product photometrics, control sequence, utility tariff, demand charges, rebates, and measured results before buying.
The lumen method is designed for enclosed indoor spaces where reflected light contributes significantly to illuminance. Outdoor areas like parking lots and building exteriors require point-by-point calculations that account for fixture beam angles and mounting geometry without reflected light. This calculator is not suitable for outdoor lighting design. Use manufacturer photometric software or IES point-by-point methods for exterior applications.
Disclaimer: This planning screen provides simplified indoor lumen-method arithmetic only. Actual lighting performance and compliance depend on current IES guidance, manufacturer photometric data, exact room geometry, surface conditions, controls, adopted code edition, AHJ interpretation, and qualified lighting/electrical review.

Learn More

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Lighting Design Basics: Lumen Method Source Boundaries

Lumen Method concepts, Room Cavity Ratio, CU and LLF source boundaries, fixture-spacing cautions, energy-code planning rows, and retrofit assumptions.

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