About this calculator

Made for the planning stage

Recessed Light Calculator helps you estimate recessed light count, spacing and placement in a rectangular indoor room. It is for homeowners and anyone comparing a first layout before discussing the details with an installer or lighting designer.

Change the room dimensions, fixture specifications or target light level to see how the plan changes. Review the 2D layout, then save the inputs and coordinates as CSV, PNG or a printed report. The aim is to make those choices easier to understand—not to turn an estimate into an installation design.

Recessed Light Calculator is the publishing name for this independent tool. Site maintenance covers the calculation code, explanatory pages and source catalog; it is not a professional lighting-design review. Questions about a value or result can be sent through the correction contact.

How the estimate works

The calculator uses a simplified lumen-method estimate for maintained average illuminance: fixture count × lumens per fixture × utilization factor × light loss factor ÷ room area.

Auto mode looks for a complete grid that meets the light-count estimate and the assumed spacing limits. Presets and Custom mode keep your selected light positions even when the estimated brightness is below your target.

The starting count is room area × target illuminance ÷ (lumens per fixture × UF × LLF), rounded up. Use m² with lux, or ft² with foot-candles (fc). To convert ft² to m², multiply by 0.09290304. It is not always the final Auto count: completing a grid and meeting the spacing cap can require more lights.

To calculate each step for your own room, use the lumen calculator: it separates light reaching the workplane from total fixture output and lets you compare UF and LLF assumptions before planning a layout.

The Auto spacing cap is the smaller of mounting height × spacing ratio and the approximate beam diameter at the reference plane. Mounting height means ceiling height minus reference-plane height. Equal-cell grids use half-spacing wall offsets; these describe positions, not installation clearances or verified uniformity.

The IES lumen-method definition explains the general relationship between lighting, room characteristics and average workplane illuminance. It is a reference for the concept, not an endorsement of this calculator or a source for all its default values. This implementation uses a fixed utilization factor rather than modelling the room’s reflections.

Reproduce the calculation

These software checks compare known inputs with the calculator’s own model. They do not validate a room’s measured lighting, glare or installation suitability. Examples reviewed: 2026-10-19.

All three use Auto mode, Custom specs of 850 lm, 90° and 10 W per light, with UF 0.55, LLF 0.8 and spacing ratio 1.5. Outputs below are generated from the same calculation code as the tool.

For the default room, 12 × 12 ft is 144 ft², or 13.37803776 m². Using lux, 13.37803776 × 150 ÷ (850 × 0.55 × 0.8) ≈ 5.37, rounded up to 6 lights before the Auto grid and spacing checks.

Example links replace the current calculator layout. You can then edit the inputs. A kitchen workplane target describes general illumination, not countertop shadows; the bedroom target is an editable starting point, not a bedside reading requirement.

What the checks cover

• Light count and grid dimensions, with average illuminance checked against count × lumens × UF × LLF ÷ area.
• Feet/meters and lux/foot-candle conversions without changing the physical room or target.
• Invalid inputs, named-example loading, editable layouts, Reset, and consistency with exported plan data.

A passing software check establishes consistency with this model—not accuracy against a real installation. No field measurements are claimed for these examples.

Where the data comes from

Room light-level targets

The Room / use presets come from a BRE residential lighting modelling study. Editable starting values from a residential modeling study; not regulatory minima or verified current IES recommendations.

These are average targets across the calculation plane, not a minimum at every point. Kitchen, bathroom and home-office presets describe general lighting—not cabinet shadows, mirror-face illumination or individual task areas. You can enter a different target.

Fixture specifications

The 12 catalog entries below use manufacturer product pages or specification sheets for lumens, beam angle and watts. Values are per light, not per pack. Each entry identifies its reference configuration and the date its source was checked; ASIN identifies a listing rather than a manufacturer part number.

Published nominal output is used where noted. Values are not interpolated across color temperatures or dimming levels. Inclusion is not a product ranking, an availability guarantee or confirmation of installation suitability.

Entering Custom specs? The spec-sheet reading guide shows how to match a complete model and setting to the required lumens and beam angle, with watts optional, before copying numbers into the calculator.

What we assume—and what we leave out

The following fixed values belong to this calculator. They are planning assumptions, not universal lighting requirements or values certified by the references above.

Utilization factor (UF)

0.55

Light loss factor (LLF)

0.8

Spacing / mounting height ratio

1.5

Floor = 0 ft. The suggested 2.5 ft (0.75 m) table/desk and about 3 ft (0.9 m) worktop heights are calculator assumptions, not heights specified in the source tables. Use your actual surface height.

UF represents the share of complete-fixture lumens reaching the selected calculation plane. It is fixed here, not calculated from room cavity ratio (RCR), surface reflectance or a manufacturer’s coefficient-of-utilization (CU) table.

Match the factor to the lumen basis. Lamp-based CU includes luminaire efficiency; for absolute photometry, IES pairs room utilization factor (utilance) with luminaire lumens instead. Applying a lamp-based CU to already-delivered fixture lumens can count the fixture-efficiency loss twice.

LLF applies a separate allowance for light-output depreciation and dirt accumulation over time. AGi32’s explanation of light loss describes these contributing factors.

The heatmap, minimum, maximum and uniformity estimates use the same 32 × 32 sampling grid and a symmetric beam approximation scaled to the lumen-method average. They are illustrative estimates, not measured illuminance or an IES-file photometric simulation.

The tool does not assess obstructions, detailed reflections, glare, electrical circuits or code compliance. Total watts are a sum of fixture power only. Review the final layout, product suitability and installation requirements with a qualified professional.

Source checks & updates

The dates above record source checks, not continuous monitoring or a claim that a document is the latest edition. Manufacturer specifications may change; check the linked source for the exact model and configuration before purchasing.

The calculator, this page and exported reports use the same room-target and fixture catalogs. Manufacturer attribution applies only to the listed specifications. Choosing Custom specs keeps your numbers but removes the manufacturer identity.

For any discrepancy, the useful details are the model or room preset, the value in question, and the supporting source with its version or date. You can send a correction; source-backed values and calculator assumptions are kept separate so that each can be checked on its own terms.

Recent published changes

• 2026-10-08Navigation & copy. The living-room header FAQ now goes to that guide’s answers; fixture-size and spec-sheet summaries were clarified.
• 2026-10-08Guides. Added the fixture-size, bedroom and spec-sheet guides, including reproducible bedroom examples.
• 2026-10-05Examples & input restoration. Added the homepage’s three-step instructions and complete kitchen examples; selecting the same kitchen example again restores its inputs.