Reef light coverage calculator

What the calculator is doing

The calculator begins with a broad source class rather than pretending every reef LED behaves identically. Compact sources use a nominal 50cm planning spread, broad 60cm-class sources use 60cm and wide systems use 90cm. It then reduces that span for a mixed or SPS-led plan and for deeper target positions. Finally, it preserves overlap between adjacent sources instead of placing headline coverage boxes edge to edge.

This produces a cautious source count across the coral-bearing length and front-to-back width. A separated-island layout can add another source when one central fixture would otherwise be asked to illuminate opposing structures. Every assumption is deliberately visible. The result does not choose a brand, channel schedule or coral placement for you.

Why tank litres are not an input

Lighting is a spatial problem. A shallow 100-litre frag tank can be long and wide, while a 100-litre cube can be compact and deep. The same volume therefore presents different spread, depth and shadow constraints. Nominal litres also include areas with no coral and ignore rock height. Using volume as the primary rule creates precise-looking but poorly targeted answers.

Enter the internal display length and front-to-back dimension. Then measure only the zone where coral is intended. If the aquascape occupies 45cm inside a 60cm tank, enter 45cm rather than buying light for empty glass. If coral may eventually grow across the full display, use the mature planned zone rather than the launch-day frag positions.

How to choose a source class

Why coral plan changes the result

The soft-coral, mixed and SPS selections are not universal PAR targets. They are planning margins. A demanding full-width SPS system has less tolerance for weak edges and mature self-shadowing than a central soft-coral island. The model therefore reduces the usable span as demand rises. It does not claim every SPS coral needs the same intensity or that all soft corals should be treated alike.

Check species-specific husbandry separately and include acclimation history. A coral moved from a low-light system can be stressed even when its new position falls inside a commonly quoted range. The calculator estimates layout; the final placement still needs measurements, observation and stable water conditions.

Why depth changes the result

The target-depth input is the distance from the water surface to the deepest important coral position—not simply the glass height. Greater distance normally reduces intensity at the target, but the exact relationship depends on optics, water and mounting. The calculator applies a conservative step-down rather than inventing a PAR conversion from watts.

If the result adds a light because of depth, compare that route against a higher-output fixture in the same geometric class. Do not assume adding watts to one central fixture fixes the ends, and do not lower a light below approved guidance merely to raise one bottom reading. Distribution and intensity remain separate constraints.

How the source count is rounded

The first source receives one conservative planning span. Each additional source contributes slightly less than a full span because neighbouring fields need overlap. This prevents a pair of nominal 60cm footprints from being presented as perfect 120cm end-to-end coverage. The overlap is a planning allowance, not a claim about any particular optic. A real map may support wider spacing or show that the proposed geometry needs another angle.

The model also checks front-to-back width. Most conventional tanks need one row, but a genuinely wide coral zone can trigger a second row. Before accepting that result, compare a wider fixture, a different mounting height and targeted fill bars. A calculator should expose an expensive layout for investigation, not pressure you into buying every source it can count.

What not to do with the result

Do not search for the cheapest product in the chosen class, multiply it by the displayed number and order immediately. Product footprints are not standardised, mounting accessories can change the field and the same model may have different guidance for mixed and SPS systems. Open the manufacturer documentation for the exact generation and record the stated height, spread, depth and power.

Do not interpret the combined wattage as a target. It is simply the sum of representative label ceilings used for the ownership-cost example. More sources can run at lower settings and create better distribution; fewer high-power sources can still leave hard shadows. The objective is a measurable field with safe headroom, not the largest total wattage.

Finally, do not use the tool to justify a sudden programme change around established livestock. If it suggests a different layout, preserve the previous setup until the new arrangement is installed and measured. Use acclimation, monitor coral response and keep the change reversible.

How to use the result

1. 01

   Draw the proposed centres

   Place the suggested number of sources over the coral zone with overlap. Mark rocks, braces, rear chambers and places where mounts can physically sit.

2. 02

   Build a product shortlist

   Choose exact fixtures whose documented spread, depth guidance and mounting system fit the geometry. Compare complete mounted prices and UK support.

3. 03

   Install conservatively

   Begin within manufacturer guidance at a reduced programme, preserve the old schedule where applicable and avoid combining the light change with unrelated major changes.

4. 04

   Map the real aquarium

   Measure the intended coral positions, centre overlap, edges and structural shadows. Reposition sources before using output as the only correction.

Understanding the cost estimate

The electricity figure is a maximum-label example. It multiplies the number of sources by a representative class wattage, ten hours per day, 365 days and 26.11p/kWh. Real fixtures normally ramp and run below combined channel maximum, so actual use may be lower. Replace the estimate with the exact product and programme in the full electricity calculator, ideally using a plug-in energy monitor.

Fixture price is excluded because models within a class vary widely and UK promotions move quickly. Build the installed budget from the fixture, arm or hanging kit, rail, shades or diffusers, controller accessories and every extra source. A two-light layout can outperform one stretched fixture, but its mounts and future replacement paths belong in the decision.

Worked example: a 90cm mixed reef

The tank is 90 × 50cm, but the coral rock occupies 78 × 42cm and the deepest target is 45cm. Selecting a broad 60cm source and mixed reef reduces the conservative planning span below the headline claim. The calculator suggests two sources along the length with useful overlap rather than one unit centred over the glass.

The owner then compares two XR15-class units with two lower-cost compact fixtures. The premium route offers more headroom; the compact route may suit two restrained islands. The correct answer comes from the exact mounted cost and a map, not from the calculator declaring a product winner. Space is reserved for a future fill bar, but it is not purchased until coral growth or measurement demonstrates the need.

Limitations that remain

The model cannot see rock overhangs, water clarity, surface agitation, lens design, diffusers, room glare, spectrum or the response of an individual coral. It also cannot know whether a manufacturer’s stated spread describes maximum visible light, a mixed-reef field or a measured target at a specific depth. Treat its count as the beginning of research, and retain a written note of every assumption used.

Use the exact manufacturer specification and manual for the chosen fixture. If evidence is missing, keep more margin, restrict demanding coral to measured positions or choose a reversible modular layout. Reef Bench does not convert a product’s watts into imaginary PAR.

Frequently asked questions