Coral PAR Requirement Calculator

Enter your surface PAR, placement depth, and coral type to estimate PAR at the coral's position and check whether it falls within the ideal range.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Surface PAR
Input the Photosynthetically Active Radiation (PAR) measured at the water surface, directly under your light fixture, in µmol.
2
Specify Depth from Light
Enter the vertical distance in inches from the light source to where your coral is placed in the tank.
3
Select Coral Type
Choose the type of coral you are placing (Softies, LPS, SPS, Tridacna Clams, or Zoanthids) to define its PAR requirements.
4
Review PAR Placement Status
Examine the PAR at placement, its status (Optimal, Too Low, Too High), and recommendations for adjustment.

Example Calculation

An aquarist has 400 µmol PAR at the surface and wants to place an LPS coral 12 inches deep. They need to know if the PAR is suitable.

Surface PAR

400 µmol

Depth from Light

12 in

Coral Type

LPS

Results

220 µmol PAR at Placement

Tips

Account for Refraction and Reflection

Remember that actual PAR delivered to coral can be affected by water surface agitation and tank wall reflections. While this calculator estimates direct light, real-world conditions might slightly alter the true PAR reading.

Gradually Acclimate New Corals

If moving a coral to a significantly higher PAR zone, acclimate it gradually over several days or weeks by starting it deeper and slowly raising it. This prevents bleaching from light shock, even if the final PAR is within its optimal range.

Consider Light Spectrum

While PAR measures intensity, corals also benefit from a balanced light spectrum. Ensure your lighting provides sufficient blue, red, and green wavelengths for photosynthesis and coloration, not just high PAR numbers.

Optimizing Coral Placement with PAR Requirements

The Coral PAR Requirement Calculator is an indispensable tool for aquarists aiming to create a thriving reef environment. This calculator estimates the Photosynthetically Active Radiation (PAR) at your coral's specific placement depth, determining if it's optimal, too low, or too high for its species. For example, if you have 400 µmol PAR at the surface and place an LPS coral 12 inches deep, the calculator might show 220 µmol PAR at placement, indicating it's too high for LPS corals (which prefer 75-175 µmol). This precise analysis helps prevent coral bleaching or starvation, ensuring optimal health and vibrant growth for your reef inhabitants in 2025.

Optimizing Photosynthetically Active Radiation for Corals

Optimizing Photosynthetically Active Radiation (PAR) for corals is critical for their survival and vibrant growth, as PAR fuels the photosynthesis of their symbiotic zooxanthellae. Different coral types have distinct light demands; for instance, SPS corals like Acropora typically require high PAR (200-400 µmol), while LPS corals such as Euphyllia thrive in medium light (75-175 µmol), and soft corals or zoanthids prefer lower PAR (25-100 µmol). Excessive PAR can lead to coral bleaching, where the algae are expelled, while inadequate PAR causes starvation. Maintaining consistent PAR levels within these species-specific ranges is therefore paramount for healthy growth and brilliant coloration in a reef aquarium.

Estimating PAR Attenuation Through Water

The Coral PAR Requirement Calculator uses an exponential decay model, an approximation of the Beer-Lambert Law, to estimate how Photosynthetically Active Radiation (PAR) attenuates as it penetrates water. This model accounts for the absorption and scattering of light by the water column, which reduces light intensity with increasing depth.

The primary formula for PAR at placement is:

placement PAR = surface PAR × EXP(-0.05 × depth from light)

Here, surface PAR is the reading directly under the light, depth from light is in inches, and EXP is the exponential function (e^x). The constant 0.05 represents an average attenuation coefficient for typical reef tank water. This calculated placement PAR is then compared against the min PAR and max PAR requirements specific to the chosen coral type.

💡 When selecting your lighting system, our Aquarium LED Wattage per Gallon Calculator can help ensure you have sufficient power to achieve your desired surface PAR levels.

Assessing PAR for an LPS Coral at Depth

Let's consider an aquarist setting up a new reef tank and determining the ideal placement for an LPS coral:

Surface PAR: The light fixture provides 400 µmol PAR at the water surface.
Depth from Light: The aquarist plans to place the LPS coral 12 inches below the light.
Coral Type: LPS (with a typical PAR range of 100-200 µmol).

The calculation proceeds as follows:

Estimated PAR at Placement: 400 µmol × EXP(-0.05 × 12 inches) = 400 × EXP(-0.6) ≈ 219.52 µmol.
Comparison to LPS Range: The calculated 219.52 µmol is above the LPS maximum of 200 µmol.
Placement Status: Too High.

The result indicates that the LPS coral, if placed at 12 inches, would receive too much light. The aquarist should consider moving the coral deeper to reduce the PAR intensity to its optimal range.

💡 Beyond intensity, the duration of light exposure is also crucial. Our Aquarium Light Duration by Tank Type Calculator helps you set appropriate photoperiods for your specific tank.

Optimizing Photosynthetically Active Radiation for Corals

Limitations of PAR Measurement and Calculator Estimates

While the Coral PAR Requirement Calculator provides a valuable estimate, it's important to acknowledge situations where simple PAR calculations might be insufficient for truly optimal coral placement. Factors such as water clarity, which can fluctuate due to detritus, algae, or dissolved organic compounds, can significantly alter the actual light penetration. Tank geometry, including reflections off tank walls and the shadowing effect of rockwork or other corals, can create unexpected hot spots or dim areas. Furthermore, the light spectrum emitted by a fixture, even at a consistent PAR, can affect coral health. For example, a light heavy in green wavelengths might show high PAR but be less photosynthetically efficient for zooxanthellae. These variables underscore the necessity for aquarists to conduct regular in-tank PAR meter readings and observe coral behavior for precise, real-time adjustments.

Frequently Asked Questions

What is PAR (Photosynthetically Active Radiation) in reef tanks?

PAR, or Photosynthetically Active Radiation, refers to the spectrum of light (400 to 700 nanometers) that photosynthetic organisms, including corals and their symbiotic zooxanthellae, use for photosynthesis. In reef tanks, PAR is measured in micromoles per square meter per second (µmol m⁻² s⁻¹), indicating the intensity of light available for energy production. It is the most critical metric for assessing lighting adequacy for corals, as insufficient PAR leads to starvation, while excessive PAR can cause bleaching and stress, making precise measurement essential for coral health.

Why do different coral types have different PAR requirements?

Different coral types have different PAR requirements primarily due to their natural habitat and evolutionary adaptations. SPS (Small Polyp Stony) corals, often found in shallow, high-light environments, typically require high PAR (200-400 µmol). LPS (Large Polyp Stony) corals, from deeper or more shaded areas, thrive in medium PAR (75-175 µmol), while soft corals and zoanthids, from even deeper or turbid waters, need low PAR (25-100 µmol). These varying demands reflect the efficiency of their zooxanthellae and their ability to cope with intense light, making species-specific lighting crucial for survival.

What are the risks of too low or too high PAR for corals?

Too low PAR for corals can lead to starvation, as their symbiotic zooxanthellae cannot produce enough energy through photosynthesis to sustain the coral, resulting in browning, tissue recession, and eventual death. Conversely, too high PAR can cause photoinhibition and coral bleaching, where the zooxanthellae are expelled due to light stress, leaving the coral's white skeleton exposed. Both extremes are detrimental to coral health, underscoring the importance of providing PAR within the species-specific optimal range to prevent stress and promote vibrant growth.

How does water depth affect PAR levels in an aquarium?

Water depth significantly affects PAR levels in an aquarium due to light attenuation, meaning light intensity decreases exponentially as it travels through water. This phenomenon is governed by the Beer-Lambert Law, where water absorbs and scatters light, especially at longer wavelengths (red and orange). Consequently, corals placed deeper in a tank will receive substantially less PAR than those at the surface. The rate of attenuation also depends on water clarity, with clearer water allowing deeper light penetration, necessitating careful coral placement based on their specific light requirements at various depths.