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Aquarium PAR Calculator — Light Intensity for Planted Tanks

Calculate LED wattage and PAR target for your planted aquarium by plant demand. PAR (photosynthetically active radiation) is the scientific standard for planted-tank lighting — watts-per-gallon is a rough proxy at best.

Calculator

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LED wattage needed
30–60W · target 35-50 PAR
PAR values are at substrate depth. Popular LEDs: Fluval 3.0 (low-medium), Chihiros WRGB II (medium-high), Kessil A series (high).

How to use the PAR calculator

  1. Enter tank volume + depth — Depth is critical — PAR falls fast with distance. Enter actual substrate-to-waterline depth.
  2. Select plant demand — Low, medium, or high-tech. This sets the target PAR at substrate.
  3. Read recommended wattage + PAR — Calculator returns watts + μmol/m²/s target.
  4. Match to fixture specs — Find an LED with published PAR-at-depth ≥ your target.

Why PAR beats watts-per-gallon

The watts-per-gallon rule dates from the fluorescent-tube era, when most aquarium bulbs had similar spectrum and efficiency. A 20 W T12 florescent over a 20-gallon was low-medium light; doubling wattage doubled intensity. Modern LEDs shattered this consistency: a 20 W high-efficiency LED can deliver 3× the PAR of a 20 W budget LED because of superior reflector design, LED-die quality, and lens-angle optics. Watts-per-gallon is now a weak rule of thumb at best.

PAR measures what plants actually use — photons in the 400–700 nm range absorbed by chlorophyll. Two fixtures with identical PAR at the substrate will grow plants at the same rate regardless of wattage, bulb type, or spectrum (within reason). This is why reputable LED manufacturers publish PAR-at-depth charts and why hobbyist-reviewed PAR databases (Reef2Reef's LED database, Barr Report) are more useful than manufacturer-stated lumens.

PAR targets for common planted-tank plant groups

Plant groupMinimum PARIdeal PARCO2 needed?
Mosses (java, flame, christmas)1015–25No
Anubias, java fern1520–30No
Cryptocoryne, vallisneria2030–45No
Amazon sword, ludwigia repens2535–50Optional
Carpeting plants (monte carlo, HC)4050–70Yes
Red-form stems (AR mini, ludwigia red)5060–90Yes

Why tank depth matters more than surface wattage

Water absorbs light at roughly the same rate as any semi-transparent medium — about 50 % of surface PAR is lost in the first 12 inches (approximately the Beer-Lambert law applied to aquarium water, accounting for moderate tannin content and light refraction through the lens). A fixture delivering 100 PAR at the waterline gives about 40 PAR at 12 in, 25 PAR at 18 in, and just 15 PAR at 24 in.

This is why deep tanks (75, 90, 125-gallon) often struggle with carpeting plants — even high-wattage fixtures can't deliver the 50+ PAR needed at 20 inches of depth. The solutions are: use multiple smaller fixtures (better coverage than one big one), suspend the fixture 2–3 inches above the water to avoid water-surface refraction, or choose taller plants that grow into the middle/upper water column where PAR is higher.

Practical PAR recommendations by tank size

  • 10–20 gal nano (depth 10–12 in): Fluval Nano, Chihiros C2 RGB, or Finnex Stingray — 20–30 W delivers medium PAR everywhere.
  • 29–40 gal standard (depth 16–18 in): Fluval 3.0, Twinstar 450E — 30–50 W with 24/30 in length.
  • 55–75 gal (depth 18–21 in): Two Finnex Planted+ 24/7 CC fixtures or one Chihiros WRGB II 90 — 50–90 W total.
  • 125+ gal (depth 24–30 in): Multiple high-output fixtures (Kessil A160, AquaIllumination Blade) or specialized PAR-heavy LEDs, 150+ W total, ideally suspended.

Frequently Asked Questions

What is PAR in aquarium lighting?

PAR (Photosynthetically Active Radiation) measures the intensity of light wavelengths plants actually use for photosynthesis — roughly 400–700 nm — in units of μmol photons/m²/second. PAR is more accurate than watts or lumens for comparing aquarium plant light because it excludes wavelengths plants can't use. Modern LED reviews and reef forums report PAR as the gold-standard metric.

How many watts per gallon do planted aquariums need?

Rough LED guideline: 0.5–1 W/gal for low light (15–30 PAR at substrate), 1–2 W/gal for medium (35–50 PAR), 2–4 W/gal for high-tech (50–80+ PAR). Legacy T5 HO fluorescents at roughly 1 W/gal often reach medium PAR. Raw watts is a weak proxy — fixture design (lens angle, LED efficiency, tank depth) affects actual PAR far more than wattage alone.

What PAR does each plant type need?

Low-light plants (anubias, java fern, bucephalandra, moss): 15–30 PAR. Medium (amazon sword, cryptocoryne, rotala rotundifolia, vallisneria): 35–50 PAR. High-light (HC cuba, monte carlo, alternanthera reineckii, blyxa, red rotala species): 50–80+ PAR with CO2 injection. Above 80 PAR without CO2 causes algae explosions.

How do I measure PAR without buying a meter?

PAR meters cost $150–$400 (Apogee MQ-200 is the hobby standard). Alternatives: match LED fixtures to published PAR data from reputable manufacturers (Finnex, Twinstar, ChiHiros, NICREW AI). Look for PAR-at-depth charts from the manufacturer — they measure PAR at 12, 18, 24 inches from the fixture. Set photoperiod to 6–8 hours to start, then adjust based on algae vs. plant growth.

How long should the aquarium light photoperiod be?

6–8 hours daily is the sweet spot for most planted tanks. Under 6 hours plants can't accumulate enough energy; over 10 hours algae outcompetes plants. High-tech CO2 tanks can handle 8 hours at high intensity; low-tech non-CO2 tanks do better at 6–7 hours to reduce algae pressure.

Can I use regular room LEDs for plants?

Only in shallow 5–10 gallon nanos with low-light plants like anubias and moss. Household LEDs lack the red/blue spectrum plants prefer, have poor canopy penetration at depth, and don't deliver enough PAR below 18 inches. Dedicated aquarium LEDs (Finnex Planted+, Fluval 3.0, Chihiros WRGB II, Twinstar) cost $80–$250 and vastly outperform household fixtures at comparable wattage.

Does PAR drop off quickly with tank depth?

Yes — dramatically. PAR falls roughly to 40 % of surface intensity at 12 inches deep, 20 % at 18 inches, 10 % at 24 inches. A deep 75-gallon tank (21 in deep) needs a fixture rated for 2–3× the PAR a shallow 20-long (12 in deep) does for the same substrate-level intensity. Always use PAR measured at your actual substrate depth, not surface PAR.

Sources & References

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