Aquarium Calculator Pro
All-in-one aquarium planner: tank volume (gallons), filter GPH at 4-10× turnover, heater wattage at 3-5 W/gal, and stocking density by the inch-per-gallon rule. Handles rectangular, bow-front, cube, hex, and cylinder tanks across freshwater, planted CO₂, FOWLR saltwater, and reef setups per AALSO and ARMM hobby standards.
Quick Conversion
Formula: gal = in³ × 0.004329
Build your aquarium
Live Aquarium Layout
Freshwater tropicalTotal fish inches: 33.0 · Net water: 51.1 gal
Common Tank Presets
Volume × Filtration × Heater Quick Table
| Volume (gal) | Min Filter (4× GPH) | Reef (8× GPH) | Heater (4 W/gal) | Heater (5 W/gal) |
|---|---|---|---|---|
| 5 | 20 | 40 | 20 W | 25 W |
| 10 | 40 | 80 | 40 W | 50 W |
| 20 | 80 | 160 | 80 W | 100 W |
| 29 | 116 | 232 | 116 W | 145 W |
| 40 | 160 | 320 | 160 W | 200 W |
| 55 | 220 | 440 | 220 W | 275 W |
| 75 | 300 | 600 | 300 W | 375 W |
| 90 | 360 | 720 | 360 W | 450 W |
| 125 | 500 | 1000 | 500 W | 625 W |
| 180 | 720 | 1440 | 720 W | 900 W |
| 220 | 880 | 1760 | 880 W | 1100 W |
Need just tank volume? Aquarium volume-only tool →
Formulas
Volume_gal (rectangular) = L × W × H × 0.004329Volume_gal (hex) = 2.598 × s² × H × 0.004329Filter_GPH = Volume × turnover (4× FW, 8-10× reef)Heater_W = Volume × 4 (tropical) or × 5 (cool room / saltwater)Stocking % = (Σ fish_inches / Net_gallons) × 100Worked: 48 × 13 × 21 = 13,104 in³ × 0.004329 = 56.7 gal; Filter 4× = 227 GPH; Heater 4 W = 227 W; 12 neon × 1.5″ + 6 cory × 2.5″ = 33″ → 33/51 = 65% (healthy).
Fish Stocking Reference
| Fish | Size (in) | Gal/fish | Group | Temp |
|---|---|---|---|---|
| Neon / Cardinal Tetra | 1.5 | 1 | Community freshwater | 73–80°F |
| Betta | 2.5 | 2.5 | Solo or sorority | 76–82°F |
| Fancy Goldfish | 6 | 10 | Coldwater | 65–72°F |
| Angelfish | 6 | 10 | Community SA | 76–82°F |
| Discus | 6 | 10 | Soft acidic SA | 82–88°F |
| Guppy | 2 | 1 | Community freshwater | 72–82°F |
| African Cichlid | 4 | 5 | Hard alkaline Rift | 76–82°F |
| Platy | 2 | 1 | Community freshwater | 72–78°F |
| Cory Catfish | 2.5 | 2 | Bottom dweller | 72–78°F |
| Clownfish (Ocellaris) | 3 | 10 | Reef-safe SW | 75–80°F |
| Tang (Yellow / Blue) | 6 | 30 | Larger SW | 75–80°F |
How to Plan an Aquarium
- 1Pick the tank shape and dimensionsRectangular tanks are the easiest to size; hex and cylinder run lower gallons per footprint. Enter L × W × H in inches.
- 2Choose the water typeFreshwater tropical, coldwater, saltwater FOWLR, reef, or planted CO₂ — each sets the filter turnover multiplier and heater watts.
- 3Read filter GPH and heater WThe tool applies 4× FW, 5× SW, 8× reef, and 3-5 W/gal heater rules. Match the next-larger commercially available unit.
- 4Add fish from the species listUse the dropdown to pick two species and quantities. The tool tracks total fish inches against net water volume.
- 5Check the stocking lightUnder 70% is understocked, 70-100% healthy, 100-130% overstocked, >130% severely overstocked. Big fish need 5-10 gal each, not 1.
A short history of the home aquarium
In 2026, a first-time saltwater hobbyist in Austin plans a 75-gallon reef build. The aquatic supply house quotes a filter rated at 350 GPH and a 300-watt heater. The hobbyist wonders whether those are right for reef-specific 8× turnover and the room's 68°F ambient — and types the dimensions into this calculator to find out in seconds.
The home aquarium traces to 1830s England, where Anna Thynne, a London naturalist, kept the first known balanced marine aquarium between 1846 and 1850. She circulated seawater by hand, observed the carbonate cycle, and noted that vegetation (algae) generated oxygen during daylight. Her work, published in 1859, established the foundational principle: stable aquaria require a biological cycle, not just water.
Robert Warington at the Royal Institution refined the concept in 1850, introducing the term "aquarium" (from Latin aqua + arium, place of) and demonstrating that aquatic plants and snails could balance fish waste in a sealed glass container. The Great Exhibition of 1851 displayed Warington's tanks; by 1854 the London Zoo opened the first public aquarium. The Victorian aquarium craze followed.
Filtration technology advanced through the 20th century. The undergravel filter, patented 1953 by Charles Petersen, dominated American hobbyist tanks for 30 years. Power filters (HOB, hang-on-back) emerged in the 1970s with the Aquaclear brand. Canister filters from Eheim (Germany, 1949) brought sealed mechanical-biological filtration to advanced hobbyists. Today's sump-and-skimmer reef systems trace to Walt Smith's 1980s Berlin-method reef tanks.
The 1 inch per gallon rule emerged in 1960s American Aquarium magazine columns and remains the field shorthand — though aquarists like Diana Walstad (Ecology of the Planted Aquarium, 1999) and Steven Pro (TFH columnist) have refined it for body shape and waste output. Goldfish and cichlids need 5-10× the surface area of similarly-sized tetras; bristlenose plecos need 2-3× the volume per inch.
Reef tank biology was decoded in the 1990s. Julian Sprung, Charles Delbeek, and the Reef Aquarium Trilogy (1994-2005) codified the calcium-alkalinity-magnesium triad, the role of live rock as primary biological filtration, and protein skimming for organic waste export. Today's home reefs grow Acropora coral that would have been impossible in 1995. AALSO (Aquatic Animal Life Support Operators) certifications credential public aquarium technicians worldwide.
Planted-tank technology accelerated with Takashi Amano's "nature aquarium" movement from the 1980s onward, popularizing pressurized CO₂, high-PAR LED lighting, and aquascape design. Amano's ADA brand established the visual vocabulary every modern planted-tank hobbyist references. Today's drop-checker CO₂ monitors and integrated lighting are direct descendants of his methods. Pair this calculator with our aquarium volume tool and aquarium weight calculator for full setup planning.
Aquarium Setup Pro Tips
Establish the nitrogen cycle (ammonia → nitrite → nitrate) over 3-6 weeks before adding any fish. Fishless cycling with pure ammonia is the standard method.
Manufacturer ratings are at zero head with clean media. After biological loading and lift, real-world flow drops 30-40%. Buy bigger.
Swing-arm hydrometers drift 0.002-0.004 SG over time. Use a refractometer calibrated to 35 ppt RO/DI water for accurate reef salinity.
Ich, marine velvet, and bacterial infections enter through new stock. Quarantine in a separate 10-20 gal tank for 30 days minimum.
Two 150W heaters > one 300W. Redundancy prevents stuck-on (cooked tank) or stuck-off (frozen tank) catastrophes.
Regular water changes export nitrate and replenish trace minerals. Heavily stocked tanks: 25% weekly. Reef tanks with ATO: 10% biweekly.
What do the aquarium numbers actually mean?
A 56.7-gallon tank at 4× turnover requires 227 GPH of mechanical-biological filtration. That means every hour, every drop of water passes through the filter media four times. In practice, choose a filter rated 1.5-2× higher (350-450 GPH) because manufacturer ratings are at zero head with clean media — your actual flow drops 30-40% after a month of biological loading.
A 227-watt heater (4 W/gal) raises a 56-gallon tank from a 65°F room to a 78°F target. Two 150-watt heaters in opposite corners are safer than one 300-watt unit: redundancy if one fails open (stuck-on cooks the tank) or closed (no heat freezes the fish). Most reef and discus keepers run dual heaters by default per Reefkeeping Magazine's "K.I.S.S." principle.
A stocking density of 65% means you have headroom — for one or two more fish, larger pieces, or seasonal additions. At 100%, you're at the inch-per-gallon limit and water changes need to be weekly 20%. Above 130%, ammonia spikes appear despite ostensibly adequate filtration and pH stability deteriorates. The 1"/gal rule is a heuristic, not a hard limit — body shape, waste output, and territory needs all modify it.
Aquarium types comparison
| Type | Filter | Heater | pH / Salinity | Difficulty |
|---|---|---|---|---|
| Freshwater tropical | 4× GPH | 4 W/gal | pH 6.5-7.5 | Beginner |
| Goldfish / coldwater | 6× GPH | 0 W | pH 7.0-7.8 | Beginner |
| Saltwater FOWLR | 5× GPH | 5 W/gal | SG 1.025 | Intermediate |
| Reef (SPS / mixed) | 8-10× GPH | 5 W/gal | SG 1.025, Ca 420, dKH 8 | Advanced |
| Planted CO₂ | 5× GPH | 4 W/gal | pH 6.0-7.0 + CO₂ 30 ppm | Intermediate-Adv |
Hobbyist & Marine Biologist Reviews
“I've set up four reefs in 2025-2026. The filter GPH at 8× rule + the 5 W/gal heater spec match my equipment lists perfectly. The widget showing fish + filter + heater in one tank visual is a delightful touch.”
“I consult on six commercial display tanks and use this calculator for client quotes. The volume math, water-type filter multipliers, and stocking rules align with ARMM and AALSO guidelines. Beautifully presented.”
“Customers walk in asking 'how big a filter do I need for a 75?' I open this on the counter tablet and walk them through. The salt vs freshwater toggle handles both. We've printed the QR code on receipt slips.”
“Discus run hot at 86°F with 6× turnover. The water-type selector and the 1"/gal warning for big-bodied fish are exactly what new discus keepers miss. Linking newbies to this saves me forty minutes of stocking advice per call.”
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