Aquarium Calculator
Use this aquarium calculator calculator to understand your numbers quickly and make clearer decisions with confidence.
Calculator Mode
What Is an Aquarium Calculator?
An aquarium calculator is an essential tool for fishkeepers at every level — from setting up your first 10-gallon community tank to designing a sophisticated 125-gallon reef. It solves the core math problems that arise when buying, setting up, and maintaining a fish tank: how much water does it actually hold, how heavy will it be when full, how many fish can I safely keep, and what size heater and filter do I need?
The aquarium calculator above provides 3 specialized modes: Volume & Weight (calculate tank volume in gallons and liters for rectangular, cylindrical, hexagonal, and bow-front tanks with substrate weight and total weight estimates), Fish Stocking (determine safe stocking levels by fish category with a per-species breakdown table), and Equipment Guide (size your heater based on temperature differential, find the correct filter GPH, and estimate lighting requirements). Supports both US imperial (inches/gallons/°F) and metric (cm/liters/°C) units.
Multi-Shape Volume Calculator
Most aquarium calculators only handle rectangular tanks. This tool supports rectangular, cylinder, hexagon, and bow-front shapes using exact mathematical formulas (π for cylinders, 3√3/2 for hexagons). Results shown in both gallons and liters with water weight and substrate weight.
Fish Stocking by Category
Go beyond the basic "1 inch per gallon" rule. The stocking guide provides targeted recommendations for nano fish, community fish, cichlids, goldfish, bettas, and reef setups — each with their own space requirements and biological considerations.
Complete Equipment Sizing
Choose the right heater wattage based on the temperature difference between your room and desired tank temperature. Size your filter by tank type turnover rate. Calculate total lumens needed for your lighting goal — from fish-only to high-light planted to reef setups.
Tank volume formulas, fish stocking rules, equipment sizing guide, and common tank sizes reference. See tank sizes table →
Tank Volume Formulas: All Shapes
The volume formula depends on your tank's shape. All formulas assume measurements in inches and convert to US gallons using the exact definition: 1 US gallon = 231 cubic inches. For metric calculations, dimensions in centimeters divide by 1,000 to get liters.
① Rectangular Tank (Most Common)
Example: 55-gallon standard tank (48" × 13" × 21"):
48 × 13 × 21 = 13,104 in³ ÷ 231 = 56.7 gallons (214.6 liters)
Water weight (freshwater): 56.7 × 8.34 = 472.7 lbs (214.5 kg)
② Cylinder Tank
Example: Cylinder tank, 18" diameter, 24" tall:
π × (9)² × 24 ÷ 231 = π × 81 × 24 ÷ 231 = 6,107.3 ÷ 231 = 26.4 gallons (100.0 liters)
③ Hexagonal Tank
Example: Hexagonal tank, 12" side length, 18" tall:
(3√3/2) × 144 × 18 ÷ 231 = 2.598 × 144 × 18 ÷ 231 = 6,736.4 ÷ 231 = 29.2 gallons (110.3 liters)
④ Bow-Front Tank (Approximate)
Bow-front tanks curve outward in the front panel. The exact additional volume depends on the curve depth, but a 10% addition to the rectangular volume gives a reliable approximation for standard bow-front configurations. For precision measurements, consult your tank manufacturer's specifications.
Fish Stocking Guide & Rules
Overstocking is the number one cause of poor water quality and fish health problems in home aquariums. The famous "1 inch of fish per gallon" rule is a starting point, but experienced fishkeepers know it has serious limitations — it doesn't account for the fish's body mass (a 6-inch oscar produces far more waste than 6 neon tetras), territorial needs, or surface area for gas exchange.
| Fish Category | Stocking Rule | Min Tank Size | Examples | Key Notes |
|---|---|---|---|---|
| Nano fish (<1.5") | 1 inch/gallon | 5 gallons | Ember tetra, chili rasbora, pygmy corydoras | High bioload per inch; need heavily planted tanks; sensitive to water params |
| Small community (1.5–3") | 1 inch/1.5 gal | 10 gallons | Neon tetra, guppy, platy, molly, danio | Most forgiving; active schoolers need 6+ per species |
| Medium community (3–5") | 1 fish/5–10 gal | 20 gallons | Angelfish, rainbowfish, bala shark, larger barbs | Territorial during spawning; consider space per individual |
| Small cichlids (4–6") | 1 pair/15–20 gal | 29 gallons | Ram, apistogramma, firemouth, convict, kribensis | Aggressive during breeding; provide caves and line-of-sight breaks |
| Large cichlids (8–15") | 1 fish/50+ gal | 75 gallons | Oscar, jack dempsey, flowerhorn, dovii | Extreme waste producers; quality canister filters essential |
| Fancy goldfish | 20 gal first + 10 each | 20 gallons | Oranda, ryukin, telescope, ranchu, bubble eye | Cool water 65–72°F; massive waste; need strong filtration |
| Common goldfish | 50+ gal each | Pond | Common, comet, shubunkin, wakin | Grow to 12"+ in ponds; NOT suitable for home aquariums long-term |
| Betta splendens | 1 male/tank minimum 5 gal | 5 gallons | Halfmoon, crowntail, plakat, delta | Males fight to death; female sorority needs 20+ gal with heavy plants |
| Marine reef fish | 1 inch/10–15 gal display | 30 gallons | Clownfish, tang, damsel, dottyback, wrasse | Bioload varies; tangs need 100+ gal; research before buying |
Heater Sizing Calculator
The wattage of your aquarium heater depends on how far above room temperature you need to maintain the tank. The larger the temperature difference, the more watts-per-gallon you need:
| Temp Difference (ΔT) | W/gal Rate | Typical Situation | Example: 55 gal |
|---|---|---|---|
| ΔT ≤ 10°F (5.6°C) | 2.5 W/gal | Warm climate; room typically 70°F+; tank needs 78–80°F | 138 W → 150W heater |
| ΔT ≤ 20°F (11°C) | 5 W/gal | Temperate climate; room 60–65°F; tank needs 78–80°F | 275 W → 300W heater |
| ΔT > 20°F (11°C+) | 7.5 W/gal | Cold climate; unheated room; tropical fish temperature target | 413 W → 2 × 200W heaters |
Filter Flow Rate Guide
The filter flow rate is expressed as GPH (gallons per hour) — how many times your filter turns over the entire tank volume per hour. More turnover means better filtration, but excessive flow can stress fish and damage plants:
| Aquarium Type | Turnover | 5 Gal | 29 Gal | 55 Gal | 125 Gal |
|---|---|---|---|---|---|
| Nano / Shrimp | 4–6× | 20–30 GPH | 116–174 GPH | 227–340 GPH | 497–746 GPH |
| Heavily Planted | 4–6× | 20–30 GPH | 116–174 GPH | 227–340 GPH | 497–746 GPH |
| Community Freshwater | 6–8× | 30–40 GPH | 174–232 GPH | 340–454 GPH | 746–994 GPH |
| African Cichlid | 8–10× | 40–50 GPH | 232–290 GPH | 454–567 GPH | 994–1,243 GPH |
| Goldfish | 8–10× | 40–50 GPH | 232–290 GPH | 454–567 GPH | 994–1,243 GPH |
| Reef / Marine | 10–20× | 50–100 GPH | 290–580 GPH | 567–1,134 GPH | 1,243–2,485 GPH |
* These are total GPH targets. Note that manufacturer GPH ratings are tested with no media, clean filters, and optimal hose positions. Real-world performance is typically 20–30% lower — factor this in when choosing a filter model.
Common Tank Sizes Reference
Standard aquarium sizes in the United States follow common dimension patterns. Note that the nominal gallon rating is marketing — the actual calculated volume is usually slightly different. Always use the actual interior dimensions for your calculations:
| Name | Dimensions (in) | US Gallons | Liters | Water (lbs) | Full Est. (lbs) | Best For |
|---|---|---|---|---|---|---|
| 5 Gallon | 16×8×10 | 5.5 | 20.8 | 45 | 75 | Betta, shrimp, nano |
| 10 Gallon | 20×10×12 | 10.4 | 39.3 | 87 | 135 | Nano fish, breeding |
| 20 Gal Long | 30×12×12 | 18.7 | 70.8 | 156 | 225 | Low bottom-dwellers, planted |
| 20 Gal High | 24×12×16 | 19.9 | 75.3 | 166 | 240 | Community, planted |
| 29 Gallon | 30×12×18 | 28.1 | 106.3 | 234 | 335 | Community, smaller cichlids |
| 40 Breeder | 36×18×16 | 44.9 | 169.9 | 374 | 530 | Turtle, planted, breeding |
| 55 Gallon | 48×13×21 | 56.7 | 214.6 | 473 | 670 | Community, African cichlids |
| 75 Gallon | 48×18×21 | 78.5 | 297.2 | 654 | 920 | Large community, medium cichlids |
| 90 Gallon | 48×18×24 | 89.7 | 339.6 | 748 | 1,050 | Large cichlids, reef starter |
| 125 Gallon | 72×18×22 | 123.4 | 467.2 | 1,029 | 1,430 | Large predators, reef, show tank |
| 180 Gallon | 72×24×25 | 187.5 | 709.8 | 1,564 | 2,150 | Large reef, predator community |
* Full weight estimates include water + gravel substrate (2" depth) + glass tank + equipment. Actual weights vary. Always place your aquarium on appropriate furniture rated for the weight — the floor structure must support 10–20 lbs/sq ft for large tanks.
Frequently Asked Questions
🐟How accurate is the "1 inch of fish per gallon" rule?
The rule is a rough starting point that works reasonably well for small fish under 3 inches. It breaks down badly for larger fish: a single 10-inch oscar produces as much waste as roughly 50 neon tetras, would demolish a 10-gallon tank, yet technically "fits" the rule in 10 gallons. For fish over 4 inches, use the fish-specific requirements listed by experienced keepers for that species, not the inch-per-gallon rule. The surface area rule (1 inch of fish per 12 sq inches of water surface) is more biologically sound since it accounts for gas exchange.
🐟How much does a full fish tank weigh?
A full freshwater aquarium weighs approximately 10–11 lbs per gallon (water = 8.34 lbs, plus substrate ~1.5 lbs, glass ~0.5 lbs, equipment ~0.5 lbs). Saltwater tanks weigh slightly more (8.55 lbs/gal for the water). Practical estimates: 10 gal ≈ 110 lbs, 29 gal ≈ 335 lbs, 55 gal ≈ 670 lbs, 75 gal ≈ 920 lbs, 125 gal ≈ 1,430 lbs. Always place large aquariums against a wall (over a floor joist) and use a dedicated aquarium stand rated for the weight.
🐟What size heater do I need for my aquarium?
The heater wattage depends on your room temperature and the desired tank temperature. For most tropical fish (78°F) in a room at 68–72°F, use 5 watts per gallon. For a 55-gallon tank in a 70°F room targeting 78°F (ΔT = 8°F): 55 × 2.5 = 138 watts → buy a 150W heater. Always buy one size up from your calculation. For tanks over 50 gallons, use two heaters (each half the total wattage) at opposite ends for even heating and safety redundancy.
🐟What size filter do I need for my fish tank?
Choose a filter that processes your tank volume at least 6–8 times per hour for community tanks. For a 55-gallon community tank: 55 × 7 = 385 GPH minimum. Since manufacturer GPH ratings are measured under ideal conditions, real-world performance is 20–30% lower — so buy a filter rated for 500+ GPH for a 55-gallon community tank. Cichlid, goldfish, and heavily stocked tanks need 8–10× turnover; heavily planted tanks with CO₂ injection benefit from lower flow (4–6×) to preserve dissolved CO₂.
🐟How many gallons does my oddly-shaped aquarium hold?
For rectangular tanks: (Length × Width × Height in inches) ÷ 231 = US gallons. For cylinder tanks: π × (Diameter÷2)² × Height ÷ 231. For hexagonal tanks (where S = side length): (3√3/2) × S² × Height ÷ 231. For bow-front tanks: calculate as rectangular and add approximately 10% for the bow curve. Use the Volume & Weight mode in the aquarium calculator above to compute any of these shapes instantly in both gallons and liters.
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