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.

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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.

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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.

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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.

Aquarium calculator infographic: tank volume formulas for rectangular cylinder and hexagon shapes, fish stocking rules comparison (1 inch per gallon, surface area rule, goldfish rule), equipment sizing guide (heater watts per gallon by temperature differential, filter turnover rates by tank type), reference table of common tank sizes from 5 gallons to 125 gallons with dimensions gallons liters and water weight

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)

Volume (gal) = L × W × H ÷ 231

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

Volume (gal) = π × (Diameter ÷ 2)² × H ÷ 231

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

Volume (gal) = (3√3 ÷ 2) × S² × H ÷ 231

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)

Volume (gal) ≈ L × W × H ÷ 231 × 1.10

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:

Watts Needed = Volume (gal) × W/gal Rate

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

Pro tip for large tanks: For tanks over 50 gallons, use two heaters of half the required wattage placed at opposite ends of the tank. This provides redundancy (a dead heater won't let the tank crash before you notice), more even heat distribution, and lower risk of a stuck-on heater cooking your fish.

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:

Filter GPH = Tank Volume (gal) × Turnover Rate (×/hr)

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.

Screen Size Calculator →
Planning your aquarium room? The screen size calculator helps you choose the right TV or monitor size for the same room as your aquarium setup — calculate physical dimensions, PPI density, and compare screens to find the perfect display for viewing your tank from a comfortable distance.
Data Transfer Calculator →
Documenting your aquarium setup? Calculate how long it takes to copy your tank photos and videos to backup storage. Particularly useful for time-lapse photographers and aquarists who record tank journals — estimate transfer times for large video files from your camera or GoPro.
Date Calculator →
Track your aquarium maintenance schedule. Calculate the exact number of days since your last water change, filter cleaning, or livestock addition. Monitor fish quarantine periods (typically 2–4 weeks), medication cycles, and the nitrogen cycle progress (typically 4–6 weeks).
Percentage Calculator →
Calculate water change percentages (25% of 55 gallons = 13.75 gallons), medication dosing (5 mL per 10 gallons for your 75-gallon tank), and treatment dilutions. Also useful for calculating the cost percentage of livestock vs equipment in your aquarium budget.
Download Time Calculator →
Modern aquarium controllers and monitoring systems (Apex, GHL, Reef-Pi) use large firmware updates and database files. Use the download time calculator to estimate how long updates will take over your home network connection before scheduling maintenance windows.
Minecraft Stack Calculator →
Building an aquarium room in Minecraft? Many aquarists recreate their fish tanks in-game for layout planning. The Minecraft stack calculator helps you tally items needed for large builds — glass blocks, water buckets, decorative blocks — and organize your inventory efficiently.