You’ve got a space, a fish, or a budget. What you need is the number. This aquarium size calculator tells you the volume in gallons or liters, the dimensions for any standard tank, and whether a given size actually works for your fish — without digging through manufacturer spec sheets.
Enter your tank dimensions to calculate volume, or pick a gallon size to see exact dimensions.
Standard Aquarium Sizes and Dimensions
If you’re shopping for a tank rather than measuring one you already own, this table covers every common size you’ll find at a pet store.
| Gallons | Length | Width | Height | Approx. Weight (Full) |
|---|---|---|---|---|
| 5 gal | 16″ | 8″ | 10″ | 62 lbs |
| 10 gal | 20″ | 10″ | 12″ | 111 lbs |
| 20 gal (High) | 24″ | 12″ | 16″ | 225 lbs |
| 20 gal (Long) | 30″ | 12″ | 12″ | 225 lbs |
| 29 gal | 30″ | 12″ | 18″ | 330 lbs |
| 40 gal (Breeder) | 36″ | 18″ | 16″ | 458 lbs |
| 55 gal | 48″ | 13″ | 20″ | 625 lbs |
| 75 gal | 48″ | 18″ | 21″ | 850 lbs |
| 90 gal | 48″ | 18″ | 24″ | 1050 lbs |
| 100 gal | 72″ | 18″ | 19″ | 1100 lbs |
| 120 gal | 48″ | 24″ | 24″ | 1400 lbs |
| 125 gal | 72″ | 18″ | 22″ | 1400 lbs |
| 180 gal | 72″ | 24″ | 24″ | 2100 lbs |
Weight includes water, gravel, rocks, and glass. Plan your floor and stand accordingly.
How to Calculate Aquarium Volume
The formula depends on your tank shape. Most aquariums are rectangular, which makes the math simple.
Rectangular tank:
Volume (gallons) = (Length × Width × Height) ÷ 231
231 is the number of cubic inches in one US gallon. If your measurements are in centimeters, divide by 1000 first to get liters, then multiply by 0.264 for gallons.
Example: A 48″ × 13″ × 20″ tank = 48 × 13 × 20 = 12,480 cubic inches ÷ 231 = 54 gallons (that’s your 55-gallon tank — manufacturers round up).
Cylinder/bow-front tanks:
Volume = π × radius² × height ÷ 231
Bow-front tanks are trickier because the bow adds volume the basic formula misses. Use the calculator above — it handles bow-front geometry automatically.
Why Your Tank Is Never Exactly the Listed Gallons
A “55-gallon” tank holds about 54 actual gallons of water. Subtract gravel (typically 1–2 gallons for a 2″ substrate bed), decorations, and the air gap at the top, and you’re working with closer to 48–50 gallons of usable water volume. This matters a lot for stocking density and dosing medications.
Aquarium Size by Fish Type
Getting the right aquarium size isn’t just about what fits in your room — it’s about what your fish need to stay healthy. These are the minimums. More is always better.
Betta Fish Aquarium Size
Minimum: 5 gallons. You’ll see 1-gallon bowls sold specifically for bettas. Don’t use them. A betta in under 5 gallons can’t maintain stable water parameters, and you’ll be doing partial water changes every 2–3 days just to keep ammonia from spiking. A 10-gallon tank is genuinely comfortable for a betta with a few small tank mates.
Goldfish Aquarium Size
Single fancy goldfish: 20 gallons minimum. Each additional goldfish: add 10 gallons. Common/comet goldfish are pond fish. They reach 12″+ and produce enormous amounts of waste. A 55-gallon tank is a reasonable starting point for two commons — anything smaller is short-term housing at best.
Angelfish Aquarium Size
Minimum: 29 gallons for a single angelfish, 55 gallons for a pair or small group. Angelfish are tall fish — they need height, not just floor space. The 29-gallon’s 18″ height works; a 20-gallon high (16″) is borderline.
Oscar Aquarium Size
Minimum: 75 gallons for one adult Oscar. Oscars reach 12–14″ and produce heavy waste loads. A juvenile Oscar will look fine in a 30-gallon for 6 months, then outgrow it fast. Buy for the adult size, not the fish you bring home.
Arowana Aquarium Size
Minimum: 250 gallons for an adult silver arowana. These fish reach 3–4 feet. Most hobbyists keep juveniles in 75–125 gallon tanks with a plan to upgrade — or sell the fish before it maxes out.
Discus Aquarium Size
Minimum: 50 gallons for a group of 6. Discus are schooling fish that do poorly alone or in small groups. They also need stable, warm water (82–86°F), which means a reliable heater sized for the actual tank volume.
Flowerhorn Aquarium Size
Minimum: 75 gallons for one adult. Flowerhorns are aggressive and territorial — you’re usually keeping one per tank anyway.
What Size Aquarium Should a Beginner Start With?
10 or 20 gallons. Not 5, not 55.
Here’s why: small tanks are harder to maintain, not easier. A 5-gallon tank swings from clean to dangerous in 24 hours if you overfeed or miss a water change. A 20-gallon long gives you enough water volume that the chemistry stays stable while you’re still learning, without the $300+ equipment costs of a 55-gallon setup.
The 20-gallon long is the better pick over the 20-gallon high for beginners because the longer footprint gives more surface area for gas exchange and more swimming room for most fish.
Aquarium Size FAQs
How do I figure out what size aquarium I have?
Measure the inside length, width, and height in inches. Plug those numbers into the calculator above. If you can’t measure the inside, measure the outside and subtract 0.5″ per wall for standard glass thickness (1″ for acrylic over 75 gallons).
How many gallons is a size 10 aquarium?
A “size 10″ means 10 gallons. The dimensions are 20″ long × 10″ wide × 12” tall. Actual water volume after substrate and décor is closer to 8–9 gallons.
What is the size of a 29-gallon aquarium?
30″ long × 12″ wide × 18″ tall. The 29-gallon is a popular upgrade from the 20-gallon because the extra 2″ of height makes it work for taller fish like angelfish. It uses the same footprint as a 20-gallon long, so many 20-gallon stands fit it.
What size heater do I need for my aquarium?
The general rule is 5 watts per gallon for tanks in rooms at 68–72°F. For a 20-gallon tank: 100-watt heater. For a 55-gallon: 200–250 watts (two 150-watt heaters on opposite ends is more reliable than one 300-watt). Larger tanks lose heat faster relative to their edges, so under-heating is a common mistake on 75-gallon+ setups.
What is the best size aquarium for a saltwater reef?
30–75 gallons is the sweet spot for most people entering reef keeping. Under 30 gallons, parameter swings are dangerous for coral. Over 75 gallons, equipment and maintenance costs scale fast. A 40-gallon breeder (36″ × 18″ footprint) is a favorite for its width-to-depth ratio — it’s easy to reach the back glass for maintenance.
Are bow front aquarium sizes different from standard tanks?
Yes. Bow-front tanks have a curved front panel that adds volume the standard L×W×H formula doesn’t capture. A “46-gallon bow front” is actually about 46 gallons accounting for the curve — but if you measure it as a rectangle, you’ll calculate about 40. Always use a bow-front specific formula or the calculator above when sizing heaters, filters, and chemical doses.
Aquarium Weight Calculator — Why It Matters
A filled aquarium weighs roughly 10 lbs per gallon (water alone). Add gravel at 1.5 lbs per pound of substrate, plus the glass, stand, and equipment.
| Tank Size | Water Weight | Total (est.) |
|---|---|---|
| 10 gal | 83 lbs | ~111 lbs |
| 20 gal | 167 lbs | ~225 lbs |
| 55 gal | 459 lbs | ~625 lbs |
| 75 gal | 625 lbs | ~850 lbs |
| 125 gal | 1,041 lbs | ~1,400 lbs |
Most residential floors handle 40 lbs per square foot. A 125-gallon tank on a 2′ × 6′ footprint concentrates about 117 lbs/sq ft at the stand legs — well over that limit. Tanks over 75 gallons should sit against a load-bearing wall or on a reinforced floor joist.
How to Use This Aquarium Size Calculator
- To find volume from dimensions: Enter your tank’s inside length, width, and height. Select your unit (inches or centimeters). Hit calculate. You’ll get gallons, liters, and an estimated weight when full.
- To find dimensions from gallons: Select “standard size lookup,” enter your target gallon size, and get the industry-standard dimensions for that tank.
- To check fish compatibility: Enter your tank volume and select your fish species from the dropdown. The calculator cross-references minimum space requirements and tells you whether you’re understocked, at capacity, or overstocked.
Looking for help with other home or hobby calculations? Try the Concrete Price Calculator for tank stand builds, or the General Lighting Calculator for sizing aquarium light coverage.