Aquarium Volume Calculator Fish Tank Gallons & Liters for Any Shape, Plus Real Water Volume and Weight
A “55-gallon” tank rarely holds 55 gallons of water. Measure any shape (rectangle, bow front, cylinder, half cylinder, corner, hexagon or oval) and get the true water volume after glass, substrate, rockwork and the gap at the top. You’ll also see filled weight, floor load, filter and heater sizes, salt and water-conditioner doses.
By Manzoor Ahmad · Updated October 18, 2026
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Real-water adjustments
Glass only applies to outside measurements. Typical glass: 4–6 mm up to 20 gal, 6–10 mm for 40–75 gal, 10–12 mm or more for bigger tanks.
Water conditioner dose
Copy the numbers from your bottle. Many concentrated conditioners treat 50 US gallons per 5 mL.
Actual water volume
—US gallons
Equipment sized for this tank
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How do I calculate aquarium volume?
For a rectangular tank, multiply length × width × height. In inches, divide by 231 to get US gallons. In centimetres, divide by 1,000 to get liters. A 48 × 13 × 21 inch tank is 13,104 in³ ÷ 231 = 56.7 US gallons (214.7 L). That’s the gross volume, though. After subtracting glass, substrate, decor and the gap below the rim, the same tank holds only about 43.5 gallons of water, and that’s the number to use for medication, salt and conditioner doses.
Key facts at a glance
- 1 US gallon = 231 in³ = 3.785 L. 1 imperial gallon = 4.546 L. 1 liter = 1,000 cm³ (NIST).
- Real water is usually 15–25% less than the tank’s rated size, once glass, substrate, rock and the air gap are subtracted.
- Freshwater weighs about 8.34 lb per US gallon (1 kg/L). Seawater at 35 ppt is about 2.5% heavier.
- Filled tanks weigh roughly 10–12 lb per gallon with glass, substrate and rock. Residential floors are designed for a 40 psf live load in living areas (IRC R301.5).
- Common sizing rules: filter 4–6× tank volume per hour, heater 3–5 W per gallon, 10–25% water change weekly.
How to measure your fish tank
- Pick the shape and your unit (inches, cm, feet or mm). Not sure? A tank with a curved front panel is a bow front, and a quarter-circle that fits into a corner is a corner tank.
- Measure the outside of the glass, ignoring the trim if you can, and keep “Outside” selected. The calculator subtracts your glass thickness on every side and at the bottom. For acrylic or inside measurements, choose “Inside”.
- Add real-world details: how far below the rim you keep the water, how deep the substrate is, and roughly what share of the tank is taken up by rock and decor (5% for light décor, 10–20% for heavy aquascapes or reef rock).
- Read the results. Use actual water volume for medication, salt and conditioner doses, and total filled weight to choose a stand and a safe spot on the floor.
Aquarium volume formulas for every tank shape
All formulas give cubic units. Divide cubic inches by 231 for US gallons, or cubic centimetres by 1,000 for liters. H is the water height.
Rectangle / cube
V = L × W × HMost tanks. A cube is the same with equal sides.
Bow front
V = H × (L × W + segment)Segment = R²/2 × (θ − sin θ), where R = (L²/4 + s²) ÷ 2s, s is the bow depth (centre depth − side depth) and θ = 2·asin(L/2R).
Cylinder
V = π × (D/2)² × HColumn and bubble tanks. D is the diameter.
Half cylinder
V = ½ × π × (D/2)² × HFlat back against a wall. D is the width of the flat back.
Corner (quarter circle)
V = ¼ × π × R² × HR is the length of each straight back side.
Hexagon
V = (3√3 ÷ 2) × a² × Ha is the length of one panel (edge) of a regular hexagon.
Oval / elliptical
V = π × (L/2) × (W/2) × HOval tanks, tubs and small preformed ponds.
Worked example: what a “55-gallon” tank really holds
- Outside size: 48 × 13 × 21 in = 13,104 in³ ÷ 231 = 56.7 US gal gross.
- Minus 6 mm glass (all sides and the bottom): the inside holds 53.5 gal when filled to the brim.
- Minus a 1 in gap at the top, 2 in of substrate and 5% rock and decor: about 43.5 US gal (165 L) of actual water.
- Filled weight: about 550 lb with water, substrate, rock and glass, before the stand and lid.
These are the calculator’s default values. Dosing for “55 gallons” here would overdose the real water by about 30%.
Standard fish tank sizes: dimensions, real volume and weight
Typical outside dimensions for common US glass tanks. Brands vary by up to about half an inch, so measure yours to be sure. “Gross” is length × width × height ÷ 231. “Water” assumes 6–10 mm glass, a 1 in gap, 2 in of substrate and 5% décor.
| Tank | Dimensions (L × W × H) | Gross volume | Real water (approx.) | Filled weight (approx.) |
|---|---|---|---|---|
| 5 gallon | 16 × 8 × 10 in 41 × 20 × 25 cm | 5.5 gal · 21 L | ≈ 3 gal · 13 L | ≈ 60 lb · 30 kg |
| 10 gallon | 20 × 10 × 12 in 51 × 25 × 30 cm | 10.4 gal · 39 L | ≈ 7 gal · 26 L | ≈ 100 lb · 50 kg |
| 15 gallon | 24 × 12 × 12 in 61 × 30 × 30 cm | 15.0 gal · 57 L | ≈ 10 gal · 38 L | ≈ 150 lb · 70 kg |
| 20 gallon high | 24 × 12 × 16 in 61 × 30 × 41 cm | 19.9 gal · 76 L | ≈ 14 gal · 55 L | ≈ 190 lb · 90 kg |
| 20 gallon long | 30 × 12 × 12 in 76 × 30 × 30 cm | 18.7 gal · 71 L | ≈ 12 gal · 47 L | ≈ 180 lb · 80 kg |
| 29 gallon | 30 × 12 × 18 in 76 × 30 × 46 cm | 28.1 gal · 106 L | ≈ 21 gal · 79 L | ≈ 270 lb · 120 kg |
| 40 gallon breeder | 36 × 18 × 16 in 91 × 46 × 41 cm | 44.9 gal · 170 L | ≈ 32 gal · 121 L | ≈ 440 lb · 200 kg |
| 40 gallon long | 48 × 13 × 16 in 122 × 33 × 41 cm | 43.2 gal · 164 L | ≈ 31 gal · 116 L | ≈ 430 lb · 190 kg |
| 55 gallon | 48 × 13 × 21 in 122 × 33 × 53 cm | 56.7 gal · 215 L | ≈ 43 gal · 161 L | ≈ 560 lb · 250 kg |
| 65 gallon | 36 × 18 × 24 in 91 × 46 × 61 cm | 67.3 gal · 255 L | ≈ 52 gal · 198 L | ≈ 650 lb · 300 kg |
| 75 gallon | 48 × 18 × 21 in 122 × 46 × 53 cm | 78.5 gal · 297 L | ≈ 60 gal · 227 L | ≈ 760 lb · 350 kg |
| 90 gallon | 48 × 18 × 24 in 122 × 46 × 61 cm | 89.8 gal · 340 L | ≈ 69 gal · 261 L | ≈ 880 lb · 400 kg |
| 120 gallon | 48 × 24 × 24 in 122 × 61 × 61 cm | 119.7 gal · 453 L | ≈ 93 gal · 352 L | ≈ 1,160 lb · 530 kg |
| 125 gallon | 72 × 18 × 21 in 183 × 46 × 53 cm | 117.8 gal · 446 L | ≈ 89 gal · 336 L | ≈ 1,160 lb · 520 kg |
| 150 gallon | 72 × 18 × 28 in 183 × 46 × 71 cm | 157.1 gal · 595 L | ≈ 124 gal · 470 L | ≈ 1,530 lb · 700 kg |
| 180 gallon | 72 × 24 × 25 in 183 × 61 × 64 cm | 187.0 gal · 708 L | ≈ 147 gal · 556 L | ≈ 1,810 lb · 820 kg |
How much does a full aquarium weigh?
Water is the heavy part: about 8.34 lb per US gallon (1 kg per liter) for freshwater, and roughly 2.5% more for seawater at a specific gravity of 1.025. Add glass (2.5 kg per liter of glass), substrate (sand and gravel weigh about 1.6 kg per liter, or 100 lb per ft³) and rock, and a typical 55-gallon setup comes to about 550 lb before the stand.
Floor load
The US International Residential Code designs floors for at least 40 lb/ft² of live load in living areas and 30 lb/ft² in bedrooms. Large tanks concentrate far more on a small footprint, so place them across joists, near a load-bearing wall, and ask a structural engineer about big builds.
Choose the right stand
Use a stand rated for the tank’s filled weight, and check it’s level. Uneven support stresses the silicone seams, especially on rimless tanks.
Never move it full
Even partly filled tanks can crack or twist their seams. Drain to a few inches with the fish removed before moving a tank.
Filter, heater and water change sizing
| Equipment | Common rule of thumb | Notes |
|---|---|---|
| Filter (freshwater) | 4–6× tank volume per hour | 8–10× for goldfish and big cichlids. Gentler flow for bettas. |
| Return pump (saltwater) | 5–10× per hour through the sump | Reefs add powerheads for 20–40× total circulation. |
| Heater | 3–5 W per US gallon | Use the higher end in cold rooms. Split into two heaters above about 75 gal. |
| Water change | 10–25% weekly | Match temperature and treat new tap water first. |
| Salt mix (reef/marine) | ≈ 36 g per liter for 35 ppt | Brands differ, so always confirm with a refractometer (SG ≈ 1.025–1.026). |
Ratings on filters are measured with no media and no head height, so real flow is lower. That’s one reason the calculator gives a range rather than a single number.
A pharmacist’s notes: dose fish medications by real water volume
Most aquarium treatment errors I’ve seen come down to arithmetic, not the medicine. Here’s how to dose accurately:
- Use actual water volume, not the tank’s label. Dosing the rated 55 gallons into about 43.5 gallons of real water gives roughly 30% more medicine than intended. With copper, formalin or some antiparasitics, that margin matters.
- ppm means mg per liter. A 1 ppm dose in 165 L of water is 165 mg of active ingredient. That’s a quick way to sanity-check any product label.
- Measure liquids with an oral syringe, not capfuls. Caps vary and small tanks need fractions of a millilitre.
- Remove activated carbon and switch off UV during treatment unless the label says otherwise. Both can remove or break down many medications.
- Copper is toxic to shrimp, snails and corals, and it lingers in silicone and substrate. Treat sick fish in a separate hospital tank whenever you can.
- Don’t use human antibiotics in a tank without veterinary advice. Wrong doses encourage resistant bacteria and can wipe out your biological filter.
Aquarium gallons to liters conversion chart
| US gallons | Liters | Imperial gallons | Water weight |
|---|---|---|---|
| 1 gal | 3.8 L | 0.8 | 8 lb · 4 kg |
| 2.5 gal | 9.5 L | 2.1 | 21 lb · 9 kg |
| 5 gal | 18.9 L | 4.2 | 42 lb · 19 kg |
| 10 gal | 37.9 L | 8.3 | 83 lb · 38 kg |
| 20 gal | 75.7 L | 16.7 | 167 lb · 76 kg |
| 29 gal | 109.8 L | 24.1 | 242 lb · 110 kg |
| 40 gal | 151.4 L | 33.3 | 334 lb · 151 kg |
| 55 gal | 208.2 L | 45.8 | 459 lb · 208 kg |
| 75 gal | 283.9 L | 62.5 | 626 lb · 284 kg |
| 90 gal | 340.7 L | 74.9 | 751 lb · 341 kg |
| 125 gal | 473.2 L | 104.1 | 1,043 lb · 473 kg |
| 150 gal | 567.8 L | 124.9 | 1,252 lb · 568 kg |
| 180 gal | 681.4 L | 149.9 | 1,502 lb · 681 kg |
| 200 gal | 757.1 L | 166.5 | 1,669 lb · 757 kg |
| 300 gal | 1,135.6 L | 249.8 | 2,504 lb · 1,136 kg |
More tools for pet owners
Aquarium keepers often have more than fish at home. These dexocalc tools use the same precise approach:
Aquarium volume calculator FAQs
Measure length, width and height in inches, multiply them, and divide by 231 to get US gallons. For liters, measure in centimetres, multiply and divide by 1,000. Use inside dimensions and the real water height for the true water volume.
55 US gallons is 208.2 liters. A typical 55-gallon tank measuring 48 × 13 × 21 inches is about 56.7 gallons (214.7 L) gross, but holds only around 44 gallons (about 165 L) of actual water once glass, substrate and decor are subtracted.
Rated sizes are based on outside dimensions. Glass thickness, the gap below the rim, substrate, rock and decor all take up space, which usually reduces real water by 15–25%.
Treat it as a rectangle (length × side depth) plus a circular segment for the bow. Enter the length, the depth at the sides, the depth at the centre of the bow and the height, and the calculator handles the segment geometry.
Volume = π × radius² × height. For a 16 inch diameter, 24 inch tall cylinder: 3.1416 × 8² × 24 = 4,825 in³ ÷ 231 ≈ 20.9 US gallons.
The water alone in a standard 48 × 18 × 21 inch 75-gallon tank weighs roughly 500–620 lb, depending on how full it is and how much substrate and decor it holds. With glass, substrate and rock, a filled 75 typically reaches around 750–850 lb before the stand.
Most freshwater tanks do well with a filter rated for 4–6 times the tank volume per hour. A 40-gallon tank needs about 160–240 GPH. Heavily stocked goldfish or cichlid tanks benefit from 8–10 times.
A common rule is 3–5 watts per US gallon, using the higher figure in cool rooms. A 55-gallon tank usually needs a 200–250 W heater, ideally split into two smaller heaters for safety.
Natural seawater averages about 35 ppt salinity. Most reef salt mixes need roughly 36 g per liter of water (about 1.2 lb per 4 US gallons) to reach this, but brands differ. Mix in a separate container and confirm with a refractometer.
Use actual water volume. Dosing by the tank’s rated size overdoses the real water, often by 20% or more, which can harm sensitive fish and invertebrates.
A US gallon is 3.785 L (231 in³) and an imperial (UK) gallon is 4.546 L, about 20% larger. UK tank sizes and product doses often use imperial gallons or liters.
Only as a rough starting point for small, slim fish. It ignores adult size, body bulk, activity and waste output. Research each species’ adult size, and use real water volume and filtration to judge stocking.
Manzoor Ahmad
Pharmacist & health writer · 13+ years of web development and SEO experience
Manzoor is a pharmacist with more than nine years of experience in dosing calculations, unit conversion and evidence-based health writing. That’s exactly the precision aquarium dosing demands. This calculator uses exact NIST unit definitions, published geometry for each tank shape, and widely used hobby sizing rules, and every assumption is shown so you can adjust it to your setup.
Sources
- NIST Special Publication 811, Appendix B: conversion factors for the US gallon, imperial gallon and cubic inch.
- USGS Water Science School: water density.
- Woods Hole Oceanographic Institution: average ocean salinity of about 35 ppt.
- JLC: calculating live loads, IRC Table R301.5 residential floor loads.
Disclaimer: This aquarium volume calculator gives estimates based on your measurements and stated assumptions. Real volume, weight and equipment needs vary with construction, décor and livestock. Always follow product labels for medications, conditioners and salt mixes, and consult a qualified professional about floor loading for large aquariums.