Aquarium Volume Calculator
Tank volume, stocking level and water changes.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Gross volume
- 78.5 US gallons
- Actual water volume
- 70.7 gallons (268 L)
- Weekly water change
- 17.7 gallons
- Filter turnover target
- 353 gph (5× per hour)
- Approximate filled weight
- 747 lb
Quick answer
A rectangular aquarium's volume in US gallons is length × width × height in inches ÷ 231, because one US gallon is 231 cubic inches. In metric, length × width × height in centimeters ÷ 1,000 gives liters. A 48 × 13 × 21 in tank (a standard "55") holds about 56.7 gallons gross, and roughly 10% less once substrate and decor go in.
What the Aquarium Volume calculator does
Enter the tank's length, width and height in inches and this calculator gives its gross volume, an estimate of the real water volume after substrate, rock and the gap below the rim (taken as 90% of gross), the size of your weekly water change, a filter flow target of five tank volumes per hour, and the approximate filled weight.
Knowing the true water volume matters more than the number on the box. Medication, water conditioner, salt for marine tanks and fertilizer doses are all given per gallon or liter, and overdosing a small tank is easy. The weight figure tells you whether your stand and floor can take it.
Measure inside the glass if you can; outside measurements include the glass thickness and slightly overstate volume. For round, bow-front or irregular tanks, a volume calculator for the matching shape gives a better estimate. Ponds and very large systems follow the same arithmetic as the pool volume calculator.
The formula
Gross gallons = L × W × H ÷ 231 Water gallons ≈ 0.9 × gross Liters = gallons × 3.785 Filled weight (lb) ≈ water gallons × 8.34 + gross gallons × 2
| Symbol | Meaning | Unit |
|---|---|---|
| L, W, H | Inside length, front-to-back width and height of the tank | in |
| 231 | Cubic inches in one US liquid gallon | in³/gal |
| 0.9 | Allowance for substrate, decor and the unfilled space under the rim | — |
| 8.34 | Weight of one US gallon of fresh water | lb/gal |
| 2 | Rough allowance for glass, substrate and decor per gallon of tank | lb/gal |
The 231 in³ US gallon is defined in US law (NIST Handbook 44). UK readers using imperial gallons should divide liters by 4.546 instead. The 10% deduction is a common hobby rule of thumb; a deep sand bed or large rockscape can take 15–20%. The weight estimate is for fresh water and does not include the stand; salt water is about 2.5% heavier. This page also replaces the former aquarium volume calculator.
Worked examples
20-gallon long
A 30 × 12 × 12 in tank, a popular size for community fish and shrimp, with 20% weekly water changes.
Inputs
- Length30 in
- Width12 in
- Height12 in
- Weekly water change20 %
Result
- Gross volume18.7 US gallons
- Actual water volume16.8 gallons (64 L)
- Weekly water change3.4 gallons
- Filter turnover target84 gph (5× per hour)
- Approximate filled weight178 lb
Sold as a "20 gallon", it holds closer to 17 gallons of water once set up. Dose conditioner and any medication for the real figure, not the label.
Standard 55-gallon
A 48 × 13 × 21 in tank with 25% weekly changes.
Inputs
- Length48 in
- Width13 in
- Height21 in
- Weekly water change25 %
Result
- Gross volume56.7 US gallons
- Actual water volume51.1 gallons (193 L)
- Weekly water change12.8 gallons
- Filter turnover target255 gph (5× per hour)
- Approximate filled weight539 lb
About 51 gallons (193 L) of water and roughly 540 lb on a 4 × 1 ft footprint, before the stand. Place it on a purpose-built stand, ideally near a load-bearing wall.
10-gallon starter tank
A 20 × 10 × 12 in tank with 30% weekly changes.
Inputs
- Length20 in
- Width10 in
- Height12 in
- Weekly water change30 %
Result
- Gross volume10.4 US gallons
- Actual water volume9.4 gallons (35 L)
- Weekly water change2.8 gallons
- Filter turnover target47 gph (5× per hour)
- Approximate filled weight99 lb
Small tanks swing in temperature and water chemistry faster, so larger regular water changes help. A filter rated around 50 gph is plenty.
Volume and weight of common tank sizes
Dimensions are typical outside measurements for standard glass tanks in the US; brands vary by up to an inch. Water volume uses the 90% allowance.
| Tank | L × W × H (in) | Gross volume | Water (approx.) |
|---|---|---|---|
| 10 gallon | 20 × 10 × 12 | 10.4 gal (39 L) | 9.4 gal (35 L) |
| 20 long | 30 × 12 × 12 | 18.7 gal (71 L) | 16.8 gal (64 L) |
| 29 gallon | 30 × 12 × 18 | 28.1 gal (106 L) | 25.2 gal (96 L) |
| 55 gallon | 48 × 13 × 21 | 56.7 gal (215 L) | 51.1 gal (193 L) |
| 75 gallon | 48 × 18 × 21 | 78.5 gal (297 L) | 70.7 gal (268 L) |
Stocking, filtration and water changes
- Stocking: the old "one inch of fish per gallon" rule only works for small, slim fish. Goldfish, cichlids and plecos produce far more waste per inch, and active swimmers need tank length, not just volume. Research each species' adult size and minimum tank size.
- Filter flow: the calculator targets five turnovers per hour. Four to six suits most community tanks; goldfish and heavily stocked tanks benefit from more. Rated flow on the box is measured with no media, so real flow is usually lower.
- Water changes: 10–25% a week is a common routine for freshwater tanks. Larger changes are fine if the new water matches the tank's temperature and is dechlorinated.
- Heater sizing: a common starting point is 3–5 watts per gallon, at the higher end in cold rooms.
Is your floor strong enough
A full tank concentrates a lot of weight on a small area. The 55-gallon example weighs around 540 lb on roughly 4.3 square feet, well over 100 lb per square foot, while typical US residential floors are designed for a uniform live load of 40 lb per square foot. Floors usually cope because joists share the load, but on upper floors or with tanks above about 75 gallons, place the tank across the joists, near a load-bearing wall, and consider asking a builder or structural engineer. Always use a stand rated for the tank, never ordinary furniture.
How to use the Aquarium Volume Calculator
- 1Measure the tank's inside length, front-to-back width and height in inches (divide centimeters by 2.54).
- 2Enter the percentage of water you change each week.
- 3Read the gross and real water volume; use the real figure for medication and conditioner doses.
- 4Check the filter target and filled weight before buying a filter or stand.
Frequently asked questions
How do I calculate aquarium volume in liters?
Multiply length × width × height in centimeters and divide by 1,000. A 60 × 30 × 36 cm tank holds 64.8 liters gross.
Why does my tank hold less than its rated gallons?
Ratings are based on outside dimensions, and substrate, rocks and the gap under the rim take up space. Expect 10–15% less water than the label.
How much does a 55-gallon tank weigh when full?
Roughly 540–625 lb including water, glass, gravel and decor, depending on how much hardscape you add. Water alone in 51 gallons weighs about 426 lb.
How many fish can I put in a 20-gallon tank?
It depends on species more than gallons. As a rough guide, a 20-gallon long can house a small school of 8–10 small tetras or rasboras plus a few bottom dwellers, but not goldfish or cichlids.
How big a filter do I need?
Choose one rated for four to six times the tank's water volume per hour, so about 200–300 gph for a 55-gallon tank.
References
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices — National Institute of Standards and Technology
- International Residential Code, Table R301.5 (minimum uniformly distributed live loads) — International Code Council
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error