What this calculator does
Find out how much water your fish tank really holds, and how much that water weighs. You need that number every time you dose water conditioner or medicine, pick a filter or heater, or plan how many fish the tank can support. The size printed on the box is the size of the glass box itself. The water inside is less, because of the glass and the space left at the top. This calculator works out the actual water volume from your measurements.
It covers the four common tank shapes: rectangular, cylinder, bow front (a rectangle with a curved front panel) and regular hexagon. If you measured the outside, enter the glass thickness. Also enter how far below the rim the water sits. Along with gallons and liters, it gives the weight of the water. That tells you how much load the stand and floor will carry.
Where it's used
Why this math matters outside the classroom:
- Fishkeeping. A hobbyist finds a 24 × 12 × 16 in tank filled to 15 in holds 24 × 12 × 15 ÷ 231 = 18.7 gallons before dosing conditioner.
- Cylinder volume. Students find that a 16 in wide, 20 in tall cylinder tank holds π × 8² × 20 ≈ 4,021 in³, about 17.4 gallons.
- Floor loading. Before buying a 75 gallon tank, a renter checks that 75 × 8.34 ≈ 626 lb of water is acceptable for the floor.
- Science classroom. A teacher setting up a class tank converts 60 × 30 × 33 cm of water to 59.4 liters.
- Reef aquariums. A reef keeper uses 1.025 kg per liter to estimate that 200 L of saltwater weighs about 205 kg.
Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.G.B.6, 8.G.C.9. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose the tank shape and whether you measured in inches or centimeters.
- Enter the measurements. For a bow front, measure front to back at the flat ends and at the middle of the bow.
- If you measured the outside, enter the glass thickness (enter 0 if you measured inside). Then enter how far below the top the water level sits.
- Choose freshwater or saltwater, then read the water volume and weight.
What the inputs mean
- Depth at the sides
- Front to back at either end, where the glass is flat.
- Depth at the center
- Measure straight from the back glass to the furthest point of the curved front.
- Width across opposite flat sides
- For a regular hexagon, measured flat side to flat side.
- Glass thickness
- The glass is taken off each side wall, and once off the height for the bottom pane. Typical glass is 1/4 to 1/2 in (6–12 mm) thick, and acrylic tanks are similar.
- Water level below the top
- Most tanks run with the water an inch or so below the rim, under the lid or trim. Use 0 to get the volume filled to the brim.
- Water
- Saltwater is heavier for its size (denser). At a specific gravity of 1.025, each liter weighs about 1.025 kg.
Formula and method
rectangle floor area = inside length × inside width cylinder floor area = π × (inside diameter ÷ 2)² hexagon floor area = (√3 ÷ 2) × (width across flats)² bow front area = length × side depth + circular segment (chord = length, bow = center depth − side depth) volume = floor area × (height − bottom glass − gap); 1 US gal = 231 in³; 1 L = 1,000 cm³ water weight = liters × 1.000 kg/L (fresh) or 1.025 kg/L (salt)
In the formulas, π is pi, the circle number. The width across flats is the distance between two opposite flat sides of the hexagon. For a bow front, the chord is the straight line across the front, and the bow is how far the curve sticks out past the flat ends.
Every tank shape here has straight up-and-down sides. So its volume is the area of the floor times the depth of the water. The real work is finding the floor area. It can be a rectangle, a circle or a regular hexagon. For a bow front, it's a rectangle plus the curved slice in front of it.
The bow is treated as part of a circle. From the bow's depth s and the chord length c, the circle's radius is c² ÷ (8s) + s ÷ 2. The area of the curved slice (a circular segment) then comes from the standard formula for it. Finally, the volume is converted with exact factors: 231 cubic inches per US gallon and 16.387064 cm³ per cubic inch.
Worked example
A rectangular tank measures 30 × 12 × 12 in inside, and the water sits 1 in below the top.
- Floor area: 30 × 12 = 360 in².
- Water depth: 12 − 0 − 1 = 11 in.
- Volume: 360 × 11 = 3,960 in³; 3,960 ÷ 231 = 17.1 US gallons, or 64.9 L.
- Freshwater weight: 64.9 L × 1 kg/L = 64.9 kg, about 143 lb.
Result: The tank holds about 17.1 gallons (64.9 L) of water weighing about 143 lb. Filled to the brim, it would hold 18.7 gallons.
Understanding the result
When measuring water treatments or medicine, use the water volume, not the size printed on the tank. Then take off a little more for gravel, rock and decorations. They can push out (displace) 10% or more of the water.
A filled tank is heavy. The water alone weighs about 8.3 lb per gallon, and glass, gravel and rock add more. Put large tanks on a stand made for them, ideally near a load-bearing wall.
Assumptions and limitations
- The walls go straight up and the floor is flat.
- The glass is the same thickness on every pane.
- Freshwater weighs 1.000 kg/L and saltwater weighs 1.025 kg/L.
- Gravel, rock, decorations and equipment take up room in the water, and they aren't subtracted.
- Corner tanks (quarter-round or pentagon), rimless designs with braces, and irregular shapes aren't covered.
- The weight shown is for the water only. It isn't a strength rating for a stand or floor.
Frequently asked questions
How do I work out how many gallons my fish tank holds?
Multiply the inside length, width and water depth in inches, then divide by 231. A 36 × 18 × 16 in tank filled to 15 in holds 36 × 18 × 15 ÷ 231 = 42.1 gallons.
Why is my tank's real volume less than its label?
Labels usually describe the outside size of the glass box. The glass, the space below the rim and the gravel all take up room. So the water volume is often 10–20% lower.
How much does a 20 gallon aquarium weigh?
20 gallons of fresh water weigh about 167 lb (75.7 kg). Add glass, gravel and decorations, and a set-up 20 gallon tank commonly weighs over 200 lb.
How do I measure a bow-front tank?
Measure the length from left to right, the depth front to back at the flat ends, the depth at the middle of the bow, and the height. The calculator adds the curved section to the rectangular part.
How this calculator is checked
Method: Prism volume (floor area × water depth) for rectangle, circle, regular hexagon and bow front (rectangle plus circular segment); exact conversions 231 in³ per US gallon and 2.54 cm per inch; water densities 1.000 kg/L (fresh) and 1.025 kg/L (typical marine specific gravity). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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