Aquarium Volume & Weight Calculator
Water volume in gallons and liters, real water weight, substrate displacement and floor load — for rectangular, cylinder and bowfront tanks.
Water volume in gallons and liters, real water weight, substrate displacement and floor load — for rectangular, cylinder and bowfront tanks.
Common tank sizes (nominal)
Tank shape
Measured from the bottom rim to the water line. Leave 1 inch below the rim for a standard fill.
Water type
Enter your tank dimensions to see volume, water weight and floor load.
Frequently asked questions
How many gallons is my aquarium?
Multiply the inside dimensions in inches, then divide by 231 — the number of cubic inches in a US gallon. A 36×18×16 inch tank holds 10,368 cubic inches, which is 44.9 gallons. For liters, multiply centimeters the same way and divide by 1,000. Remember that a filled-to-the-brim number overstates real capacity: most tanks run about an inch below the rim, and gravel, rocks and driftwood displace several more gallons.
How much does a 55 gallon aquarium weigh?
Water alone weighs about 459 lb — 55 gallons times 8.345 lb per gallon. Add roughly 60–75 lb of substrate, 25–50 lb of double-strength glass, and a stand, and a stocked 55 gallon system commonly lands between 600 and 700 lb. That is why the floor under it matters more than the tank: our calculator adds a floor-load estimate so you can compare it against typical residential floor ratings.
How much water does substrate displace?
Only the solid grains displace water — water fills the gaps between them. With the industry-standard 45% solid fraction assumed here, a 1-inch bed in a 48×18 inch tank displaces about 1.7 gallons; a 2-inch bed displaces roughly double that. The calculator subtracts the displaced amount from installed water volume automatically, so your water weight stays honest on deep planted-tank beds.
Can my floor hold a 75 gallon tank?
A stocked 75 gallon aquarium commonly weighs 800–1,000 lb. Modern US residential floors are designed for 40 pounds per square foot of live load, and a tank placed perpendicular to the floor joists — especially against a bearing wall — spreads that weight across several joists and usually passes with margin. Long tanks parallel to joists concentrate load on fewer members. When a tank sits in a basement over a steel beam or on slab, load is rarely a concern; for upper floors with long spans, ask a structural engineer.
How do I measure a bowfront tank?
Measure the straight length, width and height as usual, then measure the bow: how far the center of the front glass bulges forward beyond the line connecting the two side panels. The calculator adds a curved-segment allowance of roughly 57% of bow depth times width times height to the rectangular volume. Bowfront tanks typically gain 5–10% capacity over a same-footprint rectangle.
How to calculate aquarium volume
Every aquarium volume figure comes from the same move: turn the tank into a box (or a cylinder), compute its volume, and convert that volume into gallons or liters. The calculator above handles the three shapes hobbyists actually own, then goes one step further and converts volume into weight — the number that really decides whether a tank is safe where you want to put it.
Rectangular tanks are the simplest case. Multiply inside length by inside width by the water height in inches, then divide by 231, the number of cubic inches in a US gallon. Use the water height rather than the tank height: a standard fill leaves about an inch of glass above the waterline, and that air gap is volume your fish never see. For liters with metric tape measures, multiply length by width by height in centimeters and divide by 1,000.
Cylinder and bowfront tanks need small corrections. A cylinder's volume is π times the radius squared times height — a 12 inch diameter, 14 inch tall column holds about 6.9 gallons at the brim. A bowfront is a rectangle plus a curved bulge along the front pane; the calculator approximates that bulge as (π/2 − 1) × width × bow depth × height, which tracks manufacturer ratings within a couple of percent.
Worked example — a 40 gallon breeder at real fill:
1. Footprint: 36 × 18 = 648 square inches. 2. Water at 15 of 16 inches: 648 × 15 = 9,720 cubic inches. 3. Volume: 9,720 ÷ 231 = 42.1 gallons. 4. Weight: 42.1 × 8.345 = about 351 lb of water — before a single grain of sand goes in. 5. Add a 2 inch gravel bed and the water drops to about 39.6 gallons while the bed itself adds 75 lb of load.
Water weight and floor safety
Water is heavy in a way that surprises people: a US gallon weighs 8.345 lb, so volume converts to weight almost like a billing rate. Saltwater is slightly heavier at 8.554 lb per gallon because dissolved salt raises density about 2.5%. The calculator uses those exact figures, which is why its weight numbers run a little higher than the rounded 8 lb-per-gallon rule of thumb.
Floor load is weight spread over area, measured in pounds per square foot (PSF). US building codes design residential floors for 40 PSF of live load in occupied rooms. A 75 gallon tank on a 48 × 18 footprint imposes roughly 145 PSF locally — far above 40 — but that is not automatically a failure: the load only becomes dangerous if the joists under the small footprint cannot hand it off to neighbors. Place the tank perpendicular to the joists or near a bearing wall and several joists share the work; place it parallel in the middle of a long span and one joist takes nearly all of it.
Practical checks before you fill: prefer ground floors and basements, orient long tanks perpendicular to joists, set the tank on a level stand with a full-deck top, and spread load with a plywood pad on slippery floors. If the tank exceeds about 90 gallons on an upper floor of typical joist construction, a one-hour consult with a structural engineer is cheap insurance. Water damage claims from tank failures are among the most expensive DIY accidents in the hobby.
Estimates are for planning only; figures assume uniform glass walls and standard densities. Confirm critical installations with a professional.
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