Fish Tank Volume Calculator
Free fish tank volume calculator for 6 shapes. US gallons (231 cu in each), liters, UK gallons, water weight at 8.34 lb/gal, plus heater sizing.
This fish tank volume calculator converts inside dimensions — in inches or centimeters — into US gallons, liters, and UK gallons for six shapes: rectangular, cube, bowfront, cylinder, half-cylinder or corner, and hexagonal. The math is fixed by definition: a US gallon is exactly 231 cubic inches, so a 48 × 12.75 × 21 inch tank works out to 55.6 gallons brim-full, though the water you can actually use is meaningfully less. Set the fill-level slider to where your waterline really sits and the tool also reports water weight at 8.34 lb per US gallon and a heater range at the standard 3–5 watts per gallon. Whether you’re confirming a floor can take a 750 lb 75-gallon setup, sizing a heater, or dosing medication that demands true volume, start from a measured number instead of the sticker on the box — the difference is routinely 10–25%.
How many gallons is my fish tank?
Measure the inside length, width, and water height in inches, multiply the three together, and divide by 231 — the exact number of cubic inches in a US gallon. A 36 × 18 × 16 inch tank holds 44.9 gallons. For bowfront, cylinder, corner, or hexagonal tanks, enter your measurements in the calculator above and it handles the geometry, returning US gallons, liters, and UK gallons at any fill level.
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Fish Tank Volume Calculator
Key Information
| Parameter | Details |
|---|---|
| Cubic inches per US gallon | 231 (exact) |
| Weight of water per US gallon | 8.34 lb |
| Filled-tank planning rule | ≈10 lb per gallon |
| Typical heater sizing | 3–5 W per gallon |
Frequently Asked Questions
How many gallons is my fish tank?
Measure the inside length, width, and water height in inches, multiply the three together, and divide by 231 — the exact number of cubic inches in a US gallon. A 36 × 18 × 16 inch tank holds 44.9 gallons. For bowfront, cylinder, corner, or hexagonal tanks, enter your measurements in the calculator above and it handles the geometry, returning US gallons, liters, and UK gallons at any fill level.
How much does a full 75-gallon fish tank weigh?
Plan on about 750 pounds using the 10-lb-per-gallon rule. The water alone is 626 lb (75 × 8.34 lb per gallon), the empty glass tank adds roughly 90–140 lb, and substrate, hood, and equipment make up the rest — published figures for complete 75-gallon setups run 700–850 lb. Place a tank this size across floor joists and near a load-bearing wall.
Why does my 55-gallon tank hold less than 55 gallons?
Because the label reflects rounded exterior dimensions, not water. Inside the glass, a standard 48.25 × 12.75 × 21 inch 55-gallon holds about 48.75 gallons — 89% of the sticker — once glass thickness and a fill line an inch below the trim are counted. Add a 2-inch gravel bed displacing about 4 gallons of water, plus a few more gallons for rock and driftwood, and true water volume lands near 42 gallons, roughly 24% under the advertised size.
Are these calculators free to use?
Yes, all calculators on CalcCorp are completely free — no registration, no login, no hidden charges. Results are calculated instantly in your browser and we do not store any of your data.
How accurate are these calculations?
Our calculators use standard financial formulas updated with the latest tax rates, interest rates, and government policies for 2026. Results are accurate for planning purposes but should be verified with a professional for final decisions.
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Last updated: March 2026