How many gallons does a 55 gallon tank hold?
48 × 13 × 21 in · freshwater planning
55 gallon · tank volume
The working
48 × 13 × 21 = 13,104 cubic inches. Divide by 231 = 56.73 US gal. Multiply by 3.78541 = 214.74 L.
| Fill assumption | US gal | Litres |
|---|---|---|
| 80% of geometric | 45.38 | 171.79 |
| 85% of geometric | 48.22 | 182.53 |
| 90% of geometric | 51.05 | 193.26 |
| 100% of geometric | 56.73 | 214.74 |
First decisions for this medium setup
For this medium rectangle, work from the base upward: every inch of height represents 2.7 gallons. Marking a waterline makes repeat fills more consistent. A tall rock or a wood arrangement displaces an unknown amount; a running total from measured containers is more useful than assigning decor a guessed percentage.
55 gallon equipment and layout plan
A low, extended rectangle puts more of the layout along a horizontal swimming lane. The 48 × 13-inch base has an aspect ratio of 3.69:1. A lid or light must span roughly 48 inches; measure rim openings and bracket adjustment before ordering.
| Volume interpretation | US gallons |
|---|---|
| Marketed nominal size | 55 |
| Rounded rectangle, before glass | 56.73 |
| Inside panes, brim full model | 53.33 |
| Inside panes, 1 inch below rim | 50.76 |
| 2-inch bed envelope, before decor | 5.4 |
For the inside-pane comparison, 47.5 inches of length times 12.5 of width times 20.75 of height gives 53.33 gallons. This assumes 0.25-inch glass and a brim-full fill. Leave room for the real waterline; decor displacement requires measurement, and water remains between substrate grains.
Moving up from the 50 gallon changes geometric gallons before glass from 53.3 to 56.73: an increase of 6.43%. Budget for the larger requirement rather than reusing the smaller plan unchanged.
Assumptions and limits
Geometric volume uses the listed planning dimensions. Glass, the fill line, substrate and decor reduce usable water; 80–90% is only a rough range. A nominal name is not a measurement.
Adjust measurements with the tank volume calculator
Questions before you buy
What dimensions does this 55 gallon calculation use?
It uses a rounded 48 × 13 × 21 inch rectangle. Actual dimensions vary by brand and frame; measure your own tank.
What is the tank volume estimate for a 55 gallon tank?
56.73 US gallons · 214.74 litres
Equipment to check
Compare aquarium measuring tape. Check dimensions, capacity and product instructions before buying.
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