20 Gallon Long Aquarium dimensions
The low, wide 20: the same 30-inch length as a 29 gallon but only 12¾ in tall — the usual choice for bottom-dwellers, shrimp and reptile setups. Dimensions are Aqueon's published exterior figures.
Measuring your own tank? The rectangular tank volume calculator works out gallons at any water depth.
| Spec | Value |
|---|---|
| Length | 30¼ in |
| Width | 12½ in |
| Height | 12¾ in |
| Geometric capacity | 20.9 gal |
| Nominal rating | 20 gal |
| Liquid weight (brim-full) | 174 lb of water |
| Empty tank weight (manufacturer) | 25 lb |
| Full weight (manufacturer) | 225 lb |
Nominal rating versus real geometry
Rated 20 gal. Computed from the dimensions in the table above: 20.9 gal. The geometry comes out 0.9 gallons over the label — 4.4 %. Exterior dimensions include the glass and trim, so the real water volume is lower than this brim-full figure, usually by a few percent.
What this weighs on your floor
| Water, brim full | 174 lb |
| Plus glass, substrate, rock and stand (≈ 25 % allowance) | ≈ 218 lb gross |
| Footprint | 30¼ in × 12½ in = 2.6 ft² |
| Load over its own footprint | ≈ 83 lb/ft² |
| IRC design live load, habitable room (not a bedroom) | 40 lb/ft² |
| IRC design live load, sleeping room | 30 lb/ft² |
That is roughly 2.1× the 40 lb/ft² design live load in IRC Table R301.5, over the tank's own footprint — what that does and does not mean is set out in the aquarium reference.
What is the same for every tank of this kind (floor loading, from rated size to actual water volume, what governs an aquarium's rated size) is set out once on the sizes hub, in the aquarium reference and the common questions for every tank.
Dimension drawing
FAQ
What are the dimensions of a 20 gallon long aquarium?
Aqueon lists its Standard 20 Long (SKU 100110021) at 30¼ × 12½ × 12¾ in (length × width × height, exterior). Other makers' tanks with the same rating can differ by an inch or so — check your tank's label or spec sheet.
How much does a full 20 gallon aquarium weigh?
About 174 lb of water filled to the brim, before the glass, substrate, rock and stand. With a typical allowance for those, plan for roughly 218 lb.
How many gallons does a 20 gallon aquarium really hold?
Brim-full, the exterior dimensions give 20.9 gal. In use, with an inch of air gap and a substrate bed, expect several gallons less — enter your own water depth in the rectangular tank volume calculator.
What is the difference between a 20 long and a 20 high?
Same rating, different box. Aqueon's 20 Long is 30¼ × 12½ × 12¾ in; the 20 High is 24¼ × 12½ × 16¾ in. The long has about 25 % more floor area, the high about 4 in more water depth.
Sources and standards
- 2026 Aqueon Product Catalog — Critical dimensions for all aquariums (p. 57) and Standard Aquariums (p. 7)Aqueon / Central Garden & Pet (manufacturer)https://www.aqueon.com/-/media/project/oneweb/aqueon/files/us/product-catalog/2026-aqueon-catalog.pdf“20 Long 30¼ x 12½ x 12¾ 25 225” — exterior length × width × height in inches, empty and full weight in lb. Aqueon's critical-dimensions diagram measures trim to trim.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-c
- International Residential Code, Table R301.5 — Minimum uniformly distributed live loadsInternational Code Council (free read-only access at codes.iccsafe.org)https://codes.iccsafe.org/s/IRC2021P2/chapter-3-building-planning/IRC2021P2-Pt03-Ch03-SecR301.5
- Methodology — the partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/
Every linked source above was retrieved and checked on 2026-11-01.