100 Gallon Aquarium dimensions
100 is not a size the big mass-market brands make. The figures here are from Custom Aquariums, a US glass-tank builder that publishes both the model rating and the real water it holds: 100 rated, 85 actual.
Measuring your own tank? The rectangular tank volume calculator works out gallons at any water depth.
| Spec | Value |
|---|---|
| Length | 48 in |
| Width | 18 in |
| Height | 24 in |
| Geometric capacity | 89.8 gal |
| Nominal rating | 100 gal |
| Liquid weight (brim-full) | 749 lb of water |
| Empty tank weight | Varies by maker — see the manufacturer's spec sheet |
| Glass thickness (manufacturer) | ⅜ in, all panels |
Nominal rating versus real geometry
Rated 100 gal. Computed from the dimensions in the table above: 89.8 gal. The geometry comes out 10.2 gallons short of the label — 10.2 %. Published exterior dimensions and internal volume are not the same thing, and glass thickness and the trim account for most of the gap. Custom Aquariums itself lists this tank as “Model Gallons 100 / Actual Gallons 85” — the rating is a size class, not the water it holds.
What this weighs on your floor
| Water, brim full | 749 lb |
| Plus glass, substrate, rock and stand (≈ 25 % allowance) | ≈ 936 lb gross |
| Footprint | 48 in × 18 in = 6 ft² |
| Load over its own footprint | ≈ 156 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 3.9× 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 100 gallon aquarium?
Custom Aquariums lists its 100 gallon 24″H × 48″L × 18″D glass aquarium at 48 × 18 × 24 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 100 gallon aquarium weigh?
About 749 lb of water filled to the brim, before the glass, substrate, rock and stand. With a typical allowance for those, plan for roughly 936 lb.
How many gallons does a 100 gallon aquarium really hold?
Brim-full, the exterior dimensions give 89.8 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 about a 72 × 18 × 18 inch 100 gallon tank?
That footprint is often quoted for a 100, but we could not find a current manufacturer's specification for it, so it is not listed. At those dimensions the box would hold 101 gallons brim-full — check it in the rectangular calculator.
Sources and standards
- 100 Gallon 24″H × 48″L × 18″D Glass Aquarium — product specificationsCustom Aquariums (manufacturer)https://www.customaquariums.com/product/100-gallon-24h-x-48l-x-18d-glass-aquarium-basic-package/“Model Gallons 100 / Actual Gallons 85”; approximate outside glass dimensions, add ¾ in to length and depth for the aluminium frame; ⅜ in glass.
- 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.