Aquarium Volume & Water Weight Calculator
Use this fish tank size calculator for rectangular aquariums. Enter internal dimensions for the most accurate result.
Why aquarium dimensions matter more than “tank gallons”
Most hobbyists shop by gallon size first, but fish and plants experience the shape of the aquarium. A long, shallow tank and a tall, narrow tank can hold similar volume while behaving very differently.
- Footprint (length × width) controls swimming room and territory space.
- Height affects lighting needs, aquascaping choices, and maintenance effort.
- Total volume influences stability, dosing, and filtration sizing.
That’s why an aquarium dimensions calculator is useful: you can test multiple dimension combinations quickly and choose the best tank for your goals.
How this aquarium volume calculator works
For a rectangular tank, gross volume is:
Volume = Length × Width × Height
Then the calculator converts that into liters, US gallons, and UK (imperial) gallons. It also estimates real water volume after:
- Leaving headspace at the top (fill level), and
- Subtracting displacement from sand, rocks, wood, and décor.
Finally, it estimates water weight for freshwater or saltwater setups, helping you check floor load and stand requirements.
Choosing smart dimensions for your setup
1) Community freshwater tanks
Prioritize horizontal swimming space. A longer tank is usually better than a tall one for tetras, rasboras, danios, and most beginner communities.
- Good approach: moderate height, generous length.
- Benefit: easier gas exchange and better fish behavior.
2) Cichlid and territorial species
More floor area means more room for territories and visual barriers. Width becomes especially important once rockwork is added.
3) Planted aquariums
Very tall tanks require stronger lighting and can complicate trimming and maintenance. Mid-height tanks are often easier to light and manage.
4) Reef tanks
Reef keepers often balance front-to-back depth for coral placement with enough height for aquascape architecture and flow patterns.
Quick reference: common rectangular aquarium dimensions
| Nominal Size | Typical Dimensions (in) | Approx. Volume |
|---|---|---|
| 20 Long | 30 × 12 × 12 | 18.7 US gal |
| 29 Gallon | 30 × 12 × 18 | 28.0 US gal |
| 40 Breeder | 36 × 18 × 16 | 44.9 US gal |
| 55 Gallon | 48 × 13 × 21 | 56.8 US gal |
| 75 Gallon | 48 × 18 × 21 | 70.7 US gal |
Manufacturers use nominal labels, so actual internal capacity can differ once glass thickness and trim are considered.
Practical planning tips before you buy
- Use internal dimensions if possible for better volume accuracy.
- Plan for true water volume, not gross volume, when dosing fertilizers, medication, or salt mixes.
- Account for weight: water weighs about 8.34 lb per US gallon (freshwater), before glass, rocks, and equipment.
- Measure your space with stand, filter clearance, and maintenance access included.
Frequently asked questions
Do I use outside or inside tank dimensions?
Inside dimensions are best. Outside dimensions include glass thickness, which slightly overstates usable water volume.
Why is my “55 gallon” not actually 55 gallons?
Nominal tank names are marketing labels. Real internal capacity depends on manufacturing dimensions and glass thickness, and your actual water volume is lower after headspace and décor displacement.
What’s the difference between US and UK gallons?
1 US gallon is smaller than 1 UK (imperial) gallon. This calculator shows both so you can follow guides from either region.
Bottom line
A good aquarium dimensions calculator helps you move from guesswork to planning. Test a few length/width/height combinations, compare real water volume, and choose a tank shape that supports your fish, your aquascape style, and your maintenance routine.