Water Weight: The Non-Negotiable
- US gallons: 8.34 lb per gallon (3.785 L per US gallon; water ~ 1 kg/L).
- Formula: Water weight (lb) = gallons × 8.34 (metric: kg ~ liters × 1.0)
Examples (water only):
10 gal → ~83 lb
20 gal → ~167 lb
29 gal → ~242 lb
40 gal → ~334 lb
55 gal → ~459 lb
75 gal → ~626 lb
120 gal → ~1,001 lb
These numbers are just the water. Substrate, glass, rock/wood, and equipment push the real total higher.
Substrate Weight: Depth & Footprint Matter
Substrate weight depends on footprint area and depth, not the gallon size. A 36" × 18" tank with 1.5" of sand holds more sand than a 30" × 12" tank at the same depth.
Step-by-step:
- Footprint area (in²): Length (in) × Width (in)
- Substrate volume (in³): Footprint × Depth (in)
- Convert to ft³: divide by 1728 (in³ per ft³)
- Multiply by bulk density (lb/ft³)
Typical bulk densities (dry, approximate):
Silica sand: ~95�105 lb/ft³
Aquarium gravel: ~90�110 lb/ft³
(Varies by grain size/voids; use a range.)
Worked Example (55-gallon standard, 48" × 13" footprint, 1.5" sand):
Footprint: 48 × 13 = 624 in²
Volume: 624 × 1.5 = 936 in³
In ft³: 936 ÷ 1728 ~ 0.542 ft³
Weight (sand @ 100 lb/ft³): 0.542 × 100 ~ 54 lb
So a filled 55g with 1.5" sand adds ~ 54 lb just in substrate.
Hardscape & Glass (Don�t Forget These)
Glass/acrylic tank (empty): rough ballpark
- 10g: ~10�15 lb
- 20�40g: ~25�45 lb
- 55g: ~70�85 lb
- 75g: ~120�150 lb
(Varies by brand and glass thickness.)
Rocks & wood: can add 10�100+ lb quickly. Weigh big pieces or estimate conservatively.
If you don�t know the exact stand/glass/hardscape numbers, add a 10�15% buffer to your calculated water and substrate weight for safety, then refine.
Real-World Totals (Illustrative)
55-gallon standard (48" × 13" × 21")
- Water: 55 × 8.34 ~ 459 lb
- Substrate: ~ 54 lb (1.5" sand)
- Tank/glass: ~ 75 lb (typical)
- Misc + small rocks: ~ 20 lb
Estimated total: ~ 608 lb (without stand)
75-gallon (48" × 18" × 21")
- Water: 75 × 8.34 ~ 626 lb
- Substrate (1.5" sand; footprint 864 in² → 0.75 ft³): ~ 75 lb
- Tank/glass: ~ 140 lb
- Misc/hardscape: 30�60 lb
Estimated total: ~ 871�901 lb (without stand)
For a 40 Breeder (36" × 18") at 1.5" substrate, expect ~56 lb of substrate + ~334 lb water, plus glass/hardscape.
Get Exact Numbers for Your Tank.
Skip the guesswork. Enter your internal dimensions, substrate depth, and optional hardscape to get a precise total:
Aquarium Volume & Weight Calculator
Floor & Stand Considerations (Practical Safety)
- Place large tanks perpendicular to floor joists and near bearing walls where possible.
- Use a rated aquarium stand with a full-contact top for rimless tanks.
- Level the stand; shim as needed before filling.
- Use a high-density foam mat for rimless tanks if the manufacturer specifies.
Always follow manufacturer specs; when in doubt, consult a contractor for large systems.
Frequently Asked Questions
Do substrates displace water weight?
Yes�adding substrate displaces some water, so the final water volume is lower than an empty-tank calculation. For conservative planning, sum (actual water present) + (dry weight of substrate) + (glass/hardscape/equipment). Our calculator bases weight on true internal volume and the substrate depth you enter.
How much does 1 gallon of water weigh?
Approximately 8.34 lb (3.785 kg) at room temperature.
How many pounds of substrate per gallon?
A common rule is 1�2 lb per gallon for ~1�2" depth, but the accurate method uses footprint × depth × density (see above).
Will a 75-gallon tank be too heavy upstairs?
It depends on joist span, placement, and load distribution. Many hobbyists keep 75s upstairs safely with proper placement. When in doubt, consult a professional.
