Is a 90-gallon safe on a typical second-floor bedroom?

Probably — but only if the stand sits perpendicular to the joists and ideally over a load-bearing wall below. A 48 × 18 in 90-gal at ~1,000 lb total is about 56 lb/ft², over the 40 lb/ft² design code. Most floors handle this in practice; some don't. Get under there with a flashlight and a tape measure before you commit.

Why is saltwater heavier than freshwater?

Salt adds about 35 g of dissolved solids per kg of water — roughly 2.5 % more mass per gallon at SG 1.025. For an 80-gallon system that's an extra 17 lb. Small but real, and it stacks with rock and sand.

Does this account for the stand?

No. A steel stand adds 50–80 lb, a wood stand 80–120 lb. Add it to the equipment field, or assume an extra ~10 % over the bare-tank total.

Calculator

Aquarium Weight Calculator (lb, kg, Floor Load)

A 75-gallon reef weighs about 850 lb filled. Residential floor live-load design is 40 lb/ft². A short, fat tank usually clears that with room to spare; a tall narrow tank or one perpendicular to the joists can fail the math. This calculator gives you total weight in pounds and kilograms, plus floor load in lb per square foot, so you can have an honest conversation with your contractor before the build, not after.

Aquarium weight & floor load

Length (in)

Width (in)

Height (in)

Glass thickness (in)

Bottom + 4 walls; open-top assumed.

Specific gravity

Live rock (lb)

Sand (lb)

Equipment, sump water (lb)

Total filled weight

889lb

Metric

403kg

Water

607lb

Glass

162lb

Floor load

148lb / sq ft

Residential design live load ≈ 40 lb/ft².

Net water volume

70.9US gal

⚠ Above ~40 lb/ft² is the residential live-load design limit. Plan for joist reinforcement or place on a load-bearing wall.

Glass density 2.5 g/cm³ (0.0903 lb/in³). Saltwater @ SG 1.025 = 8.554 lb/gal. Live rock and equipment are user-supplied estimates — see methodology.

How this is calculated

water_lb  = (L − 2T)(W − 2T)(H − T) / 231 × 8.345 × SG
glass_lb  = (bottom + 2 long walls + 2 short walls) × 0.0903 lb/in³
total_lb  = water + glass + rock + sand + equipment
floor_psi = total_lb / (L × W)
floor_lb_per_sqft = floor_psi × 144

Glass density is 2.5 g/cm³ (0.0903 lb/in³). Freshwater is 8.345 lb/US gal; saltwater at SG 1.025 is 8.554 lb/gal. Live rock and sand weights are user-supplied — there's no clean volumetric density for either once they sit submerged with trapped air pockets and bound water. If you don't know your rock weight, weigh it on a bathroom scale before the build.

The 40 lb/ft² figure is the residential live-load design standard for typical wood-frame floors. It's a design minimum, not a hard ceiling — most floors handle considerably more once you factor in dead load already in the assembly and the fact that aquariums distribute load through a stand. But if you're above 40 lb/ft², stop and either reinforce the joists, place the tank parallel to and on top of a load-bearing wall, or move to the ground floor.

FAQ

Is a 90-gallon safe on a typical second-floor bedroom?: Probably — but only if the stand sits perpendicular to the joists and ideally over a load-bearing wall below. A 48 × 18 in 90-gal at ~1,000 lb total is about 56 lb/ft², over the 40 lb/ft² design code. Most floors handle this in practice; some don't. Get under there with a flashlight and a tape measure before you commit.
Why is saltwater heavier than freshwater?: Salt adds about 35 g of dissolved solids per kg of water — roughly 2.5 % more mass per gallon at SG 1.025. For an 80-gallon system that's an extra 17 lb. Small but real, and it stacks with rock and sand.
Does this account for the stand?: No. A steel stand adds 50–80 lb, a wood stand 80–120 lb. Add it to the equipment field, or assume an extra ~10 % over the bare-tank total.

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Sources & references

01
Randy Holmes-Farley — Specific Gravity article (water density tables)
https://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/
02
Quattrolifts — Glass weight calculator (verifies 2.5 g/cm³)
https://www.quattrolifts.com/glass-weight-calculator/
03
BRS — Live rock weight rule of thumb
https://www.bulkreefsupply.com/content/post/aquascaping-with-real-reef-rock

Last reviewed 2026-05-25