34-gallon cube

Key stats

How many gallons is a 20 × 20 × 20 in tank?

A 20 × 20 × 20 in aquarium contains 35 US gallons by outside dimensions. The math:

20 × 20 × 20 = 8000 in³
8000 ÷ 231 in³/gal = 34.63 US gal
34.63 × 3.785 = 131.1 L

In practice, actual water capacity is 31 gallons after subtracting 0.375" of glass thickness on each side. Manufacturers typically market this as a 34-gallon tank for round-number simplicity.

For a different shape (cylinder, hex, bow-front) or to enter custom dimensions, use the full aquarium volume calculator.

How much does a filled 20 × 20 × 20 in tank weigh?

• Saltwater reef (with ~31 lb live rock + 1" sand bed): 390 lb (177 kg)
• Freshwater (with 20 lb gravel, no rock): 349 lb (159 kg)
• Empty glass only: approximately 67 lb
• Floor load: 140 lb/ft² (saltwater filled)

Floor-load warning: at 140 lb/ft², this tank exceeds the 40 lb/ft² typical residential live-load rating. Place over a load-bearing wall or get a structural assessment before installing upstairs.

Full breakdown including equipment, sump, and custom rock weight: aquarium weight calculator.

Glass thickness for a 20 × 20 × 20 in tank

Using the Wisner hobby formula with a 3.8 safety factor, this tank requires:

• Annealed (regular float) glass: 12 mm minimum (0.472")
• Tempered glass: 6 mm minimum (tempered is ~4× stronger than annealed)

Most commercial tanks at this size ship one thickness step up for additional safety. See the glass thickness calculator for the full formula and Starphire (low-iron) options.

Heater wattage for a 20 × 20 × 20 in tank

In a warm room (~70 °F ambient) targeting 78 °F, this size tank needs about 93 W of heating. The conservative reefkeeping practice is to run two heaters at 50 W each: if one fails stuck-on, the other isn't powerful enough to cook the tank alone; if one fails stuck-off, the remaining one slows the temperature drop while you replace it.

For cold-room or cool-basement adjustments: heater wattage calculator.

How much sand or gravel for a 20 × 20 × 20 in tank?

• 2" sand bed: ~39 lb of CaribSea Special Grade aragonite (2 × 20-lb bags)
• 4" deep sand bed (DSB): ~79 lb
• 1" shallow / aesthetic: ~20 lb

Other substrate brands (Nature's Ocean, Tahitian Moon, Eco-Complete): substrate calculator.

FAQ

How many gallons is a 20 × 20 × 20 in tank?

A 20 × 20 × 20 in aquarium holds approximately 35 US gallons by outside dimensions (131.1 L). After subtracting a standard 0.375" glass thickness on each interior wall, the actual water capacity is about 31 US gallons. Manufacturers commonly market this as a "34-gallon" tank.

How much does a filled 20 × 20 × 20 in tank weigh?

A 20 × 20 × 20 in saltwater reef tank with ~31 lb of live rock and a 1" sand bed weighs about 390 lb (177 kg) filled. Freshwater with no rock comes in lighter, around 349 lb. Floor load is roughly 140 lb per square foot — most residential floors are rated for 40 lb/ft² live load, so confirm before placing upstairs.

What glass thickness do I need for a 20 × 20 × 20 in tank?

Using the Wisner hobby formula with a 3.8 safety factor, a 20 × 20 × 20 in tank needs at least 12 mm annealed (regular float) glass. Tempered glass can be 6 mm and still meet the same safety margin. This is the calculated minimum — most commercial tanks at this size ship with one step up for safety. See the glass thickness calculator for the full math.

What size heater for a 20 × 20 × 20 in tank?

For a tank in a warm room (~70 °F ambient), this size needs roughly 93 W of heating. For redundancy, run two heaters at 50 W each — heaters fail, and a single point of failure on tropical fish or coral is expensive.

How much sand or substrate for a 20 × 20 × 20 in tank?

A 2-inch sand bed in this footprint (20" × 20") needs approximately 39 lb of CaribSea Special Grade aragonite, or about 2 20-lb bags. For a deep sand bed at 4 inches, double that.

Is a 20 × 20 × 20 in tank good for a reef?

Yes — the 20 × 20 × 20 in footprint is well-suited to a reef tank. The width gives enough front-to-back depth for aquascaping. Height is in the SPS-friendly range for most fixtures.

Run your own numbers