Standard 75 gallon

Key stats

How many gallons is a 48 × 18 × 21 in tank?

A 48 × 18 × 21 in aquarium contains 79 US gallons by outside dimensions. The math:

48 × 18 × 21 = 18144 in³
18144 ÷ 231 in³/gal = 78.55 US gal
78.55 × 3.785 = 297.3 L

In practice, actual water capacity is 71 gallons after subtracting 0.375" of glass thickness on each side. Manufacturers typically market this as a 75-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 48 × 18 × 21 in tank weigh?

• Saltwater reef (with ~71 lb live rock + 1" sand bed): 866 lb (393 kg)
• Freshwater (with 20 lb gravel, no rock): 749 lb (340 kg)
• Empty glass only: approximately 122 lb
• Floor load: 144 lb/ft² (saltwater filled)

Floor-load warning: at 144 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 48 × 18 × 21 in tank

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

• Annealed (regular float) glass: 15 mm minimum (0.591")
• Tempered glass: 8 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 48 × 18 × 21 in tank

In a warm room (~70 °F ambient) targeting 78 °F, this size tank needs about 215 W of heating. The conservative reefkeeping practice is to run two heaters at 125 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 48 × 18 × 21 in tank?

• 2" sand bed: ~85 lb of CaribSea Special Grade aragonite (5 × 20-lb bags)
• 4" deep sand bed (DSB): ~170 lb
• 1" shallow / aesthetic: ~43 lb

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

FAQ

How many gallons is a 48 × 18 × 21 in tank?

A 48 × 18 × 21 in aquarium holds approximately 79 US gallons by outside dimensions (297.3 L). After subtracting a standard 0.375" glass thickness on each interior wall, the actual water capacity is about 71 US gallons. Manufacturers commonly market this as a "75-gallon" tank.

How much does a filled 48 × 18 × 21 in tank weigh?

A 48 × 18 × 21 in saltwater reef tank with ~71 lb of live rock and a 1" sand bed weighs about 866 lb (393 kg) filled. Freshwater with no rock comes in lighter, around 749 lb. Floor load is roughly 144 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 48 × 18 × 21 in tank?

Using the Wisner hobby formula with a 3.8 safety factor, a 48 × 18 × 21 in tank needs at least 15 mm annealed (regular float) glass. Tempered glass can be 8 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 48 × 18 × 21 in tank?

For a tank in a warm room (~70 °F ambient), this size needs roughly 215 W of heating. For redundancy, run two heaters at 125 W each — heaters fail, and a single point of failure on tropical fish or coral is expensive.

How much sand or substrate for a 48 × 18 × 21 in tank?

A 2-inch sand bed in this footprint (48" × 18") needs approximately 85 lb of CaribSea Special Grade aragonite, or about 5 20-lb bags. For a deep sand bed at 4 inches, double that.

Is a 48 × 18 × 21 in tank good for a reef?

Yes — the 48 × 18 × 21 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.

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