20 Gallon Long Tank Volume Calculator

Volume calculations account for glass thickness, fill height, and substrate displacement to provide accurate measurements.

Don't forget to measure from the inside if you're not accounting for glass thickness!

Other common errors: Using tank height instead of fill height, forgetting to convert units, and measuring curved sections on bow front tanks.

Medication dosing: Underdosing won't cure sick fish, overdosing can kill them. Most treatments require precise gallons/liters.

Water conditioners & treatments: Chlorine removers, pH adjusters, and supplements all depend on exact volume.

Reef tank dosing: Calcium, alkalinity, and magnesium need accurate volume calculations or you'll crash your tank parameters.

Water weighs 8.34 lbs per gallon (1 kg per liter). A 55-gallon tank holds ~460 lbs of water alone!

Floor support: Add glass (10-30 lbs), stand (20-100 lbs), substrate (40-80 lbs), and rocks (50-200 lbs). A "55 gallon" setup can easily exceed 700 lbs.

Safety tip: Always place large tanks perpendicular to floor joists, never parallel. Second floors and apartments need extra consideration.

Bow front tanks: The curved front adds ~10-15% more volume than the rectangular back panel dimensions suggest. We calculate this automatically using a 1.12× multiplier.

Corner/Pentagon tanks: These hold significantly less water (~30% less) than a rectangular tank with the same dimensions. Our calculator uses a 0.7× adjustment.

Hexagon & cylinder tanks: Use the diameter measurement and we'll apply the circular volume formula (π × r² × height).

Most tanks aren't filled to the brim. Typical fill height is 1-2 inches below the top trim to prevent overflow and allow surface agitation.

Reef tanks & sumps: Often run 2-4 inches below the rim to accommodate wave makers and reduce the risk of salt creep overflow.

Evaporation matters: Over a week, a 50-gallon tank can lose 5+ gallons to evaporation, changing your actual volume. Use our fill height feature for precision.

Substrate displacement: A 2-inch gravel bed in a 55-gallon tank displaces ~7 gallons. Sand displaces even more due to finer particle packing.

Live rock in reef tanks: 1-2 lbs of rock per gallon is common, displacing 10-20% of total volume. A heavily aquascaped 100-gallon reef might only hold 80 gallons of water.

Why it matters: Your "net water volume" is what matters for dosing, medication, and water changes. Our calculator shows gross volume; reduce by 10-25% for heavily decorated tanks.

Measure from the inside. Most aquarium dimensions online refer to outside measurements, but volume depends on internal space.

Glass thickness by tank size: Small tanks (10-20 gal) use 1/4" glass. Medium tanks (30-75 gal) use 3/8" or 1/2". Large tanks (100+ gal) use 1/2" to 3/4" glass.

Volume difference: A 48×18×21" tank with 1/2" glass loses nearly 3 gallons compared to the outside dimensions. For medication dosing, this matters!