Aquarium Volume Calculator

For rectangular tanks, multiply Length × Width × Height (in inches), then divide by 231 to get US gallons. Formula: Gallons = (L × W × H) ÷ 231. For example, a 30" × 12" × 18" tank = 30 × 12 × 18 ÷ 231 = 28.05 gallons. Our calculator handles this automatically for all 8 tank shapes.

Total volume is the theoretical capacity of an empty tank. Actual water volume accounts for substrate displacement—gravel, sand, and decorations take up space. A 2-inch layer of gravel in a 29-gallon tank can displace 2-3 gallons. Our calculator lets you input substrate depth and type to show actual water volume.

The traditional rule is 1 inch of fish per gallon of water. For a 29-gallon tank, that's 29 inches of adult fish. A more conservative rule is 1 inch per 2 gallons. Consider: (1) Use adult fish sizes, not juvenile sizes. (2) Goldfish need 20+ gallons for the first fish. (3) Territorial fish need more space. (4) Heavily planted tanks can handle more fish. Our fish stocking calculator shows stocking levels and warns of overstocking.

Water weighs 8.34 lbs per gallon (1 kg per liter). A 29-gallon tank holds ~242 lbs of water alone. Add 30-50 lbs for glass and 20-40 lbs for substrate, and your total can exceed 300 lbs. Our calculator estimates total weight including water, glass (based on tank depth), and substrate. Always ensure your stand and floor can support this weight.

The general rule is 3-5 watts per gallon depending on your room temperature: Cold room (under 65°F): 5 watts/gallon. Moderate room (65-75°F): 4 watts/gallon. Warm room (over 75°F): 3 watts/gallon. A 29-gallon tank in a moderate room needs ~116 watts. For tanks over 75 gallons, use two smaller heaters for safety and even heating.

Your filter should turn over the tank volume 4 times per hour (4x rule). For a 29-gallon tank: 29 × 4 = 116 GPH minimum. For messy fish (goldfish, oscars, plecos), aim for 6-8x turnover. HOB filters should be rated for at least 1.5× your tank size. Canister filters are more efficient and can be sized closer to the 4x rule.

Bowfront tanks have a curved front panel that adds volume. The formula approximates a rectangle plus a half-ellipse: Volume = (Rectangular Volume) + (π × Length/2 × Bow Depth × Height) ÷ 2. The bow typically adds 10-15% more volume than a standard rectangle of the same footprint. Our calculator handles this automatically when you select the Bowfront shape.

Glass thickness depends primarily on tank height (water pressure): Under 16": 6mm (1/4") glass. 16-20": 8mm (5/16") glass. 20-24": 10mm (3/8") glass. Over 24": 12mm+ (1/2"+) glass. Our calculator estimates glass thickness based on your tank height and factors this into the weight calculation.

Most freshwater tanks benefit from 10-25% weekly water changes. Heavily stocked tanks may need 25-50% weekly. Lightly planted tanks with few fish may get by with 10% bi-weekly. Key factors: (1) Nitrate levels should stay under 20ppm. (2) More fish = more water changes. (3) Planted tanks can go longer between changes. Our calculator shows exact volumes for 10%, 25%, and 50% changes.

For accurate volume, measure INSIDE dimensions: (1) Length: Inside left to right at the top. (2) Width: Inside front to back (also called depth). (3) Height: Inside bottom to where water will fill. Subtract 1-2" from rim for safety margin. If measuring outside, subtract 2× glass thickness from each dimension. For a standard glass tank, this is about 0.5" total.

For a natural look, aim for 2-3 inches of substrate (sloped higher in back). To calculate pounds needed: (Length × Width × Depth in inches) ÷ 10 ≈ pounds of gravel. A 30×12 tank with 2" substrate needs ~72 lbs of gravel. For planted tanks, use 3-4 inches of nutrient-rich substrate. Sand is lighter; use (L×W×D) ÷ 12 for sand calculation.

The inch-per-gallon rule states: 1 inch of fish per 1 gallon of water. It's a rough guideline with limitations: (1) Body shape matters—a 6" oscar produces far more waste than six 1" tetras. (2) Always use ADULT sizes—many fish are sold as juveniles. (3) Tall fish need more space than slim fish. (4) The conservative rule (1" per 2 gallons) is safer for beginners. Our calculator offers both options.

Most aquarium medications are dosed in milliliters per gallon or ppm (parts per million). Always: (1) Calculate ACTUAL water volume (after substrate). (2) Follow medication instructions exactly. (3) Remove carbon from filter during treatment. (4) Increase aeration during medication. Our medication calculator helps convert dosage requirements to your specific tank volume.

Lighting needs depend on plant types: Low-light plants (Anubias, Java Fern): 1-2 watts per gallon. Medium-light plants (Amazon Sword, Crypts): 2-3 watts per gallon. High-light plants (Carpeting plants, Red plants): 3-5 watts per gallon. LED wattage is more efficient—divide by 2 for comparable LED requirements. Our calculator factors in your plant level for recommendations.

Accurate volume is critical for: (1) Medication dosing—overdosing can kill fish, underdosing is ineffective. (2) Fertilizer dosing for planted tanks. (3) Salt and pH buffer calculations. (4) Fish stocking decisions. (5) Equipment sizing (heater, filter, CO2). (6) Water conditioner amounts. (7) Understanding total weight for stand and floor safety. Never guess—always calculate.