Aquarium Calculator

Calculate aquarium volume from tank dimensions, then estimate stocking capacity, heater wattage, and substrate weight. Supports rectangular, cylindrical, and bow-front tank shapes with output in both gallons and liters. Use the inch-per-gallon rule to plan freshwater communities, size heaters at 5 watts per gallon, and budget about one pound of substrate per gallon for a planted base.

Aquarium

Tank Volume Calculator

Common tank presets

Bow-front estimate adds ~12% to a rectangular volume to account for the curved front panel.

Results

How to Use

  1. 1
    Enter tank dimensions

    Pick a tank shape (rectangular, cylindrical, or bow-front), choose your unit (inches or centimeters), then enter the length, width, and height of the tank's water-holding interior.

  2. 2
    Read the derived metrics

    Read the calculated volume in gallons and liters, plus the surface area, recommended fish inches, heater wattage, and substrate weight that the tool derives from the dimensions.

  3. 3
    Compare against community presets

    Use the freshwater community presets (10 gal, 20 gal long, 55 gal, 75 gal) to compare standard tank sizes, or adjust your custom dimensions and watch every output update instantly.

About

The Aquarium Calculator turns three simple measurements into the planning numbers every fishkeeper needs before adding water, fish, or equipment. It supports rectangular, cylindrical, and bow-front tank shapes, accepts dimensions in inches or centimeters, and reports volume in both US gallons and liters so you can match imported equipment specs. Surface area is shown alongside volume because it drives gas exchange and the safe stocking density of labyrinth fish like bettas and gouramis. Knowing actual water volume also matters when dosing medications, fertilizers, and water conditioners, where overdosing can stress or kill sensitive species.

Stocking estimates use the classic inch-per-gallon freshwater rule as a quick sanity check: a 20-gallon long tank can comfortably house roughly 20 inches of small community fish such as neon tetras or harlequin rasboras. Heater wattage is calculated at 5 watts per gallon, which keeps a tank 5 to 10°F above typical room temperature, and substrate weight assumes about one pound per gallon of standard aquarium gravel. Built-in presets for 10-gallon, 20-gallon long, 55-gallon, and 75-gallon tanks let you compare your custom build against industry standards in a single click. Pair the volume output with the FishFYI species database, and you can pre-screen any community plan against minimum tank size, water type, and temperament before you ever add a single fish.

FAQ

How accurate is the inch-per-gallon stocking rule?
The 1 inch of fish per gallon guideline is a rough rule of thumb that works best for slim-bodied freshwater community fish under 3 inches, like tetras, rasboras, and small barbs. It overstates capacity for chunky, messy, or aggressive species (cichlids, goldfish, plecos), and understates it for heavily planted or oversized-filtration tanks. Use it as a starting point, then adjust for body mass, bioload, swimming style, and territory needs of each species.
How deep should aquarium substrate be?
For a basic gravel or sand bed, 1 to 1.5 inches (2.5 to 4 cm) is enough to cover the glass and anchor decor. Planted tanks typically use 2 to 3 inches (5 to 8 cm) so root systems can spread, with deeper beds at the back for visual depth. The calculator's 1 pound per gallon estimate matches a roughly 1 inch layer of standard aquarium gravel; double it for planted setups.
What size heater does my aquarium need?
A common starting point is 5 watts per gallon (about 1.3 watts per liter) for a tank kept 5 to 10°F above room temperature. Cooler rooms, larger temperature swings, or open-top tanks may need 7 to 10 W per gallon. For tanks over 50 gallons it is usually safer and faster to run two smaller heaters on opposite ends of the tank than one oversized unit, both for redundancy and even heat distribution.
What filter GPH (gallons per hour) should I target?
Most freshwater community tanks do well with a filter rated for 4 to 6 times the tank volume per hour, so a 30-gallon tank wants 120 to 180 GPH of real flow. Heavily stocked or messy-fish tanks (goldfish, cichlids, plecos) benefit from 8 to 10 times turnover. Remember that media, head height, and clogging reduce real-world flow well below the box rating, so pick a filter at the upper end of your target range.
How long does it take to cycle a new aquarium?
A fishless nitrogen cycle typically takes 4 to 6 weeks, during which beneficial bacteria colonize your filter media to convert ammonia to nitrite and then nitrite to nitrate. You can speed this up to 1 to 2 weeks by seeding the filter with media from an established tank, dosing pure ammonia to about 2 ppm, and keeping the water warm (78 to 82°F). The tank is cycled when ammonia and nitrite both read 0 ppm within 24 hours of dosing.
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