Aquarium Calculator

About This Tool

The Aquarium Calculator takes your tank dimensions and calculates everything you need to know: volume in gallons and liters, total water weight, substrate requirements, fish stocking capacity, and heater wattage. All calculations use standard aquarium formulas and rules of thumb.

Why Tank Volume Matters

Knowing your exact tank volume is essential for proper fishkeeping. Medication dosing, water treatment, stocking density, filtration sizing, and heater selection all depend on accurate volume calculations. Many tanks are listed by nominal sizes (e.g., “40 gallon”) but actual usable volume may differ once you account for substrate, decorations, and the gap between water line and rim.

The One-Inch-Per-Gallon Rule

The one-inch-of-fish-per-gallon rule is a simplified guideline for community tropical fish. It works reasonably well for small, slender fish like tetras and guppies but breaks down for deep-bodied fish like discus or messy eaters like goldfish. Heavy bioload fish need more gallons per inch. Surface area matters more than volume for gas exchange, so long, shallow tanks support more fish than tall, narrow ones of the same volume.

Substrate and Equipment

A 2-inch substrate layer is standard for most aquariums. Planted tanks may need 3-4 inches for root growth. Gravel weighs roughly 1.5 lbs per gallon of volume it displaces. For heaters, 3 watts per gallon works for rooms at 70-75 degrees F, while 5 watts per gallon is better for cooler rooms or fish requiring higher temperatures (78-82 degrees F). Filtration should turn over the tank volume 4-6 times per hour.