How Many Gallons Is My Fish Tank?
Calculate aquarium gallons from tank length, width, height, and displacement from substrate or decor.
Last updated: May 2026
Publisher
Published by EverydayCalc Editorial
Our calculator pages are built to show the formula, explain the inputs, provide examples, and highlight assumptions so readers can understand how each result is estimated.
Measure the inside dimensions
Aquarium volume is based on length, width, and water height. Outside dimensions can overstate the amount of water because glass thickness and waterline height reduce volume.
Substrate and decor reduce water volume
Gravel, sand, rocks, wood, and equipment displace water. Use the calculator result as a gross estimate, then round down slightly for actual treatment or dosing.
Gallons affect every care task
Water changes, conditioners, medications, heater sizing, filter flow, and tank weight all depend on an accurate gallon estimate.
Sources
Source boxes list references used for factual claims, safety notes, energy rates, product-sizing conventions, or official data points.