Fish Tank Calculator: every number for a healthier tank
Free fish tank calculators: tank volume is length x width x height in inches divided by 231, a heater needs about 3 to 5 watts per gallon, and most tanks want a 20 to 25 percent water change each week. Get the exact numbers for your aquarium below, freshwater or saltwater. Choosing a fish first? The fish tank size calculator gives the smallest safe tank by species, and the stocking calculator tells you how many fish your tank can hold.
Length
48 in
Width
18 in
Height
21 in
75 gallons
284 L · ~750 lb filled
Most-used calculators
Tap a tool, enter your numbers, get an answer plus the gear that fits.
Most-read gear guides
All reviews →Explore the whole hub
Calculators
18 free tools to size your tank and gear.
Explore → 🐠Fish Species
Care, tank size, and tankmates by species.
Explore → ⭐Gear Reviews
Filters, heaters, lights, test kits, and more.
Explore → 🛒Setup Shopping Lists
Complete build kits from nano to premium.
Explore → 💧Water & Care
Cycling, parameters, algae, and maintenance.
Explore → 📊Reference Charts
Quick-look tables you can bookmark.
Explore → 🛟Troubleshooting
Fix cloudy water, sick fish, and more.
Explore →Every-tank essentials
All gear reviews →
Seachem Prime Water Conditioner
Dechlorinates every refill and detoxifies ammonia and nitrite between water changes.
Check price on Amazon · Under $25
Pawfly 5 in 1 Water Master Test Kit
Liquid tests for pH, ammonia, nitrite, nitrate, and phosphate, the numbers every tank runs on.
Check price on Amazon · Under $50
VEVOR 4-Stage Canister Filter (396 GPH)
Quiet multi-stage canister filtration for 100 to 125 gallon tanks.
Check price on Amazon · Under $100As an Amazon Associate we earn from qualifying purchases. Budget bands come from our catalog; Amazon shows the current price.
Sized to real equipment
Our calculators do not just spit out a number, they translate it into the heater wattage, filter flow, and gear that actually fits your tank.
Freshwater and saltwater
From a first betta tank to a reef build, the tools and guides cover community, planted, and marine setups.
No fluff, no fear-mongering
Clear, practical advice grounded in how fishkeeping actually works, so you can make confident decisions fast.
Aquarium Setup & Maintenance Planner
Stocking planner, water-test log, cycling tracker, maintenance schedule, and more, in one printable PDF that keeps your tank on track.
Get the Planner for $39Are you an aquarium brand?
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