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Free aquarium stocking calculator for freshwater and marine planning. Enter tank volume, an editable inches-per-gallon rule, and each species’ adult length with quantity. Compare total load to a simple capacity line.
Last updated: April 8, 2026
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Many guides start near 1 in/gal for small slender freshwater fish; use lower for messy or marine fish.
Total inch-units
20.0
Rule capacity
29.0
Stocking vs rule
69%
100% = at the simple rule limit.
Rule-of-thumb only: fish shape, waste load, filtration, and swimming space matter. Research each species.
Sum
Σ (in × qty)
Matches how many “inch rules” are often explained in stores.
Line in the sand
gal × rule
You set the rule—freshwater, conservative, or marine-style presets.
At a glance
Load ÷ capacity
Over 100% means you exceeded the rule you entered.
Community
Tetras, corys, etc.
Add or remove rows as you change your stock list.
Reminder
Tank size only
A mature cycled filter changes what you can keep—this math does not.
Behavior
Not in the %
Aggression and territory need more than an inch rule.
29 gal, 1 in/gal rule — 8× neon tetra @ 1.5 in, 4× cory @ 2 in
Stocking vs rule
69%
Inch-units / capacity
20.0 / 29.0
Many beginners hear “one inch of fish per gallon” as a shorthand. This calculator makes that idea explicit: you choose how many inches of fish per gallon count as your “full” line, then compare your fish list to that line.
inch-units = Σ (adult inches × quantity)capacity = gallons × (inches per gallon rule)stocking % = (inch-units ÷ capacity) × 100%US gallons are assumed. Convert liters before entering if needed.
Slim schooling fish and heavy-bodied cichlids do not produce the same waste or need the same swimming space per inch. Use conservative rules for large fish, messy fish, or high-flow marine setups.
Managing water volume? See our pool gallon calculator and pond liner calculator.
Get a Custom Calculator for Your SiteUnder the rule line, but real tanks still need filtration, plants, and compatible species. Use this as one input among many.
Share it with others planning a new tank
Suggested hashtags: #Aquarium #Fishkeeping #FishTank #Aquascape #Calculator