TankWise aquarium calculators

Free aquarium calculators for beginners

Figure out your tank's volume, a conservative stocking level, and how much substrate to buy — before any fish are involved. Every tool here errs on the side of the animals.

Tank volume calculator

Enter the inside dimensions of a rectangular tank. For bow-fronts or cylinders, measure the closest rectangle and treat the result as approximate.

Results will appear here.

Conservative stocking estimator

Read this first. This tool uses the inch-per-gallon idea as a starting guideline only, tightened for active and territorial fish — and it refuses to apply it to goldfish at all. It cannot see your filter, your water parameters, or your fishes' personalities. Here's what the rule gets wrong. When in doubt, understock.
Use the volume calculator above if you're unsure.

Add fish (adult sizes used)

SpeciesCount

Enter your tank size, then add fish.

Substrate calculator

How many pounds of gravel, sand, or planted soil your tank footprint needs at a given depth.

1–2 in for fish-only tanks; 2–3 in for planted tanks.

Results will appear here.

Beginner guides

Common questions

How many fish can I put in a 20 gallon tank?

It depends on the species. A conservative guideline is about 20 inches of small, peaceful adult fish — but filtration, temperament, and whether the tank is cycled matter as much as the number. Try the estimator above and understock while you learn.

How do I calculate aquarium volume in gallons?

Length × width × height in inches, divided by 231. In metric: the three dimensions in centimeters divided by 1000 gives liters.

How much substrate do I need?

Length × width × depth (inches) ÷ 1728 × ~96 for gravel or ~100 for sand gives pounds. The calculator above does it for you.

Is the inch-per-gallon rule accurate?

Only as a rough starting point for small community fish. It ignores body mass, behavior, and oxygen demand, and it fails completely for goldfish and cichlids. Full explanation.

How heavy is a filled fish tank?

Water is about 8.34 lb per gallon; a filled, decorated tank runs roughly 10 lb per gallon all-in. A 20-gallon tank is around 200 lb — make sure the stand is rated for it.

Recommended starter gear

Affiliate links will appear here at deploy. As an affiliate we may earn from qualifying purchases — it never changes our advice: buy less, cycle first.