Aquariums · Use case

Aquarium volume,
in gallons.

Enter the inside dimensions of your rectangular fish tank. Get the volume in US gallons, liters, and cubic feet — plus stocking guidelines and notes on substrate displacement.

Stocking rules of thumb

How many fish?

The classic guideline is 1 inch of adult fish per US gallon of water — but it's only a rough start. It works for slim community fish like tetras and rasboras, breaks down for larger or messier species, and ignores territory needs.

Better guidelines:

  • Small community fish (under 3"): 1 inch per gallon works.
  • Larger fish (4–6"): roughly 1 inch per 2 gallons.
  • Large/messy fish (oscars, plecos): 1 inch per 3–5 gallons.
  • Bettas: 5 gallons minimum, regardless of body size.

Filtration capacity, plant load, and tank dimensions (a 20-gallon long is better for tetras than a 20-gallon tall) matter more than the gallon count alone.

Substrate displacement

The calculated volume is the gross volume of the tank. Actual water capacity is lower because gravel, rocks, and decor take up space.

2 in (5 cm) of substrate  →  subtract ~8%
3 in (7.5 cm) of substrate  →  subtract ~12%
Heavy aquascape with rocks  →  subtract 15–20%

For accurate medication dosing, measure the water you pour in when filling, or use a tank-volume calibrated dose calculator from a fishkeeping reference.

Reference

Common tank sizes.

Common name Dimensions (L×W×H) Volume
5-gallon nano16 × 8 × 10 in5 US gal · 19 L
10-gallon "starter"20 × 10 × 12 in10 US gal · 38 L
20-gallon long30 × 12 × 12 in20 US gal · 76 L
29-gallon30 × 12 × 18 in29 US gal · 110 L
55-gallon48 × 13 × 21 in55 US gal · 208 L
75-gallon48 × 18 × 21 in75 US gal · 284 L
125-gallon72 × 18 × 21 in125 US gal · 473 L
Aquarium FAQ

Common questions.

How do I calculate the volume of a rectangular aquarium?
Multiply length × width × height (all in the same unit), then convert to gallons or liters. For a 48 × 24 × 20 inch tank: 23,040 in³ ÷ 231 = 99.7 US gallons, or about 377 liters. This calculator does the conversion automatically.
Should I use inside or outside dimensions?
Use inside dimensions for water volume. Aquarium glass is typically 6–12 mm thick, so the inside is slightly smaller. The difference is small for big tanks but meaningful for nano tanks (under 10 gallons).
What if my tank is bowfront or hexagonal?
This calculator is for rectangular tanks. For bowfront, calculate the rectangular portion and add about 5–8% for the curve. For hexagonal or other shapes, the formula differs — try a polyhedron volume calculator instead.
Does substrate and decor affect water volume?
Yes. Substrate, rocks, and large decor displace water. Subtract 5–15% from the calculated volume to estimate actual water capacity. For exact dosing of medications, measure the water you add when filling.
Related reading

From the blog.

Article
Aquarium Volume: A Complete Guide →
Article
cm³ vs ml: Are They the Same? →