Aquariums · Use case

Aquarium volume,
in gallons.

Q: 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).

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)	Nominal	Full to the brim
5-gallon nano	16 × 8 × 10 in	5 US gal	5.5 gal · 21 L
10-gallon "starter"	20 × 10 × 12 in	10 US gal	10.4 gal · 39 L
20-gallon long	30 × 12 × 12 in	20 US gal	18.7 gal · 71 L
29-gallon	30 × 12 × 18 in	29 US gal	28.1 gal · 106 L
55-gallon	48 × 13 × 21 in	55 US gal	56.7 gal · 215 L
75-gallon	48 × 18 × 21 in	75 US gal	78.5 gal · 297 L
125-gallon	72 × 18 × 21 in	125 US gal	117.8 gal · 446 L

“Nominal” is the trade name the tank is sold under. “Full to the brim” is the geometric volume of those outside dimensions — what this calculator returns. The real water volume sits below both: glass thickness, the rim, substrate, rock and the gap you leave at the top typically take 10–15% out.

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.