How many litres is an ADA 60P aquarium?

A 60×30×36 cm 60P is 64.8 litres gross. With 5 cm of substrate and medium hardscape the net water volume is about 52 litres, and a 30% weekly water change is 16 litres.

How many litres is a 55 gallon aquarium?

55 US gallons is about 208 litres nominal. Measured internally at roughly 121×32×52 cm the gross volume is about 205 litres, and with substrate and hardscape about 174 litres net — so a 30% change is 52 litres, not 62.

Why do two identical tanks have different net volumes?

Substrate depth and the amount of rock and wood. The same 60P holds 55.8 litres with 3 cm of substrate, 52.2 litres with 5 cm and 46.8 litres with 8 cm — a 9 litre spread on an identical tank.

Are these dimensions internal or external?

Internal, because that is what holds water. Manufacturer specs are usually external: subtract twice the glass thickness, about 0.6 cm per side for 6 mm glass, before multiplying.

Reference

Aquarium tank sizes in litres

Q: Is a 20 gallon long really 20 gallons?

No. 20 US gallons is 75.7 litres, but a 20 long's internal volume is about 67 litres, and with substrate and hardscape it holds roughly 52 litres of water — about 14 US gallons. The label describes the outside of the box, not the water.

Q: How much water should I change in a 90P?

A 90×45×45 cm 90P is 182 litres gross and about 152 litres net, so a 30% weekly change is about 46 litres, with roughly 1.1 ml of conditioner at the 1 ml per 40 litres rate.

A 60P is 64.8 litres gross and about 52 litres net once substrate and hardscape are in. A 55 gallon is 205 litres gross and about 174 net — a 30% water change is 52 litres, not 62. Nominal sizes are labels on the box; only the net volume is what you dose against.

The table

Net volume assumes 5 cm of substrate and medium hardscape (5.5% of gross). Conditioner uses the 1 ml per 40 litres reference rate; salt uses 35 g per litre of replacement water.

Tank	Internal W×D×H (cm)	Gross	Net	Change 30%	Cond.	Salt
30C cube	30 × 30 × 30	27.0 L	21.0 L	6 L	0.1 ml	210 g
45P	45 × 30 × 45	60.8 L	50.7 L	15 L	0.4 ml	525 g
60P	60 × 30 × 36	64.8 L	52.2 L	16 L	0.4 ml	560 g
60H	60 × 45 × 45	121.5 L	101.3 L	30 L	0.8 ml	1050 g
90P	90 × 45 × 45	182.2 L	152.0 L	46 L	1.1 ml	1610 g
90H	90 × 45 × 60	243.0 L	209.4 L	63 L	1.6 ml	2205 g
120P	120 × 50 × 50	300.0 L	253.5 L	76 L	1.9 ml	2660 g
180P	180 × 60 × 60	648.0 L	558.4 L	168 L	4.2 ml	5880 g
US 5.5 gal	42 × 22 × 27	24.0 L	18.2 L	5 L	0.1 ml	175 g
US 10 gal	50 × 24 × 29	35.5 L	27.5 L	8 L	0.2 ml	280 g
US 20 long	76 × 30 × 30	66.7 L	51.8 L	16 L	0.4 ml	560 g
US 20 high	60 × 30 × 39	70.2 L	57.4 L	17 L	0.4 ml	595 g
US 29 gal	76 × 30 × 45	99.9 L	83.2 L	25 L	0.6 ml	875 g
US 40 breeder	91 × 45 × 39	161.3 L	132.0 L	40 L	1.0 ml	1400 g
US 55 gal	121 × 32 × 52	204.8 L	173.8 L	52 L	1.3 ml	1820 g
US 75 gal	121 × 45 × 52	285.0 L	242.0 L	73 L	1.8 ml	2555 g
US 125 gal	182 × 45 × 52	428.4 L	363.7 L	109 L	2.7 ml	3815 g

Dimensions are internal and rounded to the nearest centimetre, so treat gross litres as ±2%. Your tank's glass thickness, brace and substrate slope will move the net number. For your exact tank, use the volume calculator.

Why the label is not the volume

Tank names describe the outside of a box, in a 3,785 ml gallon. That is three conversions away from "how much water do I dose":

nominal 55 gal → 208 L internal 121 × 32 × 52 → 205 L gross − substrate (5 cm) → − 19.4 L − hardscape (5.5%) → − 11.3 L = net → 174 L (46 gal) 30% weekly change → 52 L

So the "55 gallon" on the box, the 205 litres you calculate from the glass and the 174 litres you actually dose into are three different numbers. Fertilizer, conditioner, salt and medication all belong to the last one.

Same tank, different substrate

Substrate depth is the single biggest source of variation between two identical tanks:

60P (60 × 30 × 36)	Gross	Substrate	Hardscape	Net
3 cm aqua soil	64.8 L	− 5.4 L	− 3.6 L	55.8 L
5 cm aqua soil	64.8 L	− 9.0 L	− 3.6 L	52.2 L
8 cm aqua soil	64.8 L	− 14.4 L	− 3.6 L	46.8 L

A deeper bed for rooted plants is not free: 8 cm instead of 3 cm takes almost 9 litres of water out of the same footprint. Dosing on the label number over-doses that tank permanently.

How to measure your own tank

Measure inside the glass, not outside. For 6 mm glass subtract about 1.2 cm from width and depth.
Height is water height, not glass height — subtract the gap under the rim or brace, usually 1–3 cm.
Substrate is an average. Sloped beds can be 3 cm at the front and 10 cm at the back; measure a few points and take the middle.
Hardscape is an estimate. Light 3%, medium 5.5%, heavy 9% of gross volume — heavier if you cannot see the back glass.
Then round to whole litres for the water change, because that is what you can measure.

Questions

How many litres is an ADA 60P?

64.8 litres gross at 60 × 30 × 36 cm, about 52 litres net with 5 cm of substrate and medium hardscape.

How many litres is a 55 gallon tank?

About 205 litres gross measured internally, roughly 174 litres net. The nominal 55 gallons is 208 litres, so the label overstates the water by about 16%.

Is a 20 gallon long really 20 gallons?

It holds about 52 litres of water — roughly 14 US gallons — once substrate and hardscape are in, against a 76 litre nominal rating.

How much water should I change in a 90P?

About 46 litres per week at 30% of the 152 litre net volume, with 1.1 ml of conditioner at the 1 ml per 40 litre rate.

Why do two identical tanks dose differently?

Substrate depth and hardscape. The same 60P ranges from 46.8 to 55.8 litres net depending on the bed.

Internal or external dimensions?

Internal — subtract twice the glass thickness before multiplying, or your litres will be a few percent high.

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