Aquarium Substrate Calculator

How much substrate does an aquarium need?

An aquarium substrate calculator finds the weight of sand or gravel needed for a tank by multiplying the inside length × width × substrate depth (all in inches), dividing by 1,728 to get cubic feet, then multiplying by about 100 lb per cubic foot. A standard 75 gallon tank (48 × 18 in) with a 2 inch bed needs 100 lb of sand.

Substrate is sold by weight, usually in 10, 20, or 50 lb bags, but you plan it by depth. This tool bridges the two, so you can buy the right number of bags before the tank is filled and you can no longer add more without stirring up a cloud.

The formula in plain words

1. Volume in cubic inches = tank length × tank width × substrate depth.
2. Volume in cubic feet = cubic inches ÷ 1,728.
3. Pounds of substrate = cubic feet × 100.

The 100 lb per cubic foot figure is built in. Most dry aquarium sand and small gravel fall close to it. Aquasoil and other baked clay substrates are much lighter, so for those, read the bag’s coverage chart or scale the result by your product’s density divided by 100.

Worked examples

75 gallon planted tank, 48 × 18 in, 2 in average depth

• Cubic inches: 48 × 18 × 2 = 1,728
• Cubic feet: 1,728 ÷ 1,728 = 1.00 ft³
• Weight: 1.00 × 100 = 100.0 lb, or two 50 lb bags

40 gallon breeder, 36 × 18 in, 3 in deep sand bed

• Cubic inches: 36 × 18 × 3 = 1,944
• Cubic feet: 1,944 ÷ 1,728 = 1.125 ft³
• Weight: 1.125 × 100 = 112.5 lb, so buy 120 lb (six 20 lb bags)

Round up to whole bags. A little spare substrate is useful for building slopes behind hardscape.

Substrate needed by tank size

Figures use common outside footprints at 100 lb per cubic foot. Inside dimensions are slightly smaller because of the glass, so these numbers carry a small built-in margin.

Tank	Footprint (in)	1 in deep	2 in deep	3 in deep
10 gallon	20 × 10	11.6 lb	23.1 lb	34.7 lb
20 long / 29 gallon	30 × 12	20.8 lb	41.7 lb	62.5 lb
40 breeder	36 × 18	37.5 lb	75.0 lb	112.5 lb
55 gallon	48 × 13	36.1 lb	72.2 lb	108.3 lb
75 gallon	48 × 18	50.0 lb	100.0 lb	150.0 lb
125 gallon	72 × 18	75.0 lb	150.0 lb	225.0 lb

Notice that the 20 long and the 29 gallon share a footprint. Substrate depends on floor area, not on how tall the tank is, which is why counting gallons alone can mislead you.

Typical depth by tank type

Setup	Usual depth	Why
Fish-only, gravel	1–2 in	Enough to cover the glass and hide waste
Planted, root feeders	2–3 in	Room for swords and crypts to root
Burrowing fish (loaches, gobies)	2 in fine sand	Soft grains protect barbels
Sloped aquascape	1 in front, 3–4 in back	Enter the average depth

How to measure your tank

• Measure the inside length and width in inches, from glass to glass.
• Ignore the tank height. It has no effect on substrate.
• For a sloped bed, measure the planned depth at the front and the back, and use the average.
• Subtract any large footprint taken by a rock pile or background only if it sits directly on the glass.

To check how much water the tank holds once the substrate is in, use the aquarium volume calculator.

Common mistakes

• Choosing by gallons. Two tanks with the same gallons can need very different amounts of sand if one is tall and narrow.
• Using sand weight for aquasoil. Lightweight soils can cover close to twice the area per pound, so you would overbuy.
• Forgetting rinse loss. Washing gravel or sand removes fine dust. Buying an extra 5–10% covers it.
• Buying play sand without checking it. Some builders use play sand for budget tanks. It must be rinsed thoroughly and checked that it is free of additives.

For odd-shaped tanks, the cubic inch calculator helps you work out the substrate volume section by section.