Aquarium Substrate Depth Calculator
Buying too little substrate mid-project is a pain, and too much is money and shelf space wasted. Enter your tank footprint and the depth you are after to get the volume in liters and cubic feet, the weight, and how many bags to buy.
Aim a touch deeper at the back if you want a sloped aquascape — this uses an even depth across the base.
Bulk density varies by product — gravel and sand run about 1.5–1.7 kg/L, aqua soils are lighter (~0.8–1.0). Adjust to your bag's stated figure.
Formula & how it works
Volume is length × width × depth in cubic inches, converted with liters = in³ × 0.01639 and cubic feet = in³ ÷ 1728.
Weight is liters × density in kilograms. Bags are the volume or weight divided by the bag size, rounded up to the next whole bag.
Worked example
A 36 × 18 in base at 2 in deep is 1,296 in³ — about 21 liters, or 0.75 cubic feet. At 1.6 kg/L that weighs roughly 34 kg (75 lb), so at 20 lb per bag you would buy 4 bags. Sold as 9 L bags instead, you would need 3.
Choosing depth and amount
Match depth to your plan
A fish-only tank with a few decorations does not need much substrate — an inch or two mainly for looks. Rooted plants prefer two to three inches so they can anchor, while carpeting plants and deep-root species may want more at the back. Deeper is not always better, since thick beds can trap waste and go anaerobic if never disturbed.
Density is the variable to check
The volume you need is pure geometry, but turning it into a weight and a bag count depends entirely on the product. Fine sand, coarse gravel, and lightweight planted-tank soils can differ a lot, so the density figure here is a starting default — read your bag's label and adjust it for a tighter estimate.
Buy a little extra
Substrate settles, gets removed during cleaning, and slopes eat more than a flat bed of the same average depth. Rounding up to whole bags usually leaves a small cushion, and keeping one spare bag on hand makes future rescapes and top-ups painless.