How much gravel or sand you need depends on two things: the footprint of the tank (length × width) and the depth of bed you want. Gallons do not come into it, which is why the familiar rule of thumb is often off. The substrate calculator works it out for any tank; this guide explains the choices behind it.
How deep should the bed be?
- Fish-only gravel: 1–2 inches (2.5–5 cm). Enough to anchor decor and cover the glass.
- Fish-only sand: 1–1.5 inches (2.5–4 cm). Thin sand beds stay oxygenated; deep, undisturbed sand can develop gas pockets.
- Planted tanks: 2–3 inches (5–7.5 cm), often sloped to 3–4 inches at the back where the taller, root-feeding plants go.
- Bare-bottom: none. Common for breeding, grow-out and hospital tanks because they are easy to clean.
The formula
The bed is a slab: footprint × depth. In inches, divide by 1,728 for cubic feet; aquarium gravel weighs about 100 lb per cubic foot and sand about 94. In centimeters, divide by 1,000 for liters and multiply by the density in kg per liter (about 1.6 for gravel, 1.5 for sand).
For a sloped bed, use the average of the front and back depths. A bed that runs from 1 inch at the front to 3 inches at the back needs the same amount as a flat 2 inch bed.
Pounds per inch of depth, by tank size
| Tank | Footprint | Gravel per inch | Sand per inch | "1 lb/gal" rule |
|---|---|---|---|---|
| 10 gallon | 20 × 10 in | 12 lb | 11 lb | 10 lb |
| 20 gallon high | 24 × 12 in | 17 lb | 16 lb | 20 lb |
| 29 gallon | 30 × 12 in | 21 lb | 20 lb | 29 lb |
| 40 gallon breeder | 36 × 18 in | 37 lb | 35 lb | 40 lb |
| 55 gallon | 48 × 13 in | 36 lb | 34 lb | 55 lb |
| 75 gallon | 48 × 18 in | 50 lb | 47 lb | 75 lb |
| 90 gallon | 48 × 18 in | 50 lb | 47 lb | 90 lb |
| 120 gallon | 48 × 24 in | 67 lb | 62 lb | 120 lb |
Multiply by the depth you want. A 2 inch gravel bed in a 55 gallon is about 72 lb, not the 110 lb the rule suggests.
Why the "1 lb per gallon" rule misleads
The rule says 1 lb of gravel per gallon of tank gives about 1 inch of depth. That is only true for tanks about 13 inches tall. Taller tanks have more gallons over the same floor, so the rule tells you to buy too much: for a 90 gallon, close to twice what you need. It is a reasonable upper bound for most tanks, but bags are heavy and not always returnable, so it pays to work it out.
Choosing a material
- Gravel: easy to vacuum and forgiving; fine for most community tanks.
- Sand: suits fish that sift the bottom (corydoras, loaches, many cichlids). Vacuum by hovering just above it.
- Aquasoil: baked clay that feeds plants and softens water. Sold by volume (liters); it breaks down over a couple of years.
- Crushed coral or aragonite: raises pH and hardness, which suits African rift lake cichlids and marine tanks, and works against soft-water fish.
Buying and preparing it
Round up to whole bags; a bed a little deeper than planned is better than a bare patch. Rinse gravel and sand in a bucket until the water runs mostly clear (aquasoils are usually not rinsed; follow the bag). Remember the bed takes up water space: enter it in the volume calculator to see the real water volume afterward.