Buying too little gravel means a second trip to the store; too much wastes money and space. The good news: you can work out the right amount in a minute, or let the substrate calculator do it.
Why the gallon rule is often wrong
You'll often hear "1 lb of gravel per gallon for every inch of depth". It works for some tanks and badly for others, because gravel covers the floor, and the floor doesn't grow with volume. A 29 gallon and a 20 gallon long have exactly the same 30 x 12 inch floor, so they need the same gravel, even though one holds 9 gallons more.
The footprint method
- Multiply the tank's length x width in inches to get the floor area.
- Multiply by the depth of gravel you want, in inches.
- Divide by 1,728 to convert to cubic feet.
- Multiply by about 100 for pounds of gravel, or about 95 for sand.
Example: a 55 gallon tank has a 48 x 13 inch floor. 48 x 13 x 2 = 1,248 cubic inches, / 1,728 = 0.72 cubic feet, x 100 = about 72 lb for a 2 inch bed.
Gravel needed for every standard tank
| Tank | Floor | 1 in | 2 in | 3 in |
|---|---|---|---|---|
| 5 gallon | 16 x 8 in | 7 lb | 15 lb | 22 lb |
| 10 gallon | 20 x 10 in | 12 lb | 23 lb | 35 lb |
| 20 gallon | 24 x 12 in | 17 lb | 33 lb | 50 lb |
| 20 gallon long | 30 x 12 in | 21 lb | 42 lb | 62 lb |
| 29 gallon | 30 x 12 in | 21 lb | 42 lb | 62 lb |
| 30 gallon | 36 x 12 in | 25 lb | 50 lb | 75 lb |
| 40 gallon breeder | 36 x 18 in | 38 lb | 75 lb | 112 lb |
| 50 gallon | 36 x 18 in | 38 lb | 75 lb | 112 lb |
| 55 gallon | 48 x 13 in | 36 lb | 72 lb | 108 lb |
| 60 gallon | 48 x 13 in | 36 lb | 72 lb | 108 lb |
| 75 gallon | 48 x 18 in | 50 lb | 100 lb | 150 lb |
| 90 gallon | 48 x 18 in | 50 lb | 100 lb | 150 lb |
| 100 gallon | 72 x 18 in | 75 lb | 150 lb | 225 lb |
| 125 gallon | 72 x 18 in | 75 lb | 150 lb | 225 lb |
How deep should the gravel be?
- 1–2 inches: most community tanks. Deep enough to anchor decor, shallow enough to clean.
- 2–3 inches: planted tanks, sloped deeper at the back.
- 1–2 inches of sand: for corydoras and other bottom sifters.
Plant substrates are sold by volume
Aquasoils are much lighter than gravel and are sold in litres, often in 3 L and 9 L bags. Use the substrate calculator with Aquasoil selected to get litres and bag counts.
Frequently asked questions
How much gravel do I need per gallon?
The old rule is 1 lb per gallon for a 1 inch bed and 2 lb per gallon for 2 inches, but it depends on the tank's floor, not its volume. Tall tanks need less, wide tanks need more.
How do I calculate how much gravel I need?
Multiply the tank's length x width x depth of gravel in inches, divide by 1,728 to get cubic feet, then multiply by about 100 lb per cubic foot.
How many bags of gravel for a 20 gallon tank?
A 20 gallon long needs about 42 lb for a 2 inch bed, which is three 20 lb bags or nine 5 lb bags.