Key numbers
- Pounds = L × W × depth (in) ÷ 1,728 × lb per ft³
- Rule of thumb: 1–2 lb of substrate per gallon (CaribSea)
- Densities (CaribSea): gravel 75, sand 100, planted 80, reef sand 85 lb/ft³
- 55 gal (48 × 13 in): 54 lb gravel or 72 lb sand at 2 in deep
- Gallons = L × W × H (in) ÷ 231
On this page
- How to use the Aquarium Gravel Calculator
- How much gravel do I need for my aquarium?
- How much gravel for common tank sizes?
- Does the 1 lb per gallon rule work?
- How deep should aquarium substrate be?
- Gravel or sand: how much does each weigh?
- Aquarium substrate in metric
- Common mistakes
- Frequently asked questions
How to use the Aquarium Gravel Calculator
- 1Enter your numbersFill in tank inside length (in), tank inside width (front to back) (in), tank height (for gallons) (in), substrate depth (in), substrate, density on your bag (optional) (lb/ft³) or bag size (lb).
- 2Check the assumptionsPounds = length × width × depth (in) ÷ 1,728 × density, the method CaribSea publishes. Densities are CaribSea's figures by product family; the figure for your product wins. Gallons = L × W × H ÷ 231 for a rectangular tank.
- 3Read the resultsThe calculator shows substrate needed, bags of 20 lb, volume, tank volume (rectangular) or rule-of-thumb check (1–2 lb per gallon), updated as you type.
- 4Use the resultRound up to what your supplier sells, and keep a little extra for waste.
Results update as you type. Back to the calculator ↑
How much gravel do I need for my aquarium?
Gravel covers the floor of the tank, so what sets the amount is the footprint and depth, not the gallons. CaribSea, a substrate maker, publishes this method:
Pounds = length (in) × width (in) × depth (in) ÷ 1,728 × density (lb/ft³)
1,728 is the number of cubic inches in a cubic foot (12 × 12 × 12). The density is the substrate’s bulk weight per cubic foot. CaribSea lists about 75 lb/ft³ for gravels and crushed coral, 80 for planted substrate, 85 for reef sand and 100 for most sands. The bag or product page for your substrate is the best figure: enter it in the calculator above to override.
- Measure the inside of the tank, length and front-to-back width. Glass thickness and the frame take up some of the outside size.
- Choose a depth. Measure from the glass up. If you want a slope, higher at the back, use the average depth.
- Run the formula or the calculator, then round up to whole bags.
Worked example: 55 gallon tank, 48 × 13 in, 2 in of gravel
- 48 × 13 × 2 = 1,248 in³ ÷ 1,728 = 0.72 ft³ (20.5 L)
- × 75 lb/ft³ = 54 lb of gravel (24.6 kg): 3 × 20 lb bags
- With sand at 100 lb/ft³: 72 lb, or 4 × 20 lb bags
- Rule-of-thumb check: 1–2 lb × 55 gal = 55–110 lb, so 54 lb is a thin-to-medium bed
How much gravel for common tank sizes?
These are typical footprints for each size; tanks from different makers vary by an inch or two, so measure yours. Weights are rounded:
| Tank | Footprint L × W × H (in) | Gallons (L × W × H ÷ 231) | Gravel 1 in (lb) | Gravel 2 in (lb) | Gravel 3 in (lb) | Sand 2 in (lb) | Sand 2 in (kg) |
|---|---|---|---|---|---|---|---|
| 5 gal | 16 × 8 × 10 | 5.5 | 6 | 11 | 17 | 15 | 6.7 |
| 10 gal | 20 × 10 × 12 | 10.4 | 9 | 17 | 26 | 23 | 10.5 |
| 20 gal long | 30 × 12 × 12 | 18.7 | 16 | 31 | 47 | 42 | 18.9 |
| 29 gal | 30 × 12 × 18 | 28.1 | 16 | 31 | 47 | 42 | 18.9 |
| 40 gal breeder | 36 × 18 × 16 | 44.9 | 28 | 56 | 84 | 75 | 34.0 |
| 55 gal | 48 × 13 × 21 | 56.7 | 27 | 54 | 81 | 72 | 32.8 |
| 75 gal | 48 × 18 × 21 | 78.5 | 38 | 75 | 113 | 100 | 45.4 |
| 125 gal | 72 × 18 × 21 | 117.8 | 56 | 113 | 169 | 150 | 68.0 |
Gravel at 75 lb/ft³, sand at 100 lb/ft³ (CaribSea figures). Note the 20 long and the 29: different volumes, same 30 × 12 in floor, so they need exactly the same gravel. The 40 breeder holds less water than the 55 but needs more substrate because its floor is wider. This is why the footprint method beats the per-gallon rule.
Does the 1 lb per gallon rule work?
As a quick check, yes. CaribSea’s general rule is 1 to 2 lb per gallon. For the long tanks in the table, 1 lb of gravel per gallon gives roughly a 1–2 in bed and 2 lb roughly 2½–4 in (a 10 gal at 10 lb is about 1.2 in; a 55 gal at 55 lb is about 2 in). It breaks down for tall tanks (too much) and wide tanks such as breeders (too little). The calculator shows both numbers so you can see the difference for your tank.
How deep should aquarium substrate be?
There is no single correct depth; it depends on what lives in the tank. Many fish-only tanks run 1 to 2 in, enough to cover the glass and anchor decorations. Tanks with rooted plants often use deeper beds, and some keepers slope the substrate from about 1 in at the front to 3 in at the back for depth of view. Burrowing fish need sand, not sharp gravel.
Whatever you choose, each inch adds weight. On a 48 × 13 in tank, every inch of gravel is about 27 lb, and every inch of sand about 36 lb. Check that your stand is rated for the full weight of water, glass, rock and substrate.
Gravel or sand: how much does each weigh?
Sand is denser than gravel, so the same depth weighs more. By CaribSea’s figures sand is about 100 lb/ft³ and gravel about 75, so a sand bed weighs a third more for the same volume. Buy by pounds, not by the number of bags, and check the bag weight: 5, 10, 20 and 40 lb bags are all common.
Aquarium substrate in metric
Work in centimetres and litres:
Litres = length (cm) × width (cm) × depth (cm) ÷ 1,000
Kilograms = litres × bulk density (kg/L)
75 lb/ft³ is 1.2 kg/L and 100 lb/ft³ is 1.6 kg/L. A 120 × 33 cm tank with 5 cm of gravel holds 19.8 L of gravel, about 23.8 kg. The calculator shows litres and kilograms for any imperial input.
Common mistakes
- Using the outside dimensions. On large tanks the frame and glass can cut half an inch off each side.
- Buying by gallons alone for tall or wide tanks; see the table above.
- Forgetting rinse loss and slopes. Dust rinses out of new gravel and a back slope uses more than a flat bed. Buy a bag extra if you are between sizes.
- Mixing up landscape and aquarium gravel. Driveway and garden stone can contain dust, sharp edges or minerals that change water chemistry. Use products sold for aquariums; for outdoor projects use the gravel calculator.
FAQ
Frequently asked questions
How much gravel do I need for a 55 gallon tank?
A 55 gallon tank with a 48 × 13 in footprint needs about 54 lb of gravel at 2 in deep (75 lb/ft³), or 81 lb at 3 in. With sand at 100 lb/ft³, 2 in takes about 72 lb. The 1–2 lb per gallon rule gives 55–110 lb.
How much gravel do I need for a 10 gallon tank?
A 10 gallon tank with a 20 × 10 in footprint needs about 17 lb of gravel at 2 in deep, or 9 lb at 1 in. With sand at 100 lb/ft³, 2 in takes 23 lb. Buy one 20 lb bag for a 2 in gravel bed, two for sand.
How many pounds of gravel per gallon for an aquarium?
CaribSea's general rule is 1 to 2 pounds per gallon. About 1 lb per gallon gives a thin bed and 2 lb a deeper one, but the footprint matters more than the gallons: a tall tank needs less than a long tank of the same volume. Use length × width × depth to be precise.
How deep should aquarium gravel be?
Many keepers use 1 to 2 in of gravel for a fish-only tank and deeper beds for rooted plants, but it is a choice, not a rule. Each inch of gravel on a 48 × 13 in tank adds about 27 lb, so decide the depth first, then calculate.
How much sand do I need for a 20 gallon tank?
A 20 gallon long (30 × 12 in) needs about 42 lb of sand for 2 in at 100 lb/ft³, or 31 lb of gravel at 75 lb/ft³. A 20 gallon high has a smaller footprint, so measure yours and enter the size.
How many bags of gravel do I need for a 75 gallon tank?
A 75 gallon tank with a 48 × 18 in footprint needs about 75 lb of gravel at 2 in deep, or 100 lb of sand. That is 4 × 20 lb bags of gravel or 5 of sand. At 3 in, gravel needs 113 lb, or 6 bags.
How do I calculate aquarium substrate in kg or litres?
Work in centimetres: length × width × depth ÷ 1,000 gives litres, then multiply by the bulk density in kg per litre. 75 lb/ft³ is 1.2 kg/L and 100 lb/ft³ is 1.6 kg/L. A 120 × 33 cm tank at 5 cm of gravel is 19.8 L, or about 23.8 kg.
2 Sources
Last reviewed October 11, 2026 by M Bilal. Found an error? Tell us.