Aquarium substrate calculator
Measure the floor inside the glass, choose how deep you want the bed at the front and back, and pick your substrate. You get the bed volume, its weight, and how many bags to buy.
How it works
A substrate bed is a slab: the floor area times the depth. If you slope it evenly from front to back, the average depth is halfway between the two, so a bed 3 cm deep at the front and 7 cm at the back takes exactly as much as a flat 5 cm bed. The weight is that volume times the material's bulk density, the weight of a loose, dry measure including the air between the grains.
Sand, gravel and crushed coral are sold by weight, so the weight is what you buy. Most aquasoils (ADA Amazonia, for example) are sold by volume in litres, so for those the calculator gives the bag count straight from the bed volume and skips the weight.
| Substrate | lb/ft³ | kg/L | Source |
|---|---|---|---|
| Aragonite or cichlid sand | 100 | 1.602 | CaribSea |
| Fine reef sand, e.g. Fiji Pink | 90 | 1.442 | CaribSea |
| Medium reef sand, e.g. Special Grade | 85 | 1.362 | CaribSea |
| Eco-Planted / Eco-Complete | 80 | 1.281 | CaribSea |
| Crushed coral or cichlid gravel | 75 | 1.201 | CaribSea |
| Generic dry sand | 97 | 1.555 | Engineering ToolBox |
| Generic dry gravel | 105 | 1.680 | Engineering ToolBox |
Worked examples
A 60 × 30 cm planted tank, sloped from 3 cm to 7 cm
The average depth is (3 + 7) ÷ 2 = 5 cm, so the bed is 60 × 30 × 5 = 9,000 cm³ = 9 L: one 9 L bag of aquasoil. That matches ADA's own guidance that one 9 L bag of Amazonia suits a 60 × 30 × 36 cm tank. Add the default 10% for spills and the calculator rounds up to two bags, so set "Extra" to 0 if you trust the measurement.
A 75 gallon reef, 48 × 18 in, 2 in of Special Grade sand
48 × 18 × 2 = 1,728 in³, which is exactly 1 ft³ (28.3 L). At CaribSea's 85 lb/ft³ that's 85 lb. With 10% extra (93.5 lb), buy five 20 lb bags.
A bow-front with a 91 cm back, 5 cm of sand
With ends 38 cm deep and a 45.7 cm centre, the floor is 3,458 cm² of rectangle plus a 469.8 cm² segment, 3,927.8 cm² in all. 3,927.8 × 5 ÷ 1,000 = 19.6 L, which at 1.555 kg/L (generic dry sand) is 30.5 kg: four 10 kg bags with 10% extra.
How deep?
| Bed | Depth | Source |
|---|---|---|
| Planted tank, aquasoil | about 5 cm (2 in) average: one 9 L bag for a 60 × 30 cm floor | ADA Amazonia Ver.2 instructions |
| Reef, shallow sand bed | 1.5–2 in (3.8–5 cm) | Marine Depot / Bulk Reef Supply |
| Reef, deep sand bed (fine sand) | 6–8 in (15–20 cm) | Marine Depot / Bulk Reef Supply |
| Coarse sand | not deeper than 2–3 in (5–7.5 cm); it clogs with detritus | Marine Depot / Bulk Reef Supply |
Limits
- Bulk density varies. Fine sand packs tighter than coarse gravel, and wet or "live" sand bags include packing water. Treat the weight as a buying estimate and keep the extra allowance.
- Uneven scapes. The slope formula assumes an even ramp from front to back. Mounds, terraces and hardscape that sits on the floor change the amount. Rock and wood resting on the glass need less substrate under them; rock placed on top of the bed needs none.
- Bow-front slopes are approximated with the same average depth, which is close but not exact for a curved front.
- Chemistry. Aragonite and other calcium carbonate sands raise pH and KH, and many aquasoils lower them. Check with the KH & GH converter after setup, and remember aquasoils make pH/KH CO₂ estimates read high (see the CO₂ calculator).
- Weight on the stand. A 2 in bed on a 48 × 18 in floor (a common 75 gallon) weighs 75–105 lb depending on the material, on top of the water weight from the volume calculator. Check your stand's rating against the total.
Common questions
How much sand do I need for a 20 gallon long?
A 20 long is sold as 30 × 12 in. A 2 in bed over that floor is 720 in³ (11.8 L, 0.42 ft³), which is 41.7 lb of aragonite or cichlid sand at CaribSea's 100 lb/ft³, or 40.4 lb of generic dry sand. With 10% extra, that's three 20 lb bags.
How much substrate do I need for a 55 gallon tank?
For a 48 × 13 in footprint and a 2 in bed: 0.72 ft³ (20.5 L), which is 72.2 lb of aragonite sand, 61.4 lb of Special Grade reef sand or 54.2 lb of crushed coral, using CaribSea's densities. Measure your own tank inside the glass, because footprints vary by maker.
Is it 1 pound of sand per gallon for each inch of depth?
Only for some tanks. CaribSea's general rule is 1 to 2 lb per gallon, but the bed depends on the floor area, not the water. One inch of 100 lb/ft³ sand is 20.8 lb in a 20 long (1.04 lb per gallon) but 36.1 lb in a 48 × 13 in 55 gallon (0.66 lb per gallon), because the 55 is mostly extra height.
How deep should aquarium substrate be?
For reefs, Marine Depot (Bulk Reef Supply) describes a shallow bed as 1.5–2 in and a deep sand bed as 6–8 in of fine sand, and advises against coarse beds deeper than 2–3 in, which clog with detritus. For planted tanks, ADA sizes one 9 L bag of Amazonia for a 60 × 30 cm tank, which is a 5 cm (2 in) average bed.
How many bags of aquasoil do I need for a 90 × 45 cm tank?
At a 5 cm average depth, 90 × 45 × 5 = 20,250 cm³, or 20.25 L: three 9 L bags (27 L), even with 10% extra. Slope it deeper at the back if you like; the average depth is what sets the amount.
References
- CaribSea, Inc. FAQ: "How many pounds do I need?" (L × W × bed depth in inches ÷ 1,728 × density: aragonite and cichlid sands 100, Eco-Planted 80, crushed coral and cichlid gravels 75, Special Grade 85, Fiji Pink 90 lb/ft³; general rule 1–2 lb per gallon)
- CaribSea, Inc. Marine substrates (Special Grade Reef: average density 85 lb/ft³, 10 and 20 lb bags)
- Engineering ToolBox: Dirt and mud densities (sand, dry 1,555 kg/m³; gravel, dry 1,680 kg/m³)
- ADA (Aqua Design Amano). Aqua Soil Amazonia Ver.2 instructions (one 9 L bag suits a W60 × D30 × H36 cm tank; don't rinse)
- Marine Depot via Bulk Reef Supply. Marine/reef aquarium sand: deep sand bed or shallow? (shallow 1.5–2 in; deep 6–8 in; avoid coarse beds over 2–3 in)
- NIST Handbook 44, Appendix C (1 ft³ = 1,728 in³ = 28.316846592 L; 1 lb = 0.45359237 kg)
Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.
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