A standard 20-gallon aquarium (24 × 12 inches) needs roughly 17 lb (7.7 kg) of gravel at a 2-inch depth. Enter your tank dimensions and desired substrate depth to get the calculation for your specific setup.
How substrate weight is calculated
The formula multiplies footprint area by substrate depth to get volume, then converts to weight using a density factor for the substrate type:
Volume (in³) = Tank length (in) × Tank width (in) × Substrate depth (in)
Weight (lb) = Volume (in³) × Density (lb/in³)
Weight (kg) = Weight (lb) × 0.45359237
Density factors used:
- Standard gravel: ~0.058 lb/in³ (dry bulk density, Aquarium Co-Op reference)
- Fine sand: ~0.052 lb/in³ (slightly lower — smaller particles but similar packing)
- Planted substrate: ~0.035 lb/in³ (porous volcanic or ceramic substrate, e.g., ADA Amazonia, Fluval Stratum)
These are approximate dry densities. Actual weight per bag varies by brand and particle size. Always round up to the next bag size when purchasing.
Recommended substrate depths
| Tank type | Substrate type | Recommended depth | Notes |
|---|---|---|---|
| Fish-only freshwater | Gravel or coarse sand | 1–2 inches | Sufficient for aesthetics and beneficial bacteria |
| Planted freshwater | Planted substrate | 2–3 inches | Root systems need depth for nutrient access |
| Cichlid / digger species | Fine sand | 2–3 inches | Deeper allows natural burrowing behavior |
| Marine / reef | Live sand or aragonite | 1–2 inches (shallow) or 4+ inches (deep sand bed) | Deep sand beds require different filtration strategy |
| Bare-bottom | None | 0 | Easier to clean; preferred for some species |
These are general industry guidelines, not prescriptions. Fish species, plant selection, and filtration setup all affect the optimal depth for your specific tank.
Common calculation examples
| Tank size | Footprint | Depth | Gravel needed |
|---|---|---|---|
| 10-gallon | 20 × 10 in | 1 in | ~11.6 lb / 5.3 kg |
| 10-gallon | 20 × 10 in | 2 in | ~23.2 lb / 10.5 kg |
| 20-gallon | 24 × 12 in | 2 in | ~33.4 lb / 15.1 kg |
| 40-gal breeder | 36 × 18 in | 2 in | ~75.2 lb / 34.1 kg |
| 55-gallon | 48 × 12.75 in | 2 in | ~71.3 lb / 32.3 kg |
| 75-gallon | 48 × 18 in | 2 in | ~100.5 lb / 45.6 kg |
Buying substrate
Aquarium substrate is typically sold in bags by weight (5 lb, 10 lb, 20 lb, 25 lb for gravel; smaller bags for specialty planted substrates). Check the bag sizes available from your supplier and round your calculated total up to the next available bag size — running short mid-setup is common and requires a second trip.
For most freshwater setups, budget slightly extra for sloping the substrate (higher in the back than the front), which can add 20–30% to the flat-depth estimate.
For calculating your total tank water volume, see the Aquarium Volume Calculator →.