How to use
- Measure the inside length and width where substrate will sit.
- Choose an average bed depth, including any planned slope in that average.
- Replace the planning density when the substrate supplier provides a bulk weight.
- Read the weight estimate and the equivalent number of 5 lb packages.
How the estimate is calculated
Substrate volume in cubic feet = inside length × inside width × average depth, all in inches, divided by 1,728. Weight = that volume × bulk density. Divide pounds by 5 and round up for equivalent 5 lb bags. The main weight rounds to whole pounds or kilograms; the detail row preserves two decimals for checking the calculation.
Example: a 30 in × 12 in footprint with a 1 in bed occupies 0.2083 cu ft. At 100 lb per cu ft the unrounded estimate is 20.8333 lb, displayed in detail as 20.83 lb and in the main card as 21 lb. That requires 5 bags of 5 lb.
Footprint versus the pounds-per-gallon shortcut
| Planning reference | Figure | How to apply it |
|---|---|---|
| LiveAquaria substrate article | About 1 lb per gallon for a 1 in bed | Multiply nominal tank gallons by depth in inches for a quick comparison |
| LiveAquaria deeper-bed example | About 2 lb per gallon for a 2 in bed | A shortcut rather than a measured-footprint formula |
| Editable bulk-density default | 100 lb per cu ft | Derived from a 50 lb / 0.5 cu ft gravel bag search summary |
For the example 20-gallon-long footprint, the shortcut gives about 20 lb at 1 in, close to the measured 20.83 lb. The shortcut cannot account for every tank shape. Decorative features that displace substrate, a steep slope, and different grain packing can move the actual weight away from either estimate.
Choosing a bed for your setup
A planted layout and a simple gravel display may call for different substrate products and depth profiles. No manufacturer instructions we checked establish a universal planted-versus-fish-only depth, so this page does not prescribe one. Pick the intended product and use its own setup guidance. Before rinsing new gravel, check the package: preparation instructions can differ for plain gravel and specialty substrates.
Common mistakes
- Using outside glass dimensions when a smaller inside footprint is available.
- Treating water capacity as an exact measure of bottom area.
- Applying the gravel density to a lightweight specialty substrate without checking its label.
Sources
LiveAquaria, Substrate: How Much Do I Need?, checked October 22, 2026, provides the gallon shortcut. The Quikrete All-Purpose Gravel bag summary supplies only a planning density basis, not a compatibility recommendation for your aquarium. No fish-health claim or universal substrate depth follows from this calculation.
Related calculators
Frequently asked questions
Does tank height affect the footprint calculation?
No. Gravel volume here depends on the bottom length, width, and substrate depth. Tank height matters only indirectly when someone uses a gallon shortcut.
Why might the gallon rule give a different answer?
Nominal water capacity does not uniquely describe bottom area, and bulk density can vary. The shortcut is a rough comparison to the dimension-based estimate.
Can I enter a sloping substrate bed?
Use an average depth that represents the footprint. For separate zones, calculate each zone and combine the weights before choosing packages.
Are the bags always five pounds?
The displayed count is explicitly a 5 lb equivalent. Divide the calculated weight by your actual bag weight and round up if the package differs.
Does the 1–3 in range tell me what my tank needs?
No. It is the calculator’s input range. Select substrate and a depth plan using instructions for your particular setup.