How much gravel for a 38 gallon tank?
36 × 12 × 20 in · freshwater planning
38 gallon · gravel & substrate
The working
36 × 12 × 2 = 864 in³. Divide by 1,728 and multiply by 100 lb/ft³ = 50 lb. Round up to 3 bags of 20 lb.
| Depth | Gravel lb / 20 lb bags | Sand lb / 20 lb bags | Aquasoil lb / L |
|---|---|---|---|
| 1 in | 25 / 2 | 25 / 2 | 15 / 7.08 |
| 2 in | 50 / 3 | 50 / 3 | 30 / 14.16 |
| 3 in | 75 / 4 | 75 / 4 | 45 / 21.24 |
First decisions for this medium setup
The 432 square inches of floor make substrate depth a meaningful purchase decision. Compare the one-inch and three-inch rows before committing to a deep planted layout. Terraces and retaining rocks can keep a rear bank from spreading, but their occupied footprint reduces the amount of loose substrate needed.
38 gallon equipment and layout plan
This upright rectangle adds vertical room without a wider bottom. The 36 × 12-inch base has an aspect ratio of 3:1. A lid or light must span roughly 36 inches; measure rim openings and bracket adjustment before ordering.
| Level bed | Sand bags, rounded up | Aquasoil bags, rounded up |
|---|---|---|
| 1 in | 25 lb / 2 × 20 lb | 7.08 L / 1 × 9 L |
| 2 in | 50 lb / 3 × 20 lb | 14.16 L / 2 × 9 L |
| 3 in | 75 lb / 4 × 20 lb | 21.24 L / 3 × 9 L |
The rear bank can rise 2 inches across the 12-inch width: a 16.67% slope. Keeping the front at 1 inch and the back at 3 uses the same 14.16 L as a flat two-inch bed across the full 36-inch length. Aquasoil is sold here in hypothetical 9 L bags; check the actual package.
Moving up from the 37 gallon changes square inches of substrate coverage from 360 to 432: an increase of 20%. Budget for the larger requirement rather than reusing the smaller plan unchanged.
Assumptions and limits
The old 1 lb/gal for roughly 1 inch and 1.5–2 lb/gal for 2 inches rules are loose shopping shortcuts. Here a 2-inch bed is 1.32 lb per nominal gallon. Footprint and bulk density explain the difference. Gravel and sand use 100 lb/ft³; aquasoil uses 60. Check bag coverage; grain size and moisture change density.
Bulk densities are stated planning assumptions; litre conversion: NIST conversion tables
How to use this plan
- Measure the inside footprint; this plan uses 36 by 12 inches.
- Choose an average depth, allowing for slopes and hardscape.
- Calculate volume and mass as above; buy about 3 20 lb bags for a level 2-inch gravel bed.
Adjust measurements with the gravel & substrate calculator
Questions before you buy
What dimensions does this 38 gallon calculation use?
It uses a rounded 36 × 12 × 20 inch rectangle. Actual dimensions vary by brand and frame; measure your own tank.
What is the gravel & substrate estimate for a 38 gallon tank?
50 lb gravel for a 2-inch bed
Equipment to check
Compare aquarium substrate. Check dimensions, capacity and product instructions before buying.
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