How much gravel for a 20 gallon long tank?

30 × 12 × 12 in · freshwater planning

41.67 lb gravel for a 2-inch bed

20 gallon long · gravel & substrate

20 gallon long equipment and layout plan

A low, extended rectangle puts more of the layout along a horizontal swimming lane. The 30 × 12-inch base has an aspect ratio of 2.5:1. A lid or light must span roughly 30 inches; measure rim openings and bracket adjustment before ordering.

Level bed	Sand bags, rounded up	Aquasoil bags, rounded up
1 in	20.83 lb / 2 × 20 lb	5.9 L / 1 × 9 L
2 in	41.67 lb / 3 × 20 lb	11.8 L / 2 × 9 L
3 in	62.5 lb / 4 × 20 lb	17.7 L / 2 × 9 L

To build a slope along the 30-inch front, calculate a shallow 1-inch strip and a deep 3-inch rear bank. Their average depth is 2 inches if the slope is linear across 12 inches. Buy 11.8 L before accounting for rocks. Soil counts assume 9 L packages, not a guaranteed bag weight.

Moving up from the 20 gallon high changes square inches of substrate coverage from 288 to 360: an increase of 25%. Budget for the larger requirement rather than reusing the smaller plan unchanged.

First decisions for this small setup

Lay out the 30-inch front edge before pouring substrate. Keep a clear strip where the siphon will reach, and place heavy hardscape securely rather than relying on loose gravel to hold it upright. Rounded-up bag totals leave surplus: do not pour every bag in just because it was purchased.

The working

30 × 12 × 2 = 720 in³. Divide by 1,728 and multiply by 100 lb/ft³ = 41.67 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	20.83 / 2	20.83 / 2	12.5 / 5.9
2 in	41.67 / 3	41.67 / 3	25 / 11.8
3 in	62.5 / 4	62.5 / 4	37.5 / 17.7

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 2.08 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 30 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 20 gallon long calculation use?

It uses a rounded 30 × 12 × 12 inch rectangle. Actual dimensions vary by brand and frame; measure your own tank.

What is the gravel & substrate estimate for a 20 gallon long tank?

41.67 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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