How many pounds of gravel do I need for a 55-gallon tank?

A standard 55-gallon (48 × 13 × 21 in) needs about 38–45 pounds of gravel for a 1-inch bed, or 55–65 pounds for a 1.5-inch bed, depending on grain density. The calculator gives you the exact number for the product you're buying.

How much gravel for a 20-gallon long?

A 20-long (30 × 12 × 12 in) needs about 17–21 pounds for a 1-inch bed. Wider footprint than a standard 20H — same gallons, more substrate.

Gravel vs sand for freshwater?

Gravel (2–5 mm grain) is the safer beginner choice — it doesn't get sucked into siphons, plants root in it easily, and corydoras can sift it without abrasion. Sand looks more natural and is required for certain species (loaches, geophagus), but it compacts and traps gas pockets if you don't stir it.

Can I use this for crushed coral?

Yes — pick "Florida Crushed Coral" or "Caribbean Crushed Coral" in the product dropdown. Note that crushed coral buffers freshwater (Cichlid-tank trick) and is too gunk-trapping for most modern reef tanks.

Why isn't pea gravel in the list?

Big-box-store pea gravel doesn't come with a published bulk density I trust. Estimates online range 90–110 lb/ft³, which is too wide to put a number on. If you must, weigh a measured bucketful and back-calculate.

Calculator

Aquarium Gravel Calculator

The "1 pound of gravel per gallon" rule is a starting point, not an answer. It overshoots tall narrow tanks and undershoots short wide ones because gravel volume depends on footprint × depth, not gallons. This calculator uses published lb/ft³ from CaribSea and similar manufacturers and gives you the exact bag count for a 1-inch, 1.5-inch, or 2-inch bed.

Substrate

Tank length (in)

Tank width (in)

Bed depth (in)

1–2 in is the common shallow-bed range. 3+ in is a deep sand bed.

Substrate product

Substrate needed

63.8lb

Metric

28.9kg

Volume

0.75cu ft

Bags (20 lb)

4bags

Bag density: 85 lb/ft³

Density source: caribsea.com

How this is calculated

cubic_inches = length_in × width_in × depth_in
cubic_feet   = cubic_inches / 1728
pounds       = cubic_feet × lb_per_cuft   // by product

Bulk densities come from CaribSea's Marine Substrates and Freshwater Substrates spec pages. Crushed coral runs lighter (~72 lb/ft³) because the chunky grains trap air gaps; fine oolitic sand packs to ~96 lb/ft³. Most aquarium gravels — Tahitian Moon Sand, Estes Marine Sand, mid-grade reef substrates — sit in the 80–90 lb/ft³ band.

Bed depth advice: under 0.5 in is cosmetic and won't anchor plants or coral bases. 1–2 in is the standard fish tank bed. 3+ in starts behaving like a deep sand bed with anoxic zones — good for established planted tanks and DSB-style reefs, problematic if you skip the rest of the setup.

FAQ

How many pounds of gravel do I need for a 55-gallon tank?: A standard 55-gallon (48 × 13 × 21 in) needs about 38–45 pounds of gravel for a 1-inch bed, or 55–65 pounds for a 1.5-inch bed, depending on grain density. The calculator gives you the exact number for the product you're buying.
How much gravel for a 20-gallon long?: A 20-long (30 × 12 × 12 in) needs about 17–21 pounds for a 1-inch bed. Wider footprint than a standard 20H — same gallons, more substrate.
Gravel vs sand for freshwater?: Gravel (2–5 mm grain) is the safer beginner choice — it doesn't get sucked into siphons, plants root in it easily, and corydoras can sift it without abrasion. Sand looks more natural and is required for certain species (loaches, geophagus), but it compacts and traps gas pockets if you don't stir it.
Can I use this for crushed coral?: Yes — pick "Florida Crushed Coral" or "Caribbean Crushed Coral" in the product dropdown. Note that crushed coral buffers freshwater (Cichlid-tank trick) and is too gunk-trapping for most modern reef tanks.
Why isn't pea gravel in the list?: Big-box-store pea gravel doesn't come with a published bulk density I trust. Estimates online range 90–110 lb/ft³, which is too wide to put a number on. If you must, weigh a measured bucketful and back-calculate.

Share / embed this calculator

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<iframe
  src="https://reefcalcs.com/embed/aquarium-gravel-calculator/"
  width="100%"
  height="720"
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  loading="lazy"
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></iframe>

Sources & references

01
CaribSea — Marine Substrates (lb/ft³ tables)
https://caribsea.com/marine-substrates/
02
CaribSea — Freshwater Substrates
https://caribsea.com/freshwater-substrates/
03
Carib Sea Eco-Complete planted substrate
https://caribsea.com/eco-complete-planted/

Last reviewed 2026-05-25