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

A standard 55-gallon (48 × 13 × 21 in) has a 624 in² footprint: about 26–35 pounds for a 1-inch bed, or 39–52 pounds for a 1.5-inch bed, across the 72–96 lb/ft³ density range. The calculator gives you the number for the closest listed product.

How much gravel for a 20-gallon long?

A 20-long (30 × 12 × 12 in) needs about 15–20 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 we 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.

Aquarium Gravel Calculator

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

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

Gear for thisAffiliate links

CaribSea Arag-Alive Special Grade reef sand

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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 spec page. Crushed coral runs lighter (~72 lb/ft³) because the chunky grains trap air gaps; fine oolitic sand packs to ~96 lb/ft³; mid-grade reef sands sit at 80–90 lb/ft³. If your gravel isn't one of these products, treat the result as an estimate.

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) has a 624 in² footprint: about 26–35 pounds for a 1-inch bed, or 39–52 pounds for a 1.5-inch bed, across the 72–96 lb/ft³ density range. The calculator gives you the number for the closest listed product.
How much gravel for a 20-gallon long?: A 20-long (30 × 12 × 12 in) needs about 15–20 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 we 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.

Where to buy

Affiliate links

Most reef tanks use aragonite sand rather than gravel. These reef-grade options ship pre-rinsed for marine use.

CaribSea Arag-Alive Special Grade reef sandLive aragonite, no rinse, 1–2 mm grain. Reef default.View on Amazon →Nature's Ocean Bio-Activ Live Aragonite sandCheaper alternative to CaribSea.View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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<iframe
  src="https://reefcalcs.com/embed/aquarium-gravel-calculator/"
  width="100%"
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Sources & references

1.
CaribSea — Marine Substrates (lb/ft³ tables)
https://caribsea.com/marine-substrates/

Last reviewed 2026-09-30