Sand or crushed coral?

Sand for SPS / mixed reef tanks — finer grain looks better, doesn't trap as much detritus, and lets fish like wrasses sift naturally. Crushed coral was popular in the 90s for buffering but it traps gunk and is a pain to clean. Most modern reef tanks run 0.5–2 mm sand.

How deep should the bed be?

1–2 inches for a shallow sand bed (SSB) — easy to clean, no anoxic zones. 4+ inches for a deep sand bed (DSB) if you want passive denitrification, but commit fully or skip it. The unstable middle (2.5–3.5 in) gets you the worst of both.

Why is pea gravel not in the list?

Pea gravel's bulk density isn't published by any manufacturer that I trust enough to put a number on. Estimates online range 90–110 lb/ft³, which is too wide a band to be useful. Use a hardware-store bag's stated weight and back-calculate from its volume if you must.

Calculator

Substrate Calculator

The old "1 lb per gallon" rule is wrong for sand. It works out to a bed too deep on tall narrow tanks and too shallow on short wide ones. What actually matters is footprint and depth. This calculator uses CaribSea's published bulk densities (80–96 lb/ft³ depending on grain size and material) and outputs pounds, kilograms, cubic feet, and bag count for 20-lb and 30-lb bag sizes.

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

Densities are taken from CaribSea's Marine Substrates page. Oolitic and finer sugar-sized substrates (96 lb/ft³) pack denser than mid-grade Special Grade Reef (85 lb/ft³); chunkier crushed corals are lighter (~72 lb/ft³) because the grain gaps trap more air. Bed depth matters: under 0.5 in is cosmetic only; 1–2 in is the standard shallow reef bed; 3+ in starts behaving like a deep sand bed (DSB) with anoxic zones.

FAQ

Sand or crushed coral?: Sand for SPS / mixed reef tanks — finer grain looks better, doesn't trap as much detritus, and lets fish like wrasses sift naturally. Crushed coral was popular in the 90s for buffering but it traps gunk and is a pain to clean. Most modern reef tanks run 0.5–2 mm sand.
How deep should the bed be?: 1–2 inches for a shallow sand bed (SSB) — easy to clean, no anoxic zones. 4+ inches for a deep sand bed (DSB) if you want passive denitrification, but commit fully or skip it. The unstable middle (2.5–3.5 in) gets you the worst of both.
Why is pea gravel not in the list?: Pea gravel's bulk density isn't published by any manufacturer that I trust enough to put a number on. Estimates online range 90–110 lb/ft³, which is too wide a band to be useful. Use a hardware-store bag's stated weight and back-calculate from its volume if you must.

Share / embed this calculator

Paste this snippet into any blog or forum post that allows raw HTML to embed the calculator.

<iframe
  src="https://reefcalcs.com/embed/substrate/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs Substrate Calculator"
></iframe>

Sources & references

01
CaribSea — Marine Substrates (lb/ft³ tables)
https://caribsea.com/marine-substrates/

Last reviewed 2026-05-25