Stocking a reef tank starts with the rock, and the classic rule of thumb scales the amount to tank volume. This calculator applies that guideline, adjusts it for how dense your chosen rock is, and estimates the cost from your own price.
How it works
The baseline is the familiar pounds-per-gallon range, scaled by a density factor for the rock type:
gallons = volume (converted from litres if needed)
base low = gallons × 1.0 lb
base high = gallons × 1.5 lb
adjusted = base × density factor
porous ≈ 1.0, standard ≈ 0.85, dense ceramic ≈ 0.65
cost = recommended weight × your price per lbDenser rock builds the same aquascape with fewer pounds, so the recommended weight drops as density rises. The recommended figure is the midpoint of the adjusted range.
Why rock type changes the numbers
Not all reef rock is equal in weight or porosity. Lightweight porous live rock — the bleached branching Caribbean or Fiji shapes beloved by reefers in the 1990s — is full of tiny caves and channels that host enormous bacterial colonies. You need more pounds of it to fill your aquascape because each pound is mostly air. Dense ceramic or dry rock like ReefRocks or CaribSea LifeRock packs significantly more mass into the same silhouette, so the pounds you need drop.
| Rock type | Density factor | Notes |
|---|---|---|
| Porous premium live rock | ~1.0 | Most biological surface area per pound |
| Standard live or dry rock | ~0.85 | Common mix of shapes and porosity |
| Dense ceramic / dry | ~0.65 | Heavier per piece; aquascape with fewer pounds |
Worked example
For a 75-gallon display tank using standard live rock at roughly $5 per pound:
- Base range: 75 to 112.5 lb (1.0 to 1.5 lb/gal)
- Density adjustment ×0.85: 64 to 96 lb adjusted
- Recommended midpoint: approximately 80 lb
- Estimated cost: 80 × $5 = $400
Swap to dense ceramic rock and the same tank might need only 50–60 lb — a real saving on weight and cost.
Practical guidance
Current reefkeeping trend toward less rock. Many modern reefers run 0.5 to 0.75 lb/gallon deliberately, relying on a deep sand bed in the sump, a refugium, and biological media like MarinePure or Seachem Matrix for the bacterial load. Less rock means better flow paths and an open aquascape with more swimming room for fish.
Curing matters. Fully cured live rock (shipped from a store) arrives biologically active but costs more and carries the risk of hitchhikers or unwanted algae. Dry rock is cheaper and pest-free but needs six to twelve weeks of curing in saltwater with an ammonia source before it is ready. Factor that maturation time into your build plan.
Aquascape before you buy. Sketch or even sketch-build your aquascape in the empty tank using cheap cardboard boxes to represent rock mass. This tells you how many pounds you actually need for your intended structure rather than buying the textbook amount and finding you have a pile, not an aquascape.
Plumbing and flow paths first. Place drain and return bulkheads before committing to rock placement so you do not block access or create dead spots behind rock pillars.
The cost estimate is a budget guide — verify the price per pound for the specific product you intend to purchase, since cured live rock and dry rock differ substantially.