Setup & Sizing
Live Rock Calculator
Estimate how much live rock (in kilograms) to buy for a reef or FOWLR marine tank, using the standard 1 kg-per-7-litres rule and two higher/lower stocking options.
Live Rock Calculator
Traditional Berlin-style filtration. 1 kg per 7 L.
Recommended weight
21.4kg
- Rule used
- 1 kg per 7 L
- Lower end (-15%)
- 18.2 kg
- Upper end (+15%)
- 24.6 kg
How much live rock for a reef tank?
The standard reef-keeping figure is 1 kg of live rock per 7 litres of water (equivalent to 1 lb per 2 US gallons). It comes from the Berlin method of the 1980s and is still the default for filtration-through-rock in most UK reef tanks.
Formula: kg = (volume / 7) x density, where density is 0.5 for an SPS aquascape, 1.0 for classic reef, or 1.5 for a fish-heavy FOWLR tank.
Modern low-rock reefs
High-end protein skimmers, refugiums and regular nitrate export mean you can run a successful reef on far less rock than 1 kg / 7 L - some premium SPS tanks use a few hundred grams for aesthetics only. Choose "Minimalist" above for that style; pick "Classic" if you're relying on rock as the primary filter.
Buying tips
Buy 10-15% over your target. Some rock will be too big or the wrong shape for your scape, and you'll want a small reserve to add fresh live rock later for pod biodiversity. Always quarantine any new cured rock for 2-4 weeks to avoid importing pests into an established reef.
Frequently asked questions
How much live rock do I need per litre?
The traditional UK rule is about 1 kg of live rock per 7 litres (or 1 lb per 2 US gallons), which gives a visually full tank and plenty of biological filtration. Modern minimalist reefs run much lighter - sometimes as little as 0.3-0.5 kg per 7 L - compensating with refugiums, skimmers and dry rock.
Should I buy live, dry or cured rock?
Dry rock is cheaper, pest-free and easy to aquascape, but takes months to become biologically active. Cured live rock is ready to filter the tank on day one but costs more and can import hitchhikers - both wanted (pods, sponges) and unwanted (aiptasia, crabs). Most UK reefers mix: 70-80% dry rock as the structure, 20-30% live for seeding.
Do I weigh live rock wet or dry?
Suppliers quote shipping weight, which is wet. A kilogram of fresh live rock loses 10-15% of its weight when dry, so if you're building with dry rock you can use a slightly lower figure or just buy a bit over the estimate.
How do I arrange live rock in a reef tank?
Leave a 2-3 cm gap between the rock and the glass on all sides so you can clean algae off and water can flow freely. Build with open arches and caves rather than a solid wall - dead zones behind piled-up rock trap detritus and fuel nuisance algae. Use reef-safe epoxy or cable ties rather than stacking loose.
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