Reef tools
Free reef tank calculators
Three reliable, in-browser tools for the math you actually do at the sump — water changes, true tank volume, and ATO reservoir sizing. No signup, no ads, no data sent anywhere.
For two-part dosing math, use BRS's official calculator — they publish the verified per-product dose constants, and we don't want to play telephone with chemistry numbers. For the theory behind dosing, see our water chemistry basics and dosing fundamentals guides.
Saltwater Water Change Calculator
Gallons of fresh saltwater needed for a percentage water change on any tank volume.
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Aquarium Tank Volume Calculator
Net water volume from internal dimensions minus rock, sand, and sump displacement.
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ATO Reservoir Sizing & Evaporation Calculator
Minimum ATO reservoir capacity for your measured evaporation rate and target unattended runtime.
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Looking for a dosing calculator?
For two-part alkalinity and calcium dosing, kalkwasser saturation, and salt mix recipes, use BRS's official calculators at bulkreefsupply.com/reef-calculator. They publish verified per-product constants directly from the manufacturers — the authoritative source for those numbers. We point you there rather than rebuild a less-accurate copy.