Calculator
Tank Volume Calculator
A "75 gallon" reef doesn't hold 75 gallons — after the sand bed, the rock, and the two inches of air at the top, it's usually closer to 60. That gap is why doses calculated on the sticker number overshoot. Here's your true water volume, plus the sand your bed depth actually needs.
Paid links: if you buy through links on this page we may earn a commission, at no extra cost to you — it never affects our ratings. How we make money.
Why true volume matters
Every dose you calculate — alkalinity, calcium, medication — scales with water volume. Use the sticker number and you're dosing water that isn't there. The same goes for stocking decisions and salt for water changes. Get the real number once, write it on the stand, and use it everywhere: the dosing calculator and the water change calculator both want this figure, not the sticker.
The assumptions (flagged, as always)
Gallons are computed at 231 cubic inches each. Sand: dry aragonite runs about 8 lb per square foot per inch of depth (bulk density ~96 lb/ft³, per CaribSea's spec sheets) — that's also the volume we subtract for the bed. Rock: dry reef rock is porous, so displacement varies; we estimate 1 lb displaces ~0.075 gallons and flag it as an estimate. Dimensions are external and glass thickness is ignored, which over-counts a standard tank by roughly 2–4% — if you want to be precise, measure inside the glass.
How deep should the sand bed be?
For most reefs: 1–2 inches. It's enough to look right, host sand-sifting critters, and stay easy to keep clean. Deep sand beds (4″+) are a deliberate filtration strategy with real maintenance implications — don't end up there by accident because a calculator told you to buy more sand. Bare-bottom is equally valid for SPS-heavy tanks chasing flow.
Planning a full build?
Volume is step one. The build cost calculator itemizes the whole system for your size and tier, and the setup guides walk specific tanks end to end.
Free — one email
The Reef Gear Cheat Sheet.
The exact skimmer / pump / light picks by tank size, plus our full build-cost worksheet — as one email, no spam.
One email with the sheet. If we ever start a newsletter, you can opt out first. Privacy