Coralkin honest saltwater & reef care

Free tool

Aquarium volume & displacement calculator

Your tank's rated size is not the amount of water in it. Rock and sand displace water — and every dose, salt mix and cleanup crew should be figured on your real water volume. Enter your dimensions and aquascape and we'll work out the gross, net and total-system volume.

1 · Tank dimensions

Measure the inside if you can; outside dimensions slightly overstate the volume (the glass has thickness, and you fill below the rim).

2 · Rock & sand (displacement)
3 · Sump (optional)

Only the actual water in the sump — not its empty size. Leave blank if you don't run one.

An honest tool, not a promise. The volume maths is exact geometry, but displacement is an estimate — the only exact way is to measure as you fill (count buckets or your top-off container). Use the total-system water figure when you dose, mix salt, size a cleanup crew or plan stock — dosing to the box's rated size is a classic way to overdose.

Displacement figures are reef-community standard estimates (rock/decor ~5–15% of volume; ~18.7 lb of dense rock ≈ 1 gal; fine sand ~60% of its bed): reef2reef & Ultimate Reef volume calculators; Reef Central / SaltwaterAquarium live-rock displacement discussions. A US gallon is exactly 231 in³; a litre is exactly 1000 cm³.

Quick answers

How much water does my tank actually hold?

Less than its rated size. Start from the exact geometry — a US gallon is exactly 231 cubic inches and a litre is exactly 1000 cubic centimetres — so a rectangular tank is length × width × height, and a cylinder is π × radius² × height. Then subtract what rock and sand displace to get your real water volume. This calculator does both from your dimensions and aquascape.

How do I calculate aquarium volume in gallons from dimensions?

Multiply length × width × height in inches to get cubic inches, then divide by 231 (a US gallon is exactly 231 in³). For example, a 48 × 13 × 21 inch tank is 13,104 in³ ÷ 231 ≈ 56.7 gallons of gross glass volume — before rock and sand. Measure the inside if you can; outside dimensions slightly overstate the volume because the glass has thickness and you fill below the rim.

How much water do rock and sand displace?

Rock and decor typically remove about 5–15% of the fill volume (up to ~20% for a heavy aquascape). By weight, about 18.7 lb of dense rock displaces roughly 1 gallon (porous rock displaces less), and fine sand displaces about 60% of the bed volume it fills. These are reef-community estimates — the only exact way is to measure as you fill, by counting buckets or your top-off container.

Which volume should I use for dosing and salt mixing?

Use your total system water — net display volume (gross minus rock and sand) plus the actual water in your sump. Dosing or mixing salt to the box's rated size is a classic way to overdose. Feed that figure into the dosing, salt-mix, cleanup-crew and stocking calculators.