What is my aquarium's true water volume?
The water your fish swim in and your doses mix into — not the number on the box, and not even the box's inside dimensions to the rim. Four subtractions and one addition get you there. Take a "75 gallon" (48 × 18 × 21 in) set up the way people actually set them up:
| Step | Gallons | Liters |
|---|---|---|
| Inside the ⅜ in glass, water 1 in below the rim | 70.6 | 267 |
| − 2 in of gravel (60% of its bulk is solid) | −4.2 | −16 |
| − 30 lb of rock and wood | −3 | −11 |
| − heater, intake, a couple of ornaments | −0.5 | −1.9 |
| True water volume | 62.8 | 238 |
| + 20 gal sump running level, 3 gal in the overflow and pipes | 85.8 | 325 |
Computed by this page's engine. Rock at 10 lb per gallon displaced; your bucket test or the water-rise method beats that estimate.
So the freshwater version of this tank holds 63 gallons of water — 16% under its name — and a dose sized for "75 gallons" lands 19% strong. With a sump it's the other direction: the same reef holds 86 gallons, and a dose sized for the display alone is 27% light. Neither error is a disaster for a water conditioner. Both are for medication or a two-part schedule (table further down).
How do I measure the water my rock displaces?
Rock is the input people guess worst, because a bag of the stuff has no honest number on it. Three ways, best first:
1. Water rise. Fill the tank to a mark, add the rock, and measure how much the level rose. Displacement is footprint × rise — on a 48 × 18 in footprint every inch of rise is 3.5 gallons (13 L). This is exact, needs a ruler and nothing else, and works after the fact: mark the level, take the rock out for a rescape, see where it settles. The calculator's "water rise" mode does the multiplication. 2. Bucket test. Fill a bucket to the brim inside a larger tub, sink the rock, measure what overflowed (or drop rock into a marked bucket and read the rise there). Slower, but it works before the tank is wet. 3. By weight. Dry aquarium rock — reef rock, seiryu, dragon stone, river cobbles — runs about 1.2 kg per liter it displaces, the hobby's "a gallon per ten pounds." Dense granite is nearly twice that; very porous rock is under it. Fine for a first pass, and what the calculator uses when weight is all you have.
It's the range that falls out of one particular tank. Run the same honest setup — ¼–⅜ in glass, an inch below the rim, two inches of gravel, half a pound of rock per labeled gallon and half a gallon of gear — across the standard sizes and the shortfall runs from 13% to 43%. Shallow tanks lose more to the same bed; tall ones lose less. And a reef with a sump isn't under at all. Compute it, don't guess it — it takes the same thirty seconds either way.
| Tank | True volume | Under label | Dose-by-label error |
|---|---|---|---|
| 5.5 gallon | 3.2 gal | 43% | +75% |
| 10 gallon | 6.9 gal | 31% | +46% |
| 15 gallon | 10.2 gal | 32% | +47% |
| 20 gallon long | 12.9 gal | 36% | +55% |
| 20 gallon high | 14.6 gal | 27% | +37% |
| 29 gallon | 21.3 gal | 27% | +36% |
| 37 gallon | 25.9 gal | 30% | +43% |
| 40 gallon breeder | 34.6 gal | 13% | +16% |
| 55 gallon | 43.9 gal | 20% | +25% |
| 65 gallon | 53.6 gal | 17% | +21% |
| 75 gallon | 62.1 gal | 17% | +21% |
| 90 gallon | 71.9 gal | 20% | +25% |
| 120 gallon | 97.2 gal | 19% | +23% |
| 125 gallon | 93.3 gal | 25% | +34% |
| 150 gallon | 129.3 gal | 14% | +16% |
| 180 gallon | 154 gal | 14% | +17% |
| 210 gallon | 181.2 gal | 14% | +16% |
Computed by this page's engine under the assumptions above; no sump. Your tank's own numbers come from the calculator.
How much does the overdose matter?
It depends entirely on what you're dosing — specifically on how wide the gap is between the dose that works and the dose that hurts. The overdose factor is the same for everything you pour into that tank; the consequences aren't.
| What you're dosing | Its safe window | Dosing by the label |
|---|---|---|
| Water conditioner (Prime, etc.) | label dose is ~5× below any harm | Harmless — dose for the label if you like. |
| Fertilizer (EI dry salts) | targets are deliberately generous | Harmless; you were going to reset with a water change anyway. |
| Copper medication | therapeutic 0.15–0.20 ppm; inverts and scaleless fish suffer well under 0.3 | A 46% overdose on a nano turns 0.20 into 0.29 ppm — inside the danger zone. |
| Reef two-part (alkalinity) | daily swings under ~0.5 dKH keep corals happy | A dose meant for +0.5 dKH lands at +0.6 on the 75 above — every day, compounding until you re-test. |
| Salt for a water change | mixing to 1.025 in a bucket is by the gallon of that bucket | Unaffected — but "10 % water change" is 10 % of the true volume. |
Overdose factors from this page's engine (the 10 gallon and 75 gallon rows of the table above); safe windows are the usual hobby targets, not this site's engine.
A ten-gallon with two inches of gravel and a few pounds of rock holds about 6.9 gallons — a dose "for 10 gallons" is 46% strong. The same setup scaled to a 125 is only 34% over. Small tanks have proportionally more glass, more gravel and more rock per gallon of water, which is exactly why medicating a nano is nervier than medicating a big tank — and why the true volume matters most where people bother with it least.
Sump, overflow and plumbing: what to add back
For a system with a sump, the dose mixes into everything the return pump touches: the display, the water in the sump at its running level (not the sump's capacity — half of it is air and the rest is the drain-down margin), the overflow box, the plumbing, and any reactor or refugium on the loop. Reefers measure two-part consumption against this system volume; forget the sump and every dose you calculate is 15–25 % light, which shows up as alkalinity that never quite gets where you aimed. The calculator's "save as my dosing volume" stores the whole system so the dashboard and the reef tools start from the right number.
Which volume should I use for what?
True (system) volume for anything dissolved: conditioner, medication, fertilizer, alkalinity, calcium, magnesium, the salt for a batch of change water sized as a percentage. Rim volume for the tank's weight — see the weight calculator; the floor doesn't care that gravel isn't water. Footprint for stocking and substrate. And when a product's label says "per 10 gallons," it means gallons of water, which is the number this page just gave you.