FishTankMath
Calculator-driven research for freshwater aquarium beginners. Volume, heater, stocking, and substrate math + 8 species deep guides + sourced setup references.
Working volume
25.7gal
97.3 L · 28.1 gal gross · 234 lb water
Total tank height. Fill-below-rim and substrate depth come below.
Working volume adjustments
Inches of air between waterline and rim. Most tanks are filled 1–2 inches below the rim.
Sand or gravel bed at the bottom. Displaces water 1:1.
Hardscape (rocks, driftwood, decor)
None: bare tank · Light: a few small ornaments · Medium: rock pile or large wood · Heavy: aquascape with substantial stone/wood
Calculators
Volume, heater, filter, substrate. The math layer.
Care guides
Cycling, setup, water parameters. Sourced, beginner-safe.
Species
8 deep-content profiles: betta, neon tetra, angelfish, more.
Equipment
Heater + filter buying guides. No best-of listicles.
Fishing game
Catch species, populate your aquarium. No signup.
Inside dimensions × cross-section ÷ 231 cubic inches per US gallon (NIST exact). Bow-front adds a half-elliptical bulge. Cylindrical and hex use π·r² and (√3/2)·F² respectively.
gallons = (L · W · H) ÷ 231
Gross is geometric capacity. Working is what you actually treat with chemicals — gross minus the air gap, substrate, and decor displacement.
Use working volume for dosing + water changes. Use gross for floor-load.
The model assumes inside dimensions you input and uses percentage-based decor displacement. Real hardscape varies a lot. For exact volume on a heavily-aquascaped tank, drain and measure.
Saltwater is ~2.5% denser; weight readout is freshwater-tuned.
| 10 gal | 20×10×12 | ~9 |
| 20 long | 30×12×12 | ~17 |
| 29 gal | 30×12×18 | ~26 |
| 55 gal | 48×13×21 | ~50 |
| 75 gal | 48×18×21 | ~70 |
Aqueon / Marineland spec sheets
What numbers do I actually plug in? My tank doesn't list 'inside dimensions.'
Measure the inside of the glass with a ruler. Length, width, and water height — not total tank height. Most tanks are filled an inch or two below the rim, and the rim itself is excluded from inside dimensions. Outside dimensions overstate volume because they include glass thickness on every side. For a typical 24-inch-long tank that's about half a gallon you don't actually have.
Why does my '20-gallon tank' only hold 17 gallons of water?
Manufacturer labels are based on outside dimensions, before accounting for glass thickness, the gap between the water line and the rim, and substrate displacement. A standard 20-gallon tank usually holds 17–18 gallons of actual water once you add a one-inch fill margin and two inches of substrate. The calculator above gives you the real number — which is the one that matters for dosing chemicals and stocking advice.
Will my floor hold a full aquarium?
Water alone is 8.34 lb per US gallon, plus the glass, stand, and substrate. A full 55-gallon setup runs about 600 lb on a 4-square-foot footprint — roughly 150 lb per square foot of point load. Standard residential floors handle that fine on the ground floor or directly over a load-bearing wall upstairs. For tanks over 75 gallons on upper floors, check your floor's specific load rating or run it past a structural engineer. Don't trust internet rules of thumb for the bigger ones.
Do I subtract the substrate from the volume?
Yes, if you care about stocking density or dosing medication. Substrate displaces water roughly 1:1 by its submerged volume — a 2-inch sand bed in a 20-gallon tank takes about 1.5–2 gallons off the actual water column. For rough beginner calculations the difference is small enough to ignore. For dosing, it matters: a fish-medication dose computed against the wrong volume can either underdose (no effect) or overdose (toxic).
I'm completely new. Should I start with a 5g, 10g, or 20g?
20-gallon long is the consensus pick for absolute beginners. Bigger water volume is more forgiving when you make mistakes — a temperature swing or ammonia spike affects 20 gallons less than 5. The 'smaller is easier to manage' intuition is wrong here. The longer breakdown lives at /guides/easiest-tank-size-for-beginners.
By Jimmy L Wu. Calculator and sourced research, not a hobbyist blog. Educational, not veterinary advice. Updated April 2026. See the editorial policy and disclosures.