Aquarium Calculator
Calculate your fish tank's volume in gallons and litres from its dimensions — plus the numbers that follow: water weight, heater wattage, stocking guideline and water-change buckets.
You might also need:Chicken Coop Size Calculator·Cat Age Calculator
Tank volume math
litres = gallons × 3.785 · water weight = gallons × 8.34 lb
A 36×18×20 inch tank is 56 gallons brim-full, ~50 gallons in practice. Knowing the true volume matters beyond bragging rights: medicine and water-conditioner doses, heater sizing and stocking plans all key off it.
Standard aquarium sizes
| Size | Dimensions (L×W×H) | Filled weight |
|---|---|---|
| 10 gal | 20×10×12″ | ~110 lb |
| 20 gal (long) | 30×12×12″ | ~225 lb |
| 29 gal | 30×12×18″ | ~330 lb |
| 40 gal (breeder) | 36×18×16″ | ~460 lb |
| 55 gal | 48×13×21″ | ~625 lb |
| 75 gal | 48×18×21″ | ~850 lb |
| 125 gal | 72×18×21″ | ~1,400 lb |
Setting up for success
- Cycle before fish: 4–6 weeks of fishless cycling establishes the bacteria that make a tank safe. Patience here prevents most beginner disasters.
- Bigger is easier: more water = slower chemistry swings. A 29-gallon is a kinder first tank than a 10.
- Stock for adult sizes: that cute 1-inch pleco wants to be 18 inches. Research before buying, and stock over weeks, not all at once.
- Backyard flock too? Size their home with the chicken coop calculator.
Frequently asked questions
How do I calculate how many gallons my fish tank is?+
Multiply length × width × height in inches and divide by 231 (cubic inches per US gallon). A standard 36×18×20 inch tank works out to 56 gallons — though the water volume is a bit less once you subtract substrate, rocks and the fill line.
How many fish can I keep per gallon?+
The old "one inch of fish per gallon" rule is a rough start for small, slim community fish — but it fails for goldfish, cichlids and anything deep-bodied. Better: research each species’ adult size and bioload, and stock lightly at first. Your filter and water-change habits matter as much as the gallons.
What size heater does my aquarium need?+
Plan 3–5 watts per gallon: a 30-gallon tank wants a 100–150 W heater. For big tanks, two smaller heaters beat one large — more even heat, and a failure won’t cook or chill the whole tank.
How much water should I change and how often?+
The standard is 20–25% weekly (or 30–50% fortnightly for lightly stocked tanks). This calculator shows the bucket count for a 25% change — the reason big tanks are, counterintuitively, easier: their chemistry drifts slower between changes.
How heavy is a full aquarium?+
Water weighs 8.34 lb per gallon, so a 55-gallon tank holds ~460 lb of water — plus glass, stand and substrate you’re near 600 lb. Check floors for big tanks, always use a purpose-built stand, and never move a tank with water in it.