Measure the inside of the glass, not the outside
The single most common mistake is measuring a tank from the outside and using the height to the rim. Glass thickness and trim add up: a standard 10 gallon tank is typically built from roughly 3.2 mm glass, and once you also leave room to keep the water level below the lid and away from fish at the surface, the usable volume is consistently 10–15% below the advertised figure. The volume calculator below assumes interior, brim-realistic measurements for exactly this reason.
The formulas the calculator uses
The maths is simple geometry plus a unit constant:
- US gallons from inches: (length × width × height in inches) ÷ 231.
- Litres from centimetres: (length × width × height in cm) ÷ 1,000.
- 1 US gallon = 3.785 litres. 1 UK/imperial gallon = 4.546 litres (about 1.2 US gallons).
Odd shapes just change the area term. Cylinders use π × radius² × height, bowfronts average the front and back depth, and a regular hexagon uses the flat-to-flat width. The tank volume calculator handles rectangular, cube, cylinder, bowfront, and hexagon shapes and reports US gallons, litres, and UK gallons at once.
Standard tank sizes and their real water volume
Tanks are sold by rounded nominal capacity, and the physical dimensions behind each label vary slightly by manufacturer. The columns that matter most are the real water volume in gallons and litres — the water you can actually dose towards:
| Nominal size | Typical dimensions (L×W×H in) | Advertised (gal) | Real water, typical setup (gal) | Real water (L) | Filled weight, est. |
|---|---|---|---|---|---|
| 2.5 gal / Nano | 12 × 8 × 10 | 2.5 | ≈ 2–2.5 | ≈ 7–9 | ≈ 25–35 lb |
| 5.5 gal | 16¼ × 8¼ × 10¾ | 5.5 | ≈ 4–5 | ≈ 15–19 | ≈ 50–65 lb |
| 10 gal (Leader) | 20¼ × 10½ × 12¾ | 10 | ≈ 8–9.5 | ≈ 30–36 | ≈ 110–130 lb |
| 15 gal | 24 × 12 × 12 | 15 | ≈ 12–14 | ≈ 45–53 | ≈ 170–200 lb |
| 20 gal High | 24¼ × 12½ × 16¾ | 20 | ≈ 16–18 | ≈ 60–68 | ≈ 230–260 lb |
| 20 gal Long | 30¼ × 12½ × 12¾ | 20 | ≈ 16–18 | ≈ 60–68 | ≈ 230–260 lb |
| 29 gal | 30¼ × 12½ × 18¾ | 29 | ≈ 23–26 | ≈ 87–98 | ≈ 330–370 lb |
| 30 gal Breeder | 36¼ × 18¼ × 12¾ | 30 | ≈ 26–28 | ≈ 98–106 | ≈ 345–385 lb |
| 40 gal Breeder | 36¼ × 18¼ × 16¾ | 40 | ≈ 33–36 | ≈ 125–136 | ≈ 460–500 lb |
| 40 gal Long | 48¼ × 12¾ × 16¾ | 40 | ≈ 33–36 | ≈ 125–136 | ≈ 460–500 lb |
| 55 gal | 48¼ × 12¾ × 21 | 55 | ≈ 46–50 | ≈ 174–189 | ≈ 625–675 lb |
| 75 gal | 48¼ × 18¼ × 21 | 75 | ≈ 62–68 | ≈ 235–257 | ≈ 850–950 lb |
| 90 gal | 48¼ × 18¼ × 24 | 90 | ≈ 75–82 | ≈ 284–310 | ≈ 1,000–1,150 lb |
| 125 gal | 72¼ × 18¼ × 21 | 125 | ≈ 104–112 | ≈ 394–424 | ≈ 1,400–1,550 lb |
Filled weights assume a standard 2-inch substrate bed, average glass thickness, and basic equipment. Reef systems with deep sand beds and live rock can run 20-30% heavier. For a precise figure, enter your exact interior dimensions into the tank weight calculator.
Why the number on the box is misleading
Advertised capacity is a brim-full estimate of the empty glass shell. In a real setup four things quietly subtract water:
- Freeboard — water sits below the rim so the tank can breathe.
- Substrate — a 2-inch bed in a 29 gallon tank can displace three or four gallons alone.
- Hardscape — rocks and driftwood push out more water than most people expect.
- Equipment — heaters, filters, and reactors all displace their own volume.
The practical rule of thumb: dose and stock against net volume. A 50-gallon-label tank that really holds 42 gallons means every medication dose computed on the label is roughly 20% too strong — a real problem with copper-based treatments and concentrated fertilisers.
Step by step: measure your own tank
- Measure inside the glass — length, width, and the height water will actually reach.
- Decide on units — inches/US gallons or centimetres/litres; the volume calculator accepts both.
- Pick the shape — rectangular, cube, cylinder, bowfront, or hexagon.
- Subtract ~10–15% for substrate, hardscape, and equipment to get net water volume.
- Use net volume everywhere — stocking (stocking calculator), weight (weight calculator), CO₂ (CO₂ calculator), and heater sizing (heater wattage calculator).
Open the Tank Volume Calculator
Enter your tank's interior dimensions and get gross volume, net water, weight, and recommended filter flow instantly — for five tank shapes in imperial or metric.
When volume really matters
Volume is the input for almost everything after setup day:
- Stocking — bioload rules are calibrated per gallon of actual water, so an inflated volume silently permits an overstocked tank.
- Weight — a filled tank's floor load is water volume times 8.34 pounds per gallon plus hardware, which is why the weight guide starts from real capacity.
- Heater and filter sizing — heater wattage scales 3–5 watts per gallon and filter flow 4–6 tank volumes per hour, both on net volume.
- Medication and fertiliser dosing — treat the water that's actually there, not the label.
Related reading: how many fish can I keep · how much does an aquarium weigh · how much substrate do I need · CO₂ injection basics.
Tank volume questions, answered.
Common questions about measuring capacity, converting units, and reconciling advertised size with real water volume.