20 Gallon Long Tank Volume Calculator

20 gallon long - popular for breeding and community tanks - Calculate exact water volume.

What's your tank shape?

Enter Your Dimensions

in
in
in

Results

US Gallons
40.4 US Gallons
(153 Liters)
Gross tank volume: 44.9 gal (170L)
Dose medications based on the Net Volume (40.4 gal).
Total System Weight: 544 lbs (247 kg)
Includes water, glass, substrate, and rocks

Verify results before use. See our disclaimer.

Recommended Equipment

Heater Needed:
~145 Watts
Filter Flow:
202–303 GPH
Based on 5–7.5× tank turnover per hour

Popular Tank Sizes & Shapes

10 Gallon40 Gallon Breeder50 Gallon1 Foot1.5 Feet2 Feet30cm Cube32cm CubeBow FrontCylinder2ft by 1ft

Quick links to pre-filled calculators for popular tank sizes and shapes.

What's Next?

Complete your aquarium setup with these helpful calculators:

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Weight Calculator →

Ensure your floor can handle the total system weight

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Stocking Calculator →

Find out how many fish can safely live in your tank

20 Gallon Long Tank Volume Calculator

Calculate the exact water volume for this aquarium size. This 20.0 gallon (76L) calculator helps you determine precise water capacity for safe medication dosing and equipment sizing.

Knowing your exact volume is crucial for accurate water treatments, filter sizing, and heater selection. We account for fill height and provide equipment recommendations based on your actual water volume.

How to Use This Calculator

  • Tank dimensions are pre-filled based on standard specifications for this size
  • Verify these match your actual aquarium (always measure from inside the glass)
  • Adjust the fill height if needed - most tanks aren't completely full
  • Switch between imperial and metric units to match your preference
  • Get instant results: exact volume, weight, and equipment recommendations

How to Measure Your Aquarium

How to measure aquarium dimensions - diagram showing length (X axis left to right), width (Y axis front to back), and height (Z axis bottom to top) with measuring tape and arrows indicating each dimension on a planted aquarium tank

What You Need

  • Measuring tape or ruler
  • Paper and pen to write down measurements

Why Accurate Measurements Matter

Knowing your exact tank dimensions is essential for calculating water volume, which affects:

  • Medication and treatment dosing
  • Fertilizer amounts for planted tanks
  • Substrate and gravel quantities
  • Fish stocking capacity and bioload
  • Filter and heater sizing

Common Mistakes to Avoid

Measuring the outside: Always measure inside the glass where water sits, not the outer frame.

Including decorations: Remove large rocks or driftwood that displace significant water volume.

Mixing units: Do not switch between inches and centimeters mid measurement.

Measuring to the top: Tanks are never filled to the brim. Measure actual water height, not glass height.

Different Tank Types

Bowfront tanks: Measure the straight back panel. The calculator accounts for the curved front.

Cylinder tanks: Measure the diameter (straight line across the widest part) and height.

Corner tanks: Measure the longest straight edges on each side.

Measuring Steps

1. Length

Measure from left to right along the front of the tank. This is the longest side when looking at the tank from the front.

2. Width

Measure from front to back. This is the depth of the tank, how far it extends away from you.

3. Height

Measure from bottom to top on the inside of the tank. Do not include the frame or lid.

Quick Reference

Standard notation:

Length × Width × Height

Example: 36 × 18 × 16 inches

Tools That Work Best

  • Metal measuring tape (most accurate)
  • Cloth tape for curved tanks
  • Ruler for small tanks under 10 gallons
  • Smartphone apps with AR measurement

Important Tips

  • Measure inside the glass: Do not include the frame thickness. Measure the actual water holding area.
  • Measure in inches or centimeters: Most tape measures show both. Pick one unit and stick with it for all three measurements.
  • Round to the nearest whole number: You do not need perfect precision. Rounding to the nearest inch or centimeter is fine.
  • Write it down: Length × Width × Height. For example: 36 × 18 × 16 inches.
Pro tip: If your tank has a center brace or rim, measure from the inside edges where the water actually sits, not the outer frame.

The Critical Importance of Accurate Aquarium Volume: A Deep Dive

In the aquarium hobby, your water volume is the canvas upon which everything else is built. However, there is a significant difference between the "advertised volume" of a tank and its "actual water capacity." For any serious aquarist, understanding this distinction is vital for the health and safety of their aquatic livestock. Precision in volume calculation is the foundation of successful long-term husbandry, affecting everything from chemistry to physics within the glass walls.

The Net Volume Reality: Beyond the Label

A standard "55-gallon" aquarium rarely holds 55 gallons of water. Once you account for the thickness of the glass (which can be 1/2 inch or more in larger tanks), the necessary space at the top for surface agitation (fill height), and the massive displacement caused by substrate, rocks, and hardscape, the actual water volume may be closer to 42-45 gallons. This 20% discrepancy is not just a trivia point—it is a critical variable in aquarium management. In smaller "nano" tanks, this displacement is even more pronounced, where a single large rock can displace 15% of the total water volume.

Precision Dosing: The Margin Between Cure and Catastrophe

The most dangerous application of inaccurate volume measurements is in medication dosing. Many common treatments, such as copper-based medications for parasites or specialized antibiotics for bacterial infections, have a narrow therapeutic window. Underdosing allows pathogens to survive and build resistance, making future treatments harder. Conversely, overdosing can be rapidly fatal to the fish you are trying to save. By using a precise volume calculator that accounts for your specific dimensions and actual fill height, you eliminate the guesswork and ensure that every drop of treatment is scaled correctly to the actual water column.

Thermal Mass and Equipment Efficiency

The larger the volume of water, the more stable the environment. Water has high specific heat capacity, meaning it acts as a thermal and chemical buffer. When you calculate your true net volume, you gain a realistic understanding of your system's "thermal mass." This knowledge is essential when sizing heaters. A heater rated for 50 gallons might struggle if your system's true volume (including a sump) is actually 65 gallons, leading to dangerous temperature fluctuations. Similarly, filter turnover rates should be based on your net volume to ensure the water is being processed efficiently through your biological and mechanical media.

Total System Weight and Structural Safety

Finally, there is the matter of physical and structural safety. Water is remarkably heavy—weighing 8.34 pounds per US gallon (1kg per liter). When you factor in the weight of the glass itself, several inches of dense substrate like sand, and large decorative rocks, a medium-sized setup can easily exceed the weight of a professional refrigerator. Accurate volume calculation allows you to estimate the total static load of your setup. This is crucial for ensuring that your aquarium stand, floor joists, and subflooring are rated to handle the load safely over decades, preventing structural damage or catastrophic tank failure.

Calculated Water Changes

Even routine maintenance benefits from knowing your exact volume. A "20% water change" on a 100-gallon tank is only 20 gallons if the tank is empty. If your displacement has reduced your water volume to 80 gallons, your 20-gallon change is actually a 25% change. While usually not harmful, in sensitive reef systems or shrimp tanks, these percentage differences can lead to osmotic shock if parameters aren't perfectly matched. Knowing your exact volume allows for perfect consistency in your maintenance routine.