Aquarium Size: Litres to Gallons Conversion Guide

If you're setting up an aquarium, you'll quickly discover that fish tank sizes are measured differently around the world. Some regions use litres (metric), while others use gallons (imperial). This can create confusion when researching fish requirements, following stocking guidelines, or purchasing equipment.

This comprehensive guide will help you navigate aquarium volume conversions with confidence.

Understanding Different Gallon Measurements

Before we dive into conversions, it's important to know there are actually two different gallon measurements:

US Gallons (US Liquid Gallon)

• Used in the United States
• 1 US gallon = 3.785 litres
• Most common in aquarium hobby globally
• Standard for American-made tanks and equipment

UK (Imperial) Gallons

• Used historically in the United Kingdom and Commonwealth countries
• 1 UK gallon = 4.546 litres
• About 20% larger than US gallons
• Less common in modern aquarium contexts

Important: Unless specified otherwise, "gallons" in the aquarium hobby typically refers to US gallons. This guide focuses on US gallons, which is the international standard for fishkeeping.

Quick Conversion Formulas

Litres to US Gallons

Formula: Litres ÷ 3.785 = US Gallons

Example: 200 litres ÷ 3.785 = 52.8 US gallons

US Gallons to Litres

Formula: US Gallons × 3.785 = Litres

Example: 55 gallons × 3.785 = 208.2 litres

UK Gallons to Litres

Formula: UK Gallons × 4.546 = Litres

Example: 50 UK gallons × 4.546 = 227.3 litres

Common Aquarium Size Conversions

Here's a quick reference chart for standard tank sizes:

Small Tanks (Nano/Desktop)

Medium Tanks (Beginner/Intermediate)

Large Tanks (Community/Display)

Extra Large Tanks (Advanced)

Why Accurate Tank Volume Matters

1. Medication Dosing

Many fish medications require precise dosing based on water volume. Incorrect calculations can:

• Underdose: Treatment fails, fish remain sick
• Overdose: Can harm or kill fish

Example: A medication calls for 1ml per 10 gallons. If you think your 200L tank is 200 gallons (it's actually ~53 gallons), you'd use 20ml instead of 5.3ml—nearly 4× the safe dose!

2. Water Conditioner Usage

Water conditioners remove chlorine and chloramine. Using too little leaves toxins, while using excessive amounts wastes money.

Cost Example: Premium water conditioner costs $15 for 500ml treating 2,000 gallons. Correct measurement saves $20+ annually per tank.

3. Fertilizer and Additives

Plant fertilizers, pH buffers, and other additives require exact volumes. Precision ensures:

• Optimal plant growth
• Stable water chemistry
• No nutrient toxicity

4. Stocking Calculations

Fish stocking guidelines depend on accurate tank volume. Misunderstanding your tank size can lead to:

• Overstocking (poor water quality, stressed fish)
• Understocking (missed opportunities for beautiful displays)

5. Filter Selection

Aquarium filters are rated by tank size. A filter rated for 50 gallons won't adequately filter a 200-litre (53-gallon) tank if you mistakenly think it's 50 gallons worth of water.

Calculating Your Actual Tank Volume

Tank manufacturers list the external dimensions, but your actual water volume is less due to:

• Substrate (gravel/sand): reduces volume by 5-10%
• Decorations and rocks: reduces volume by 5-15%
• Water level below rim: reduces volume by 5-10%
• Filter media displacement: minimal but present

Formula for Rectangular Tanks

In Centimeters: (Length × Width × Height) ÷ 1,000 = Litres

Example: 100cm × 40cm × 50cm = 200,000 ÷ 1,000 = 200 litres

In Inches: (Length × Width × Height) ÷ 231 = US Gallons

Example: 48″ × 18″ × 20″ = 17,280 ÷ 231 = 74.8 US gallons

Actual vs. Nominal Volume

A "55-gallon" tank actually holds closer to 48-50 gallons of water when accounting for:

• Substrate: -3 gallons
• Decorations: -2 gallons
• Below-rim fill: -2 gallons

Pro Tip: Measure water added during initial fill for the most accurate volume. Count gallons/litres as you fill!

Regional Differences in Fish Keeping

North America (USA, Canada)

• Primary unit: US Gallons
• Tank sizes: 10g, 20g, 29g, 40g, 55g, 75g, 125g
• Equipment rated in gallons/hour (GPH)

Europe, UK, Australia

• Primary unit: Litres
• Tank sizes: 50L, 100L, 200L, 300L, 500L
• Equipment rated in litres/hour (L/H)

Asia

• Mixed usage, increasingly metric (litres)
• Japanese brands often use litres
• Chinese manufacturers use both

Converting Fish Requirements

When researching fish care, you'll find requirements in both units. Here's how to convert common specifications:

Minimum Tank Size

Example: "Angelfish need minimum 30 gallons"

• 30 gallons × 3.785 = 113.6 litres

Example: "Discus need minimum 250 litres"

• 250 litres ÷ 3.785 = 66 gallons

Water Change Volumes

Example: "25% water change on 200L tank"

• 200L × 0.25 = 50 litres to change
• 50L ÷ 3.785 = 13.2 gallons

Filter Flow Rates

Example: "Filter rated 300 GPH (gallons per hour)"

• 300 GPH × 3.785 = 1,135 litres per hour

Example: "Filter rated 1,000 L/H"

• 1,000 L/H ÷ 3.785 = 264 gallons per hour

Tools to Make Conversion Easy

Use an Aquarium Calculator

Rather than manual conversions, use a dedicated aquarium calculator that:

• Switches between litres and gallons instantly
• Calculates stocking for either unit
• Adjusts fish requirements automatically

Try our free aquarium stocking calculator with built-in unit conversion. Toggle between litres and gallons with one click!

Smartphone Apps

Mobile apps like Fishi offer:

• Automatic unit conversion
• Tank volume calculators
• Water parameter tracking in your preferred units
• Medication dosing calculators

Conversion Charts

Print a conversion chart and keep it near your tank for quick reference during water changes and maintenance.

Common Conversion Mistakes to Avoid

1. Confusing US and UK Gallons

Mistake: Using UK gallon conversion for US-marketed products

Impact: 20% dosing error

Solution: Always verify which gallon type. US gallons are standard unless specifically marked "Imperial."

2. Using Nominal Tank Sizes

Mistake: A "75-gallon" tank holds closer to 70 gallons with substrate

Impact: Slight overstocking calculations

Solution: Use actual water volume, not manufacturer specifications

3. Rounding Too Aggressively

Mistake: "200 litres is about 50 gallons" (actually 52.8)

Impact: 5-6% error compounds with dosing

Solution: Use calculators or round to one decimal place

4. Forgetting Temperature Effects

Mistake: Water volume expands slightly when heated

Impact: Minimal for most purposes, but matters for precision dosing

Solution: Accept small variation, or measure at operating temperature

Fish Stocking by Volume

General Guidelines (Tropical Community Fish)

In Litres: 1cm of fish per 1-2 litres of water In Gallons: 1 inch of fish per 1-2 gallons of water

Example Stocking for 200L (53 gallon) Tank:

• 15 Neon Tetras (1.5cm each) = 22.5cm = 22.5L
• 8 Corydoras (5cm each) = 40cm = 40L
• 2 Honey Gouramis (5cm each) = 10cm = 10L
• Total: 72.5cm = 72.5L (~36% stocked)

This conservative approach ensures excellent water quality and happy fish.

Temperature Conversion Bonus

Since we're discussing conversions, here's the aquarium temperature conversion:

Celsius to Fahrenheit: (°C × 9/5) + 32 = °F

• 24°C = 75.2°F (tropical minimum)
• 26°C = 78.8°F (tropical ideal)
• 28°C = 82.4°F (tropical maximum)

Fahrenheit to Celsius: (°F - 32) × 5/9 = °C

• 76°F = 24.4°C
• 78°F = 25.6°C
• 80°F = 26.7°C

Practical Tips for International Fishkeepers

Buying Equipment

• US-made equipment: Specifications in gallons/GPH
• European equipment: Specifications in litres/L/H
• Asian equipment: Check packaging, often both units listed

Following Care Guides

• Convert immediately when planning
• Save conversions in your notes
• Use apps that remember your preferred units

Joining Online Communities

• Specify units when asking questions
• Most forums accept both but appreciate clarity
• Reddit r/Aquariums: Primarily US gallons
• UK Aquatic Plant Society: Primarily litres

Professional Use

Aquarium Shops

• Display pricing per gallon or litre
• Stock products labeled for regional markets
• Staff should be fluent in both conversions

Public Aquariums

• Often use metric (litres/cubic meters)
• May display both units for public
• Large volumes: cubic meters more practical

Conclusion: Master Your Measurements

Understanding aquarium volume conversions is essential for:

• ✅ Safe medication dosing
• ✅ Accurate stocking calculations
• ✅ Proper equipment selection
• ✅ Following international care guidelines
• ✅ Cost-effective additive usage

Key Takeaway: 1 US gallon = 3.785 litres. Use this conversion factor for all aquarium calculations, and when in doubt, use a calculator rather than estimating.

Ready to plan your aquarium? Use our free stocking calculator that handles conversions automatically. Switch between litres and gallons with one click, and calculate perfect stocking for 800+ fish species.

For ongoing tank management with built-in unit conversion, download the Fishi mobile app – trusted by over 100,000 aquarists worldwide!

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