Enter your tank's three dimensions, choose your unit, and get the exact capacity instantly — litres, gallons, cubic metres, and more. Works for square tanks too — just enter the same value for length and width.
Pick a unit for each dimension — they can be different
Enter your 3 measurements
Length
Width
Height
Fill Level
100%
25%50%75%Full
⚠ Please enter a valid number (greater than zero) for all three dimensions.
WaterTankCalculator.com — Rectangular Tank Results
Your Results
Total Volume
—
Litres (L)
Cubic Metres
— m³
US Gallons
— gal
UK/Imperial gallons
— gal
Cubic Feet
— ft³
Cubic Inches
— in³
Water Weight
— kg
Weight (lbs)
— lbs
Fill Level
— %
Visual Fill Level
📖 How To Use
How to Use This Calculator
Getting your rectangular tank's exact capacity takes under 30 seconds:
Enter your 3 measurementsMeasure Length, Width, and Height from the inside of the tank. Internal dimensions give the actual water storage volume — external dimensions include the walls and overstate it, often by 5% or more on small tanks.
Choose your unit per fieldUse whatever units are on your tape measure — ft, in, cm, m, or mm. Each field has its own selector. You can even mix units (e.g., length in ft, height in cm).
Set the fill level (optional)Drag the slider if you want volume at a partial fill, e.g. 75% for current water level. Leave at 100% for full capacity.
Hit CalculateResults appear instantly — litres, gallons, cubic metres, weight in kg and lbs. Copy or print them with one click.
Tip: For tanks with internal fittings or baffles, subtract their volume from the result. A 50 mm pipe running 120 cm tall displaces roughly 2.4 litres — negligible for households, worth noting for industrial use.
📐 The Formula
Rectangular Tank Volume Formula
The math is simple — multiply three numbers:
Volume = Length × Width × Height
Litres (cm inputs): V (L) = (L × W × H) ÷ 1,000
Cubic metres (m inputs): V (m³) = L × W × H
Cubic feet (ft inputs): V (ft³) = L × W × H
This calculator converts all units to centimetres internally before calculating, so mixed-unit inputs always produce correct results.
Unit Conversion Reference
Unit
Equals Litres
From Litres
1 Litre (L)
1 L
× 1
1 Cubic Metre (m³)
1,000 L
÷ 1,000
1 US Gallon
3.785 L
÷ 3.785
1 UK Gallon
4.546 L
÷ 4.546
1 Cubic Foot (ft³)
28.317 L
÷ 28.317
1 Cubic Inch (in³)
0.01639 L
× 61.024
Water Weight
1 litre of fresh water weighs about 1 kg (2.2046 lbs). The exact figure is 0.998 kg at 20 °C, so 1 kg per litre is the standard rounding used for load estimates.
Common manufactured dimensions and their calculated volumes:
Capacity
L × W × H (cm)
L × W × H (ft)
Litres
US Gal
Full Weight
250 L
80 × 60 × 52
2.6 × 2 × 1.7
250
66
250 kg
500 L
100 × 70 × 72
3.3 × 2.3 × 2.4
504
133
504 kg
750 L
120 × 80 × 80
3.9 × 2.6 × 2.6
768
203
768 kg
1,000 L
120 × 100 × 85
3.9 × 3.3 × 2.8
1,020
269
1,020 kg
1,500 L
150 × 100 × 102
4.9 × 3.3 × 3.3
1,530
404
1,530 kg
2,000 L
160 × 120 × 106
5.2 × 3.9 × 3.5
2,035
538
2,035 kg
5,000 L
200 × 160 × 158
6.6 × 5.2 × 5.2
5,056
1,336
5,056 kg
10,000 L
250 × 200 × 200
8.2 × 6.6 × 6.6
10,000
2,642
10,000 kg
Note: Actual manufacturer dimensions vary. Always verify against the supplier's data sheet.
⚠️ Common Mistakes
Common Measurement Mistakes
1. External Instead of Internal Dimensions
A 10 mm wall on each side of a 100 cm cube means each internal dimension is only 98 cm. Across all three dimensions that loses about 6% of the volume (1,000 L becomes about 941 L). Always measure inside the tank.
2. Including Dead Space
Inlet/outlet zones, sediment sumps, and internal columns displace volume. Only measure space that can actually hold water.
3. Inconsistent Units Without Conversion
Measuring length in feet and height in inches then multiplying directly gives nonsense. This calculator handles mixed units correctly — just set the right unit per field.
4. Forgetting the Overflow Level
A tank can only fill to its overflow outlet, and the float valve usually shuts off a little below that. The top few centimetres are rarely usable, so measure height up to the overflow, or use the fill slider to plan for a realistic working level.
5. Trusting the Label Over Measurement
A "1,000-litre tank" may hold 980 or 1,040 litres depending on actual internal geometry. Measure and calculate — don't rely on the label when precision matters.
❓ FAQ
Frequently Asked Questions
How do I calculate the volume of a rectangular water tank?
Multiply Length × Width × Height. If measuring in centimetres, divide the result by 1,000 to get litres. This calculator handles all conversions automatically.
Should I use internal or external tank dimensions?
Always use internal dimensions. External dimensions include the wall thickness, which overstates actual water storage. With 10 mm walls, a 100 cm cube loses about 6% of its volume, and small thick-walled or concrete tanks can lose more than 10%.
What is the difference between US gallons and UK/Imperial gallons?
A US gallon equals 3.785 litres; a UK gallon equals 4.546 litres — about 20% larger. This calculator shows both simultaneously.
How heavy is a full rectangular water tank?
Water weighs 1 kg per litre. A 1,000 L tank carries 1,000 kg of water plus the tank shell (typically 30–60 kg for polyethylene). Always confirm your roof structure can handle the total weight before installation.
How many litres per day does a family of 4 need?
It depends on what the water is for. WHO puts emergency basic needs at 7.5–15 litres per person per day, and around 50 litres covers drinking, cooking, bathing and laundry. Most developed-world households use 100–200 L per person per day, so a family of 4 typically uses 400–800 litres per day.
What tank size do I need for a 72-hour emergency supply?
CDC and FEMA recommend at least 1 US gallon (3.8 L) per person per day. For 4 people over 3 days that's 3.8 × 4 × 3 = about 46 litres (12 gallons) as the bare minimum. CDC suggests a 2-week supply if possible, which is about 213 litres for 4 people. Use our Water Stockpile Duration Calculator to check your own numbers.
How much does a full rooftop tank weigh — and is my roof safe?
Water weighs 1 kg per litre, so a full 1,000-litre tank puts 1,000 kg on your roof plus 30–80 kg for the tank shell. A 1,000 L tank sitting on a 1.2 m × 1.0 m base concentrates roughly 870 kg/m², which is several times what typical residential roofs are designed to carry. Always use a load-distributing frame and have a structural engineer verify before installation. Use our Water Tank Weight Calculator for precise load figures.
Can I use this calculator for liquids other than water?
Yes — the volume results apply to any liquid. Only the weight estimate assumes fresh water (1 kg/L). For other liquids, multiply the litre result by that liquid's density: diesel ≈ 0.85 kg/L, milk ≈ 1.03 kg/L, cooking oil ≈ 0.92 kg/L, seawater ≈ 1.025 kg/L. This makes the calculator equally useful for fuel storage, food production, and industrial tank sizing.
✅ Sources & Review
Sources and review
The conversion factors and figures on this page come from these references:
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
