Find the exact capacity of any tank — rectangular, cylindrical, spherical, horizontal, underground, and more. Get instant results in litres, gallons, cubic metres, and other units — free, no sign-up needed.
Volume and capacity calculators help you determine exactly how much water a tank can hold — expressed in litres, gallons, cubic metres, or other units. Whether you're sizing a new rooftop tank for a family home, planning a rainwater harvesting cistern, specifying a horizontal transport tanker, or simply converting between unit systems, this category has a dedicated tool for every tank shape.
Each calculator on this page handles a specific geometry using the correct mathematical formula for that shape — from the straightforward L × W × H of a rectangular tank, to the more complex combined volume of a cone-bottom or capsule tank. The tank-shape calculators accept feet, inches and metric units — even mixed in one calculation — and show the result in US gallons, litres, UK/Imperial gallons, cubic metres and cubic feet, with the water weight in pounds and kilograms. The rooftop sizing tool works from people and daily use, and the two converters from a volume. Every tool shows its working and its sources, and has been reviewed by John Mac, PE.
These tools are used by homeowners sizing household storage, plumbers specifying tank installations, civil engineers designing reservoirs, farmers planning irrigation storage, and emergency preparedness planners calculating water supply buffers. All calculators are free, require no sign-up, and work on any device.
🔍 How To Choose
How to Choose the Right Volume Calculator
The key factor is your tank's cross-sectional shape. Look at your tank from above and from the side to identify the correct category below.
Always measure internal dimensions. External dimensions include wall thickness, which overstates actual water capacity — by a few percent on a thin plastic tank, and by a third or more on a small concrete cistern. Use a tape measure from the inside wall to the opposite inside wall for the most accurate result.
✅ Sources & Review
How These Calculators Are Built and Reviewed
Every calculator in this category uses standard geometry for its shape and exact, legally defined unit conversions, and each one shows its working so you can check the numbers. The figures 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.
Last updated
❓ FAQ
Frequently Asked Questions
How do I calculate the volume of a water tank?
The formula depends on the tank's shape. For a rectangular tank, multiply length × width × height. For a cylindrical tank, use π × radius² × height. Working in feet, multiply the cubic feet by 7.48 to get US gallons; in centimetres, divide the cubic centimetres by 1,000 to get litres. Rather than doing the maths manually, select the calculator for your tank shape above — it handles all unit conversions automatically and gives you results in litres, gallons, cubic metres, and water weight at once.
What is the most common household water tank shape?
It depends on the country. Upright cylindrical plastic tanks are the most common household tanks in many parts of the world, valued for their strength — a cylinder carries internal water pressure more evenly than flat walls. Rectangular tanks are common where space is tight, such as lofts and built-in rooftop tanks, and horizontal cylinders are widely used for agricultural, fuel and transport storage.
What's the difference between US gallons and UK (Imperial) gallons?
A US gallon equals approximately 3.785 litres, while a UK/Imperial gallon equals approximately 4.546 litres — making the Imperial gallon about 20% larger. This distinction matters significantly when reading international tank specifications. For a dedicated single-unit result, use the Litres to US Gallons Converter or the Litres to UK Gallons Converter.
Should I use internal or external dimensions when calculating tank volume?
Always use internal (inside) dimensions. External dimensions include the tank wall thickness, which will overestimate the actual water storage capacity. For plastic tanks the walls are typically 5–15 mm (¼–½ in) thick, an error of a few percent; for reinforced concrete cisterns they can be 50–150 mm (2–6 in) or more, which on a small cistern can overstate the volume by a third. Measure from the inner face of one wall to the inner face of the opposite wall.
How many litres should my rooftop water tank hold?
A general rule of thumb where supply is unreliable is to store at least 1–2 days of water at 150–200 litres (40–53 US gallons) per person per day. A 4-person household would need 600–800 litres (160–210 gallons) for one day's backup, or 1,200–1,600 litres (320–420 gallons) for two days. In areas with unreliable municipal supply, 3–5 days of storage is recommended. Use the Rooftop Tank Size Calculator for a precise recommendation based on your specific household size and usage patterns.
Which calculator handles tanks that combine multiple shapes?
For tanks with a cylindrical upper section and a conical lower section (very common in chemical processing and slurry storage), use the Cone Bottom Tank Volume Calculator. For tanks shaped like a capsule — a cylinder with a hemispherical dome at each end (common in pressure vessels and food-grade storage) — use the Capsule Tank Volume Calculator. Both tools calculate the combined volume of all sections automatically.
