📐 About This Category
What Are Volume & Capacity Calculators?
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. All tools support multiple input units so you can work in whatever unit system your measurements are in, and every result is displayed in litres, US gallons, UK/Imperial gallons, cubic metres, cubic feet, and water weight simultaneously.
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.
| Tank Shape |
Best Calculator |
Key Inputs |
| Box / cuboid (rectangular) |
Rectangular Calculator |
Length, Width, Height |
| Upright cylinder |
Cylindrical Calculator |
Diameter, Height |
| Horizontal cylinder (on its side) |
Horizontal Calculator |
Diameter, Length, Fill depth |
| Round / disc-shaped |
Circular Calculator |
Inner diameter, Depth |
| Oval or race-track cross-section |
Oval / Elliptical Calculator |
Semi-axis A, Semi-axis B, Height |
| Sphere or ball-shaped |
Spherical Calculator |
Diameter or Radius |
| Cylinder with conical bottom |
Cone Bottom Calculator |
Cylinder diameter/height, Cone height |
| Cylinder with hemispherical end caps |
Capsule Calculator |
Diameter, Cylinder body length |
| Buried / underground cistern |
Underground Calculator |
Shape dimensions, Depth |
| Rooftop overhead tank (sizing) |
Rooftop Size Calculator |
Household size, Daily usage |
| Converting between volume units |
Litres ↔ Gallons Converter |
Any volume value |
Always measure internal dimensions. External dimensions include wall thickness, which overstates actual water capacity by 5–10% depending on tank material and construction. Use a tape measure from the inside wall to the opposite inside wall for the most accurate result.
❓ 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. Divide the result in 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?
Rectangular (cuboid) tanks are the most common for household rooftop storage because they make efficient use of flat roof space. Cylindrical plastic tanks are the second most popular, valued for their structural strength — a cylinder distributes internal water pressure more evenly than flat walls. Horizontal cylindrical tanks are widely used for agricultural and transport applications.
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. All calculators on this site display both values side by side, and the Litres to Gallons Converter lets you switch between them instantly.
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 by 5–10% depending on the material. For plastic tanks this difference is typically 5–15 mm per side; for reinforced concrete cisterns it can be 50–150 mm per side. 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 is to store at least 1–2 days of water per person per day at 150–200 litres per person. A 4-person household would need a minimum of 600–800 litres for one day's backup, or 1,200–1,600 litres 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.