Water Calculator
Calculate water in litres for a tank, estimate daily water intake with an editable planning factor, or convert water between litres, millilitres, cubic metres and gallons.
Quick answer
Litres = length (m) × width (m) × water depth (m) × 1,000.
Litres = π × radius² × water depth (m) × 1,000.
mL/day = body weight (kg) × the editable mL/kg planning factor you choose.
Plain unsweetened water contains 0 calories. Sweetened or flavoured drinks can be different.
Water calculator workspace
Choose what you want to calculate, enter your values, and see the result instantly.
Water calculation formulas
The correct water formula depends on what you are measuring. Tank calculations use geometric volume, while a daily intake estimate uses a body-weight planning factor. The calculator keeps these purposes separate so a tank-capacity result is not confused with a health estimate.
| Calculation | Formula | Useful result |
|---|---|---|
| Rectangular tank water | length × width × water depth | m³ × 1,000 = litres |
| Vertical cylindrical tank | π × radius² × water depth | m³ × 1,000 = litres |
| Daily intake estimate | weight kg × chosen mL/kg factor | mL/day ÷ 1,000 = L/day |
| Water conversion | 1 m³ = 1,000 L | Litres, gallons and approximate mass |
How to calculate water in a tank in litres
Measure the inside of the tank, not the outside. For a rectangular tank, multiply the internal length by the internal width and the actual water depth. If all measurements are in metres, the answer is cubic metres; multiply by 1,000 for litres.
Example: A rectangular tank 2 m long and 1 m wide with 0.8 m of water contains 2 × 1 × 0.8 = 1.6 m³, which equals 1,600 litres.
For a vertical cylinder, use the internal radius (half the diameter). A 1 m diameter tank has a radius of 0.5 m. Multiply π × radius² × water depth, then convert cubic metres to litres.
For horizontal cylinders, spheres or more specialised tank shapes, use the Liquid Volume Calculator for Tanks, which supports additional geometry.
Daily water intake calculator: what the estimate means
The daily intake mode is intentionally simple. It multiplies body weight in kilograms by an editable number of millilitres per kilogram. This makes the assumption visible instead of presenting one fixed number as correct for everyone.
Important: this is a general planning calculation, not a medical prescription. Individual fluid needs can change substantially with climate, exercise, diet, pregnancy, fever, kidney or heart conditions, medicines and other factors. Total water also comes from foods and other beverages.
If a healthcare professional has given you a fluid target or restriction, use that value rather than a generic body-weight formula. You can change the calculator's mL/kg field to match a target that is appropriate for your own plan.
Water calculator in litres: conversion facts
Litres are often the clearest unit for household tanks, bottles and everyday water quantities. One cubic metre equals exactly 1,000 litres. The converter also shows US gallons and imperial gallons because the word “gallon” does not represent the same volume in every measurement system.
The converter displays approximate water mass using 1 kilogram per litre as a practical everyday assumption. Real water density changes slightly with temperature, so use a temperature-specific density when a laboratory, industrial or engineering calculation requires higher precision.
Does water have calories?
Plain water has 0 calories. The calculator therefore does not try to compute calories for plain water. Products described as flavoured water, vitamin water, electrolyte drinks or sweetened water can contain sugar or other energy-providing ingredients, so check the product nutrition label when calories matter.
Frequently asked questions
How do I calculate water in litres?
Calculate the volume in cubic metres and multiply by 1,000. For a rectangular tank, use internal length × internal width × actual water depth.
What is the formula for water calculation in a tank?
For a rectangular tank: L × W × water depth. For a vertical cylindrical tank: π × radius² × water depth. Convert dimensions to metres before multiplying if you want cubic metres and litres.
Can I calculate a partially filled tank?
Yes. Enter the current water depth rather than the full internal height. The result also shows the percentage of the entered tank height that is filled.
Can I use this as a water intake calculator?
Yes, but treat the result as a general planning estimate. The mL/kg factor is editable because there is no single body-weight multiplier that is appropriate for every person or circumstance.
How many litres are in 1 cubic metre?
Exactly 1,000 litres.
How many calories are in water?
Plain unsweetened water contains 0 calories. Added sugar or other ingredients can add calories.