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Percent Water Calculator

Written by the percentages.co.uk team. Reviewed for accuracy.

This calculator finds the percentage of water in a substance. Use basic mode if you already know the water mass and total mass, or switch to the hydrate (crucible) mode for the A-level method of heating a hydrated salt in a crucible to drive off the water of crystallisation.

Takes about 30 secondsUpdated 17 May 2026

How it works

Percent water expresses the mass of water in a substance as a percentage of the total mass. For hydrated salts (compounds that contain water of crystallisation, written as a dot formula such as CuSO₄·5H₂O), this is calculated experimentally by heating the compound to drive off the water and recording the mass loss.

The formula

% Water = (Mass of water / Total mass of hydrate) × 100

Hydrate method: Water mass = Before heating - After heating

Hydrate mass = Before heating - Crucible mass

Why this works: Heating decomposes the hydrated salt into the anhydrous (water-free) form. The mass difference between before and after heating equals the water lost. Dividing this by the original hydrate mass and multiplying by 100 gives the percent water.

Worked examples

A sample of blue copper sulphate (CuSO₄·5H₂O) contains 3.60 g of water in a 10.00 g sample. What is the percent water?

  1. % Water: (3.60 / 10.00) × 100 = 36%
  2. Dry mass: 10.00 - 3.60 = 6.40 g

Answer: 36% water (theoretical value is 36.08% for CuSO₄·5H₂O)

A crucible experiment gives: crucible + hydrate before heating = 20.45 g, after heating = 18.57 g, empty crucible = 15.20 g. What is the percent water?

  1. Water mass: 20.45 - 18.57 = 1.88 g
  2. Hydrate mass: 20.45 - 15.20 = 5.25 g
  3. % Water: (1.88 / 5.25) × 100 = 35.81%

Answer: 35.81% water

A food sample has a total wet mass of 50 g and contains 38 g of water. What is the moisture content?

  1. % Water: (38 / 50) × 100 = 76%
  2. Dry mass: 50 - 38 = 12 g

Answer: 76% moisture content

A hydrate sample before heating weighs 8.32 g in a crucible weighing 12.40 g. After heating, the crucible + salt weighs 19.86 g. What is the percent water?

  1. Before (crucible + hydrate): 12.40 + 8.32 = 20.72 g
  2. Water: 20.72 - 19.86 = 0.86 g
  3. Hydrate mass: 8.32 g
  4. % Water: (0.86 / 8.32) × 100 = 10.34%

Answer: 10.34% water

Theoretical question: CaSO₄·2H₂O has a molar mass of 172.17 g/mol. Water of crystallisation is 2 × 18.02 = 36.04 g/mol. What is the percent water by mass?

  1. % Water: (36.04 / 172.17) × 100 = 20.93%

Answer: 20.93% water by mass

When to use this

  • A-level Chemistry practicals: The hydrate experiment is a standard A-level practical in AQA, OCR and Edexcel Chemistry where students determine the formula of a hydrated salt.
  • Food science and technology: Moisture content is a critical quality parameter for food products, affecting shelf life, texture and microbial safety.
  • Pharmaceutical analysis: Karl Fischer titration and loss on drying (LOD) tests measure water content in pharmaceutical ingredients and finished products.
  • Soil analysis: Agronomists and environmental scientists measure soil moisture content by weighing samples before and after oven drying.

Understanding the result

For hydrated salts, the theoretical percent water can be calculated from the chemical formula. If your experimental result differs significantly from the theoretical value, possible causes include: incomplete dehydration (heating for too short a time), decomposition of the anhydrous salt (overheating), or contamination.

In food science, moisture content above a critical threshold allows microbial growth. The water activity (aw) is a related but distinct concept; percent water by mass is a simpler measure that does not account for how tightly the water is bound.

Related concepts

➡ For finding the mass percentage of any solute in a solution (not just water), the mass percent calculator handles any solute-solvent combination. ➡ To calculate the mole fraction of water in a hydrated compound, the mole percentage calculator finds the mole percentage of each component. ➡ For concentration expressed in grams per volume rather than mass fractions, use the percentage strength calculator for % w/v, % w/w and % v/v solutions.

How to do this in Excel

Basic: =(A1/B1)*100 | Hydrate: =((A1-B1)/(A1-C1))*100

For basic mode: A1 = water mass, B1 = total mass. For hydrate mode: A1 = before heating, B1 = after heating, C1 = empty crucible mass.

How to do this without a calculator

For basic mode: divide water mass by total mass and multiply by 100. For the hydrate method: subtract the after-heating mass from the before-heating mass to get water lost. Subtract the crucible mass from the before-heating mass to get hydrate mass. Divide water by hydrate and multiply by 100.

Common mistakes

Not heating to constant mass

In the hydrate experiment, you must heat the crucible until two successive weighings give the same (or very similar) result. Stopping too early leaves residual water and underestimates the percent water.

Dividing by the wrong denominator

Always divide the water mass by the hydrate mass (before heating minus crucible), not by the anhydrous salt mass (after heating minus crucible). The denominator is the total original mass of the hydrated compound.

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