Percentage Strength Calculator
Written by the percentages.co.uk team. Reviewed for accuracy.
This calculator works out the percentage strength of a solution in three formats used in pharmacy and chemistry: % w/v (grams per 100 mL), % w/w (grams per 100 g) and % v/v (millilitres per 100 mL). Select the type and enter your amounts for an instant result with clear step-by-step workings.
How it works
Percentage strength expresses how much active substance is present per 100 units of the total preparation. The three standard types each use different units:
The formula
% w/v = (Mass of solute in g / Volume of solution in mL) × 100
% w/w = (Mass of solute in g / Mass of solution in g) × 100
% v/v = (Volume of solute in mL / Volume of solution in mL) × 100
- % w/v is the most common in pharmaceutical solutions: a 5% w/v saline solution contains 5 g of NaCl per 100 mL of solution.
- % w/w is used for creams, ointments and solid mixtures: a 1% w/w hydrocortisone cream contains 1 g per 100 g of cream.
- % v/v is used for alcohol content and liquid dilutions: a 40% v/v spirit contains 40 mL of ethanol per 100 mL of drink.
Why this works: All three formats use the same proportional calculation, just with different units. Multiplying by 100 converts the decimal fraction to a percentage expressed in the conventional "per 100 units" format.
Worked examples
A nurse prepares a 500 mL intravenous solution containing 4.5 g of sodium chloride. What is the % w/v strength?
- % w/v: (4.5 / 500) × 100 = 0.9%
- This is the standard isotonic saline used in hospitals
Answer: 0.9% w/v (normal saline)
A pharmacist prepares 200 g of a cream with 2 g of active ingredient. What is the % w/w strength?
- % w/w: (2 / 200) × 100 = 1%
Answer: 1% w/w cream
A bottle of whisky contains 40 mL of ethanol per 100 mL. What is the % v/v?
- % v/v: (40 / 100) × 100 = 40%
- This is equivalent to 40% ABV
Answer: 40% v/v (40% ABV)
A mouthwash solution contains 1.5 g of cetylpyridinium chloride in 250 mL of solution. What is the % w/v?
- % w/v: (1.5 / 250) × 100 = 0.6%
Answer: 0.6% w/v
A hand sanitiser contains 750 mL of isopropanol in 1,000 mL total. What is the % v/v?
- % v/v: (750 / 1000) × 100 = 75%
Answer: 75% v/v isopropanol
When to use this
- Pharmacy students and technicians: Percentage strength calculations are a core topic in BTEC and MPharm courses and appear regularly in GPhC registration assessments.
- Compounding pharmacies: Preparing topical preparations, oral solutions and dilutions from concentrated stock requires accurate percentage strength calculations.
- Laboratory preparations: Making up buffer solutions, staining solutions or reagents from solid or concentrated liquid stock.
- Alcohol-related calculations: Distillers, brewers and cider makers use % v/v to express and verify the alcohol content of their products in line with HMRC requirements.
Understanding the result
The three percentage strength types are not interchangeable. A 5% w/v solution is not the same concentration as a 5% w/w solution unless the solution density is exactly 1 g/mL (which is only true for very dilute aqueous solutions). Always specify the type when expressing percentage strength.
In UK pharmacy practice, percentage strength without a qualifier usually implies % w/v for liquids and % w/w for semi-solids, following British Pharmacopoeia conventions.
Related concepts
➡ For the specific case of mass-in-mass concentration (solute and solution both in grams), the mass percent calculator handles % w/w calculations with the option to enter solvent mass separately. ➡ To find the percentage of water in a hydrated compound or substance after drying, the percent water calculator uses mass before and after drying or crucible data. ➡ For expressing mixture composition as mole fractions, the mole percentage calculator finds the mole percentage of each component.
How to do this in Excel
=(A1/B1)*100
Put solute amount in A1 and total solution amount in B1, using consistent units for the chosen strength type (g/mL for % w/v, g/g for % w/w, mL/mL for % v/v).
How to do this without a calculator
Divide the amount of active substance by the total amount and multiply by 100. For % w/v: if there are 5 g of drug in 1,000 mL, that is 5 ÷ 1,000 × 100 = 0.5% w/v. Check your units carefully before calculating.
Common mistakes
Mixing up % w/v and % w/w
% w/v uses the volume of the final solution as the denominator; % w/w uses the total mass. For dense solutions, these differ significantly. Choose the correct type for your application.
Using the solvent volume instead of the solution volume for % w/v
The denominator for % w/v is the total volume of the final solution, not the volume of solvent added. Always make the solution to volume in a volumetric flask, not to a fixed solvent volume.
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