Adding water to a solution changes its strength but not the amount of substance in it, so concentration × volume is the same before and after: C₁V₁ = C₂V₂. To make 1 liter at 3% from a 12% stock, take 3 × 1,000 ÷ 12 = 250 mL of stock and add 750 mL of water. The calculator solves for whichever value you need – the stock to take, the final concentration or the volume to top up to – and also mixes two solutions of the same substance.
Making 1 liter from a 12% stock
| Strength you want | Stock (V₁) | Water to add | Stock : water |
|---|---|---|---|
| 6% | 500 mL | 500 mL | 1 : 1 |
| 3% | 250 mL | 750 mL | 1 : 3 |
| 1% | 83.3 mL | 916.7 mL | 1 : 11 |
| 0.5% | 41.7 mL | 958.3 mL | 1 : 23 |
Halving the strength means equal parts stock and water; going from 12% to 1% needs eleven parts of water for every part of stock.
Two ways to write a ratio
The same “1 : 4” can mean two different mixtures:
| Written as | Meaning | Stock in 1,000 mL |
|---|---|---|
| 1 : 4 (parts of stock to parts of water) | 1 + 4 = 5 parts in all | 200 mL |
| 1 in 4, or a 1/4 dilution (parts of stock in the total) | 4 parts in all | 250 mL |
Cleaning product labels usually mean the first; many lab protocols mean the second. The results show your mix both as a dilution factor and as a stock : water ratio, so you can match either.
Mixing two strengths
If you mix 200 g of a 10% solution with 300 g of a 30% one, the substance adds up: 20 g + 90 g = 110 g in 500 g, so 22%. The answer is always between the two strengths, closer to the one you used more of. It is exact with masses; with volumes it is close for watery solutions, but alcohol and some acids shrink slightly when mixed.
Good practice
Add concentrated acid to water, never water to acid, and wear eye protection. Use a measuring cylinder or a volumetric flask for the final volume. The calculator itself needs no network and works offline on a phone in the lab.
Frequently asked questions
How do I use C1V1 = C2V2?
The amount of substance is the same before and after adding water, so concentration × volume stays equal. To make 1,000 mL at 3% from a 12% stock: V₁ = 3 × 1,000 ÷ 12 = 250 mL of stock, topped up to 1,000 mL.
What does a 1:4 dilution mean?
It depends on who writes it. On many cleaning product labels, 1:4 means 1 part product plus 4 parts water (5 parts in all). In many labs, a 1:4 dilution means 1 part in a total of 4. Check which one is meant before mixing.
What is a dilution factor?
How many times weaker the result is: C₁ ÷ C₂, or V₂ ÷ V₁. Going from 12% to 3% is a dilution factor of 4.
How do I work out the concentration of two solutions mixed together?
Add up the substance in each and divide by the total: (m₁C₁ + m₂C₂) ÷ (m₁ + m₂). 200 g at 10% plus 300 g at 30% gives 22%.
Can I use any concentration unit?
Yes, as long as both concentrations use the same one: %, mol/L, mg/mL or ppm. The volumes must also share a unit; the calculator converts mL, liters, US fluid ounces, cups and gallons for you.
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