What does 1% mean in pharmacy?

In pharmacy, 1% weight/volume (w/v) means 1 g per 100 mL. This equals 10 mg/mL. So 2% = 20 mg/mL, 0.9% NaCl = 9 mg/mL. This conversion is essential when mixing % and mg/mL values.

Why do I add stock to diluent, not diluent to stock?

For most preparations you are making up to a final volume — you add stock first, then top up with diluent. However for some drugs (especially acids and concentrated solutions) you must add concentrated solution to solvent, not vice versa, to prevent exothermic reactions or precipitation.

Can I use C1V1 = C2V2 for serial dilutions?

Yes — apply the equation sequentially for each step. Confirm each intermediate concentration before using it as C1 for the next step.

Concentration & Dilution — Free Pharmacy Calculator

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The chemist has a 10% stock solution and needs to prepare a 2% topical preparation in a 200 mL bottle. How much stock and how much diluent?

Concentration & Dilution

Compounding

Calculation type

Leave the unknown field blank — it will be calculated.

C₁ — Initial conc (mg/mL)

V₁ — Initial volume (mL)

C₂ — Final conc (mg/mL)

V₂ — Final volume (mL)

C₁V₁ = C₂V₂ Example: Dilute 10 mg/mL stock to 2 mg/mL in 50 mL total
V₁ = (2 × 50) ÷ 10 = 10 mL stock + 40 mL diluent % w/v = (mass g ÷ volume mL) × 100 2 g in 100 mL = 2% w/v = 20 mg/mL

Reference: Pharmacy Board of Australia · TGA Guidelines

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Solves any variable in the dilution equation C1V1 = C2V2. Enter any three of the four values — stock concentration (C1), stock volume (V1), final concentration (C2), final volume (V2) — and the calculator finds the fourth. Essential for pharmacy compounding, IV admixture preparation, and diluting disinfectants.

2 Formula & professional reasoning

C1 × V1 = C2 × V2
C1 = stock concentration · V1 = volume of stock to use
C2 = desired final concentration · V2 = total final volume

The C1V1 = C2V2 equation expresses conservation of the amount of solute: the total moles (or mass) of drug in the stock volume equals the total moles of drug in the final diluted volume. Rearranging for any unknown gives V1 = C2V2/C1 (how much stock to use) or C2 = C1V1/V2 (final concentration). Concentrations must be in the same units before calculating — a mix of % and mg/mL will give a wrong answer.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Dilute 10% to 2% in 200 mL

Given: C1 = 10% · C2 = 2% · V2 = 200 mL · Find: V1

Working: V1 = (2 × 200) ÷ 10 = 400 ÷ 10

Answer: V1 = 40 mL stock + 160 mL diluent

💡 Take 40 mL of 10% solution and add water to make up to 200 mL total.

Standard

What concentration results from this dilution?

Given: C1 = 1000 mg/mL · V1 = 5 mL · V2 = 250 mL · Find: C2

Working: C2 = (1000 × 5) ÷ 250 = 5000 ÷ 250

Answer: C2 = 20 mg/mL

💡 Confirm: 20 mg/mL × 250 mL = 5000 mg = 1000 mg/mL × 5 mL ✓

Advanced

Final volume from stock and concentrations

Given: C1 = 500 mcg/mL · V1 = 2 mL · C2 = 50 mcg/mL · Find: V2

Working: V2 = (500 × 2) ÷ 50 = 1000 ÷ 50

Answer: V2 = 20 mL total volume

💡 Add 2 mL of stock to 18 mL of diluent to reach 20 mL total. Do NOT add 20 mL of diluent to 2 mL of stock.

4 Sanity check

C2 must always be less than C1

You cannot dilute to a higher concentration

If your calculated C2 > C1, check your inputs — the equation is likely inverted.

V1 must always be less than V2

Stock volume used must be less than final volume

If V1 ≥ V2, the calculation implies you're concentrating, not diluting.

Unit consistency

C1 and C2 must be in the same units

% with %, mg/mL with mg/mL. Never mix % and mg/mL in one calculation.

Volume check

Diluent to add = V2 − V1

You add stock to partial diluent, then make up to V2. Never add V2 of diluent to V1 of stock.

5 Common errors

Error	Cause	Consequence	Fix
Mixing % and mg/mL concentrations in the same equation	Using mg/mL for C1 and % for C2	Answer wrong by factor of 10	Convert all concentrations to the same unit before calculating. 1% = 10 mg/mL.
Adding V2 of diluent instead of making up to V2	Misunderstanding 'make to volume'	Final concentration lower than intended	'Make up to 200 mL' means the TOTAL final volume is 200 mL — add enough diluent so that stock + diluent = 200 mL
Forgetting to check chemical compatibility	Assuming any diluent works	Precipitation, drug degradation or reduced potency	Always consult AMH, Micromedex or the product PI for compatible diluents before preparation
Not labelling the prepared product immediately	Preparing multiple items	Unlabelled containers, mix-up risk	Label every container immediately upon preparation with drug, concentration, volume, prepared by, date/time, expiry

6 Reference & regulatory links

🇦🇺 Australia

🇺🇸 United States

7 Professional workflow

Common tools used alongside this one:

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Alligation Calculator

Use Alligation when you need to mix two concentrations rather than diluting one with pure diluent

→

📐

Body Surface Area (BSA)

Prepare IV admixture concentrations for chemotherapy alongside BSA-based dose calculation

→

Input	Result
10% → 2% in 200 mL	40 mL stock + 160 mL diluent
1000 mg/mL, 5 mL into 250 mL	= 20 mg/mL
1% = 10 mg/mL	conversion reference