Find how many parts of a stronger and a weaker mix you need to reach a target strength.
| Parts high | – |
| Parts low | – |
Use this calculator in pharmacy and compounding when you have two stocks, such as 90% and 40%, and you want an in-between strength such as 70%. The same method works for alcohol, cream, or any two strengths on one scale.
Alligation alternate: parts of the high strength = target − low, and parts of the low strength = high − target. Mix those parts by mass or by volume, using the same unit on both stocks. The target must sit strictly between the high and low strengths.
This is a mixing-ratio method. It does not correct for density changes, heat of mixing, or volume contraction.
High strength: The stronger of the two stocks, as a percent or any consistent strength number, for example 90.
Low strength: The weaker stock on the same scale, for example 40, or 0 if you are diluting with solvent.
Target strength: The strength you want, strictly between high and low, for example 70.
Parts high: Relative parts of the stronger stock to mix.
Parts low: Relative parts of the weaker stock to mix.
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