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Dilution refers to make a lower concentration solution from higher concentrations. Solutions usually are stored in a higher concentration, for convience of use and avoiding contamination.

The dilution fomula is:
Concentration_(stock) × Volume_(stock) = Concentration_(dilute) × Volume_(dilute)

Dilution Calculator of Mass Percentage Concentration Solution:
This calculation can be used for dilutions of solutions with concentration in Mass Percentage units, e.g. mg/ml, ug/ml, ... For dilution of molar concentration solution, like mol/L, mM, nM, please use the Dilution Calculator of Molar concentration.

E.g. The diluted NaCl solution is 300 ml, with concentration 40 ng/ml, how much 5 ug/ml NaCl stock solution is needed?
Answer:
Volume_(stock) = 300ml * 40ng/ml / 5ug/ml = 2.4ml

Dilution Calculator of molar concentration:
This calculation can be used for dilutions with molar concentration, e.g. mol/L, nmol/L, ..., click here for molar mass calculation of chemicals. Note: 1M is equal to 1mol/L.

E.g.
(1) The diluted HCl solution is 300ml, with concentration 100mM, how much 5M HCl stock solution is needed?
Answer:
Volume_(stock) = 300ml * 100mM / 5M = 6ml

(2) The stock HCl solution concentration is 5mol/L, 2ml of which was used to produce a diluted solution of 500ml, what's the concentration of the diluted solution?
Answer:
Concentration_(dilute) = 5mol/L * 2ml / (500ml) = 0.02mol/L

Serial Dilution Calculator:
This calculation is useful when make a solution of lower concentration from its higher concentration solution while the solution units of the stock and dilute are the same, e.g. both in copies/mL, mg/L ......

For example, question can be:
To get 100 ml 1 × 10⁵ diluted solution, how much 1 × 10⁷ solution is needed?
Answer:
Volume_(stock) = 100ml * 100000 / 10000000 = 1ml