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Buffer Dilution Calculator

Calculate the amount of stock solution and solvent needed for dilution

How to use the dilution calculator

There are three calculators, switched with the tabs above the form.

  1. Single dilution — enter the stock concentration, the total volume you want to make and the concentration you need. You get the volume of stock to measure and how much solvent to add.
  2. Serial dilution — enter the stock concentration and its unit, the dilution factor, the number of tubes and the volume per tube. You get the diluent for each tube, the transfer volume and a table of concentrations.
  3. Molarity — work out the grams to weigh for a molar solution, or the molarity of a mass you have already dissolved.

The formula: C1V1 = C2V2

C1 is the stock concentration, V1 the volume of stock you need, C2 the concentration you want and V2 the final volume. Rearranged, V1 = C2 × V2 ÷ C1, and the solvent is V2 − V1. Use the same unit for both concentrations — X with X, or % with % — and the volumes come out in the unit you pick.

Worked example: to make 500 mL of 1X PBS from a 10X stock, V1 = 1 × 500 ÷ 10 = 50 mL of stock plus 450 mL of water. For 100 mL of 70% ethanol from 95% ethanol, V1 = 70 × 100 ÷ 95 = 73.7 mL. Ethanol and water shrink slightly when mixed, so measure the ethanol and then make up to the 100 mL mark rather than adding exactly 26.3 mL.

Serial dilution

Each tube is diluted by the same factor as the one before it. With a factor f and a volume V per tube, put V − V ÷ f of diluent in every tube and carry V ÷ f from one tube to the next. Tube n then holds the stock concentration divided by f to the power n.

Worked example: a ten-fold series in 1 mL tubes from a suspension of 10⁸ CFU/mL means 0.9 mL of diluent in each tube and 0.1 mL transferred each time. Tube 1 holds 10⁷ CFU/mL, tube 3 holds 10⁵ and tube 6 holds 100 CFU/mL — a typical range for a plate count. Doubling dilutions for serology titres use a factor of 2: equal volumes of sample and diluent at every step.

Molarity

Mass in grams = molarity (mol/L) × volume (L) × molecular weight (g/mol). For 500 mL of 0.5 M sodium chloride (58.44 g/mol), weigh 0.5 × 0.5 × 58.44 = 14.61 g and make up to 500 mL. Working backwards, 2.5 g of glucose (180.16 g/mol) in 500 mL is 2.5 ÷ 180.16 ÷ 0.5 = 0.0278 M, about 27.8 mM. Note that normal saline, 0.9% NaCl, is a weight-by-volume percentage — 9 g per litre, roughly 0.154 M — not 0.9 M.