Lab calculators

Molarity: how much to weigh

mass = concentration × volume × molar mass

Weigh 5.844 g of NaCl, dissolve, and make up to 100 mL.

Use the formula weight on your bottle: hydrates and salts differ (for example, MgCl₂ is 95.21 g/mol, MgCl₂·6H₂O 203.30).

Dilution: C₁V₁ = C₂V₂

stock concentration × stock volume = final concentration × final volume

Take 50 mL of the stock and add 450 mL of diluent.

Serial dilution

transfer = diluent volume ÷ (dilution factor − 1)

Put 900 µL of diluent in each of 6 tubes. Transfer 100 µL from the stock into tube 1, mix, then 100 µL from each tube into the next. Remove 100 µL from the last tube so all hold 900 µL.

Centrifuge: rpm and × g

RCF (× g) = 1.118 × 10⁻⁵ × radius (cm) × rpm²

13,000 rpm with a radius of 8.5 cm is 16,060 × g.

Use the maximum radius (centre of the rotor to the bottom of the tube) from the rotor’s manual. Protocols should give × g, which is the same on every centrifuge.

Cell count (haemocytometer) and seeding

cells per mL = (cells counted ÷ large squares) × dilution factor × 10⁴

Enter the counts (a 1 : 1 mix with trypan blue is a dilution factor of 2).

Seeding

Count the cells above (or type the suspension’s concentration), then fill in the seeding.

Vessel areas are typical values; they differ between manufacturers, so check the datasheet of your plates.

DNA and RNA: concentration, purity and copies

concentration (ng/µL) = A260 × factor × dilution ÷ path (cm); dsDNA 50, ssDNA 33, RNA 40

Enter A260 (the readings should be between about 0.1 and 1.0 for a reliable value).

Instruments such as the NanoDrop already convert to a 1 cm path: leave the path at 1 for their readings.

Copy number and molarity

copies = mass (g) × 6.022 × 10²³ ÷ (length × mass per bp or nt)

Enter the amount and the length (for a plasmid, the whole plasmid).

650 g/mol per base pair (and 325 or 340 per base for single-stranded DNA or RNA) are the usual averages for the sodium salt; change them if your protocol uses another value.

Protein concentration from A280

concentration (M) = A280 × dilution ÷ (ε × path); mg/mL = M × molar mass

Enter A280, ε and the molar mass. Get ε and the mass from the protein sequence with the sequence tools.