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Example values are filled in. Replace them with your own.
Molarity
Work out how much solid to weigh, what concentration you made, or what volume to make up to.
Check the bottle label. Hydrated salts have a higher molecular weight.
Example: 250 mL of 0.5 M NaCl needs 0.5 × 0.25 × 58.44 = 7.305 g. Dissolve in about 80% of the final volume, then make up to the mark.
Dilution (C1V1 = C2V2)
Make a working solution from a stock. Use the same unit for both concentrations: M, mM, %, mg/mL or ×.
Example: 100 mL of 50 mM from a 1 M (1000 mM) stock needs 5 mL of stock and 95 mL of diluent.
CFU / mL
Turn a plate count into viable cells per mL of the original sample.
Use the average of your replicate plates.
Spread plate is usually 100 µL; pour plate is usually 1000 µL.
Example: 156 colonies from 100 µL of the 10⁻⁵ tube gives 156 ÷ (10⁻⁵ × 0.1) = 1.56 × 10⁸ CFU/mL. Count plates with 30 to 300 colonies.
Plan a serial dilution
Check the dilution each tube gives before you start pipetting.
Sterile saline (0.85% NaCl) or sterile distilled water.
% Solutions
Grams of solid (w/v) or millilitres of liquid (v/v) for a percentage solution.
Example: 500 mL of 1.5% agar needs 1.5 × 500 ÷ 100 = 7.5 g. Diluting from a stock that is not 100%, such as 95% ethanol to 70%? Use the Dilution calculator.
DNA / RNA concentration
Concentration, yield and purity from spectrophotometer readings at 260 nm and 280 nm (1 cm path length).
Most accurate between 0.1 and 1.0.
10 µL sample in 90 µL water is a factor of 10. Enter 1 for an undiluted reading.
µg/mL and ng/µL are the same number. Pure DNA gives an A260/A280 ratio of about 1.8 and pure RNA about 2.0. A lower ratio usually means protein or phenol carry-over.
RPM and RCF
Protocols give speed as × g (RCF), while many centrifuges are set in RPM. The conversion depends on the rotor radius.
Distance from the rotor centre to the bottom of the tube. Check the rotor manual or lid; microcentrifuges are usually 7 to 9 cm.
Example: 13,000 RPM on an 8.5 cm rotor is about 16,060 × g. Never run above the rotor's rated maximum speed.
Check before you prepare
These calculators are a learning aid. Confirm critical values against your protocol and with your mentor or supervisor, and follow your lab's safety rules.
Spotted an error? Email info@savitrbio.com and we will correct it.
Practise these techniques at the bench
Media preparation, dilutions, viable counts and DNA isolation, done in our Kochi lab with a mentor.
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