How to use
- Enter the starting concentration, the dilution factor and the number of steps. Ten-fold across five steps means a factor of 10 and 5 steps.
- Total volume per tube is optional. Leave it blank and you get the concentration at each step; fill it in and the transfer volume and diluent volume are worked out as well.
- Editing a cell in the Dilution factor column lets a step differ from the rest. Change any one of them and the field above switches to ‘Custom’.
- [Same factor throughout] puts them back. An empty cell follows the shared factor above.
- Results update as you type. There is no calculate button.
Formula & notes
A serial dilution reaches a wide range of concentrations by repeating the same small dilution. Going ten-thousand-fold in one step means transferring a volume too small to pipette accurately; four ten-fold steps arrive at the same place with a workable volume every time.
Each step divides the previous concentration by the factor.
- Concentration at step n
- Cₙ = C₀ ÷ (factor)ⁿ
Where the factor differs from step to step, divide by the product of the factors up to that point.
- With mixed factors
- Cₙ = C₀ ÷ (f₁ × f₂ × … × fₙ)
Once the total volume V per tube is set, the transfer and the diluent follow from it.
- Transfer
- V ÷ factor
- Diluent
- V − (V ÷ factor)
A ten-fold dilution into 1 mL means carrying 100 μL from the previous tube and adding 900 μL of diluent.
Practical notes
- Mix thoroughly at every step. Carrying an unmixed tube forward passes its error into every step after it, and because the errors of a serial dilution multiply, an early one costs the most.
- Change the tip between steps. Concentrated solution clinging to the outside of a tip raises the concentrations downstream above the calculated values.
- When the transfer falls below about 1 μL, a smaller factor over more steps is more accurate. Seven five-fold steps are often easier to handle than five ten-fold ones.
- The volumes in the table make each tube up to the total volume you set. If you need more than that from the last tube, set a larger total — every tube loses what is carried to the next one.
- Substances that adsorb readily — some proteins, oligonucleotides — can come out below the calculated value at the dilute end. Consider low-binding tubes or a carrier.