Large dilutions made in a single step force you to pipette tiny, error-prone volumes from a concentrated stock. A serial dilution spreads the job across several equal steps. This planner picks the even fold factor and hands you a tube-by-tube pipetting recipe.
How it works
The total dilution is the stock divided by the target. To split it evenly over
N steps the planner takes the Nth root: fold = (stock / target)^(1/N). Every
tube then uses the same fold, so a 100-fold dilution over three steps becomes
three steps of about 4.64-fold each.
For a fixed tube volume V, the source you carry forward is V / fold and the
diluent is the remainder. Tube 1 draws from the stock, and each later tube draws
from the tube before it. This keeps C1V1 = C2V2 satisfied at every step.
Worked example
Diluting a 1000 ng/mL stock to 10 ng/mL over three steps into 1000 µL tubes gives a 4.64-fold step. Each tube takes 1000 / 4.64 ≈ 215 µL of source plus 785 µL diluent, producing concentrations of approximately 215, 46, and 10 ng/mL.
Compare this to attempting the same dilution in one step: you would need to transfer only 10 µL of the 1000 ng/mL stock into 990 µL diluent. A 1% pipetting error on a 10 µL draw is a 0.1 µL absolute error — proportionally enormous and difficult to detect. The same 1% error on a 215 µL draw is about 2.15 µL, far less consequential.
How many steps to use
More steps means smaller absolute errors at each transfer, but also more opportunities for carryover and compounding. General guidance:
| Total dilution | Recommended steps | Typical per-step fold |
|---|---|---|
| 10 – 100-fold | 2–3 | 3– to 10-fold |
| 100 – 1,000-fold | 3–4 | ~5 to 10-fold |
| 1,000 – 10,000-fold | 4–5 | ~6 to 10-fold |
| Over 10,000-fold | 5–7 | ~7 to 10-fold |
For most analytical purposes, 5- to 10-fold steps per tube strike the best balance between transfer volume and number of steps.
Tips
- Mix each tube thoroughly before drawing into the next — carryover from an unmixed tube propagates an error through every subsequent point.
- Change tips between steps to avoid contaminating back up the series.
- Keep transfer volumes above roughly 20 µL for accuracy; if a step gives less, add another tube or use a larger tube volume.
- Use the same pipette (calibrated) for the transfer volume throughout one series; switching instruments mid-series introduces a systematic offset.
- Label tubes before you start — misordering a tube in a 7-point series is easy and hard to detect downstream.