Stock → Working Solution Planner with Pipetting Steps

You grab the 10 mM stock, and you need 200 µM working solution. One step or two? That decision drives every stock solution dilution you plan at the bench. A single-step approach sounds simpler, but look at the math: a 50-fold dilution into 100 µL final volume means pipetting 2 µL of stock. That is right at the edge of what a P2 can do accurately. Push that dilution factor higher and you are asking a pipette to measure fractions of a microliter, which is a recipe for inconsistent results.

The common mistake is forcing a huge fold-change into one step when the pipette volume drops below about 0.5 µL. At that point, the relative error from tip wetting, surface tension, and user technique dominates the measurement. Two sequential 1:10 dilutions, for example, keep every pipetting step in a comfortable range and give you a much more reproducible final concentration.

What the calculator gives you is a clear pipetting plan with feasible volumes at every step. If it flags that a single-step stock volume falls below a practical threshold, that is your cue to break the dilution into intermediate steps.

You will never recover every last microliter from a tube. Liquid clings to the inside wall, sits in the pipette tip, and hides under the meniscus. If you are dispensing into a multi-channel reservoir, add the trough dead volume on top of that. Make exactly what you need and you will run short on the last well every time.

Standard practice is 10-15% overage. The calculator applies this before back-calculating how much stock to pull. So if you need 1 mL of working solution, a 10% overage bumps the prep to 1.1 mL, and the stock volume scales accordingly. For a 96-well plate filled with a multi-channel, 15% is safer because of reservoir dead volume. For single-tube preps on the bench, 10% is usually enough.

Overage does not change the concentration of your working solution. It only changes the total volume you prepare, which means you use proportionally more stock and more diluent.

If the calculator tells you to pipette 0.05 µL, stop. No standard lab pipette handles that volume with any accuracy. The practical lower limit for a well-calibrated P2 is around 0.5 µL, and even then the coefficient of variation is high. Below 1 µL, small technique differences -- how fast you release the plunger, whether the tip touched the wall -- translate into large percentage errors.

Two fixes when the stock volume is too small:

• Increase the total prep volume so the stock aliquot scales up into a comfortable range.
• Do a two-step dilution: make an intermediate stock at a lower concentration, then dilute that intermediate to your target.

Either way, every pipetting step should land between 1 µL and the pipette maximum for best accuracy. The calculator flags volumes under 0.5 µL as a warning precisely because of this constraint.

Real bench work rarely involves a single solute. You might combine a 1000x antibiotic stock, a 100x supplement, and a 10x buffer into one working solution. Each stock has its own concentration and its own required dilution factor. The math is the same C₁V₁ = C₂V₂ applied independently to each component, but there is one constraint people forget: the sum of all stock volumes must be less than the final volume. Whatever is left over is your diluent.

If the stock volumes add up to more than the final volume, you cannot physically make that solution at those concentrations in that total volume. This usually means one of your stocks is not concentrated enough. The fix is either to use a more concentrated stock or to increase the final volume.

The calculator handles this by computing each stock volume separately, summing them, and reporting the diluent remainder. If the remainder goes negative, it tells you the combination is not feasible.

My stock volume is negative -- what happened?
Your target concentration is higher than your stock concentration. You cannot dilute something into a stronger solution. Double-check that stock and target are not swapped in the input fields.

Can I mix µM and mM units?
Only if the calculator converts them for you. If you enter stock as 10 mM and target as 200 µM, the tool needs to know those are different scales. When in doubt, convert everything to the same unit before entering values. 200 µM = 0.2 mM.

How much overage is too much?
Anything above 20% is wasteful for most bench preps. If you find yourself adding 30-50% overage to avoid running short, the real problem is likely technique or dead-volume management, not volume planning.

What if my stock is less concentrated than the target?
Then you need a more concentrated stock or a different approach entirely. Dilution can only decrease concentration, never increase it. The calculator will flag this as an error.

The diluent volume shows zero -- is that right?
It can happen when the dilution factor is 1x, meaning stock and target concentrations are equal. You are just transferring stock directly with no dilution.

The core relationship:

Rearranged to solve for stock volume:

Diluent volume is whatever remains:

When overage is included, the adjusted final volume becomes:

Substitute V_final_adj for V_final in the stock volume equation and the rest follows. These four lines cover every single-component dilution the calculator performs.

Step 1 -- Dilute 10 mM stock to 1 mM intermediate.
V_stock = (1 mM x 100 µL) / 10 mM = 10 µL of stock.
V_diluent = 100 µL - 10 µL = 90 µL buffer.
Result: 100 µL of 1 mM intermediate solution.

Step 2 -- Dilute 1 mM intermediate to 200 µM working.
V_intermediate = (200 µM x 100 µL) / 1000 µM = 20 µL of the 1 mM solution.
V_diluent = 100 µL - 20 µL = 80 µL buffer.
Result: 100 µL of 200 µM working solution.

Every pipetting step here is between 10 µL and 90 µL -- well within the accurate range of a P100 or P200.

Compare to single-step: V_stock = (200 µM x 100 µL) / 10,000 µM = 2 µL of 10 mM stock into 98 µL buffer. That 2 µL is technically feasible with a P2 or P10, but the relative error at 2 µL is substantially higher than at 10 or 20 µL. For critical assays -- dose-response curves, enzyme kinetics, anything where concentration accuracy matters -- the two-step route is worth the extra tube.

• Sigma-Aldrich -- Dilution and Solution Preparation Protocols
• IUPAC -- Nomenclature and Units for Concentration
• OpenStax Chemistry 2e -- Chapter 3: Composition of Substances and Solutions
• LibreTexts Analytical Chemistry -- Basic Tools of Analytical Chemistry