Sample Prep Calculator

The calculation is per sample volume multiplied by sample count, multiplied by one plus the overage fraction, and that one plus is the entire content of the idea. Ten percent overage is a multiplier of 1.1, not of 10 and not of 1.01. This is where the arithmetic most often slips, because an overage quoted as a percentage invites being read as a factor rather than as an increment. Ten percent of fifty millilitres is five millilitres, and fifty plus five is fifty five, which is what the factor of 1.1 is doing underneath.

It is a percentage of the requirement, not of the finished total, and the two produce answers that look almost identical. Ten percent added to a requirement of fifty gives fifty five. Ten percent of the finished volume, solved backwards, means preparing fifty divided by 0.9, which is 55.56 millilitres. The first is the correct reading and the second is the same intent computed in the wrong order, leaving a surplus of 5.56 rather than 5. Any surplus is a decision, so it is worth making it deliberately instead of arriving at it by rearrangement.

A second worked angle at a different scale. Half a millilitre each across ninety six samples is a bare requirement of forty eight millilitres, and five percent overage gives 50.4 millilitres to prepare, a surplus of 2.4 millilitres and therefore under five samples worth. That is usually the right order of magnitude for a plate where the risk is a single pipetting loss rather than a repeated transfer. Notice too that the inputs are in millilitres only, so a microlitre scale preparation has to be converted before it is entered, and the volume unit converter is the place to do that.

A flat percentage is a rough answer to a question that is not really about percentages. What it absorbs is dead volume: the film left in a pipette tip, the residual in the source vessel, the volume a tip holds but cannot deliver, the drop that stays in the well. Dead volume is an absolute quantity, roughly fixed for a given tip, so the percentage it represents falls as the per sample volume rises. A per sample loss of 0.05 mL is five percent of a one millilitre sample and half a percent of a ten millilitre sample, from the same pipette and the same careless transfer.

The loss also multiplies with the number of transfers. A sample needing three transfers carries roughly three times the loss of one needing a single transfer, so the volume required grows as sample volume multiplied by count multiplied by transfers. That product is what the overage percentage is really trying to estimate, and it is why the right overage depends on the method rather than on the number of samples. A flat figure that suits a direct transfer from a reservoir will fall short for a prepared serial set, where the serial dilution calculator is the relevant tool for the transfer structure itself.

Evaporation and temperature drift sit on top of both effects and are not modelled here at all. A prepared set left open across a long session loses solvent, which raises the concentration of every sample in it by the fraction of volume that went. Whether that matters depends on the tolerance of whatever the samples feed into, and it is entirely invisible in a total volume figure, which is the point at which it stops being an arithmetic problem and becomes a handling one. The guide to rounding and small volume precision covers the related question of how far small volume arithmetic stays trustworthy.

The count is fixed the moment it is typed, so what counts is worth settling first. Controls, blanks and the whole calibration curve are samples too, and they are the ones most often left out because they are not the thing being measured. Every point on a standard curve needs its own preparation at the same per sample volume as the unknowns, so a six point curve with duplicates has already added a dozen vessels before the unknowns begin. Where the curve levels are made by scaling one stock preparation to different volumes, the batch scaling calculator is the arithmetic for that, and each level still needs an overage of its own.

Over-counting by a handful is cheap and running short is not. A set that ends one sample short has already been made with whatever overage was chosen and cannot be topped up without disturbing what has already been dispensed. The count also has to be a whole number, so a planned number of samples gets rounded up before it is entered rather than after. If some samples are likely to be repeated, count that possibility now instead of treating it as a later problem, and remember that what comes out is a volume, so a surplus expressed in millilitres has to be divided back by the per sample volume to see how many spare samples it actually represents.

What comes out is a volume to prepare, not a method for preparing it. To actually make it you need the mass of compound that volume corresponds to and the concentration you want, and those are a different pair of calculations. Starting from a solid, the mass follows from concentration multiplied by volume, which is the stock solution calculator . Starting from a stock, the volume of stock follows from the C1V1 relationship with the diluent volume being the remainder, and the C1V1 dilution calculator makes that split in one step.

Once the stock volume and the diluent volume are known, combining them is the volume weighted average on the solution mixing calculator , which is the same reasoning run backwards: the total amount of solute is fixed by what was weighed out, so the final concentration follows from the final volume. If one preparation has to serve a second purpose at a different size then that ratio is a batch scaling problem once more. And the overage added here is a margin on the volume alone. Nothing on this page decides the concentration, checks that the required mass will dissolve in that volume, or confirms that a stock at a workable concentration exists to be diluted in the first place.