Pack size is not the same as the experiment size

The number on the label is the size of the container. The number your design consumes is a different sum: every unknown, every control, the dead volume you cannot recover, and one reasonable repeat when a plate or a gel fails for ordinary reasons. Confusing the two is how a quotation looks sufficient and the bench runs dry on a Thursday. This comparison is part of specifying a purchase so the quotation can be checked, which is the job described in how to write a laboratory sourcing enquiry. Send the two numbers, labelled, through laboratory procurement. A university laboratory can start from academic research. Place the file with a quotation request or contact. A scope can be discussed. This page states no prices.

Two quantities that share a unit

Pack size is what the seller puts in one container: a fill volume, a mass, a number of pieces, or a number of reactions as the seller defines a reaction. It is a packaging fact. You can check it on receipt by reading the label. You cannot pipette the label.

Experiment size is the amount the design consumes before you are willing to call the run finished. It includes the unknowns you mean to measure, the controls the design requires, dead volume left in the vial and in tips or tubing, and a reasonable repeat. "Reasonable" is a number you choose in advance, such as one extra plate because a sealing failure or a power interruption at the cycler is ordinary, not a licence to reorder forever.

Both quantities may be millilitres or micrograms. They answer different questions. Pack size answers "what will the box contain?" Experiment size answers "what must I have in hand to finish this design?" A quotation that matches the first and ignores the second can be commercially tidy and scientifically short.

What sits inside experiment size

Start from the protocol's working volume, which lives in your SOP. Follow that SOP and the manufacturer's product instructions for the concentration you will actually use. Do not invent a microlitre table from a brochure.

Dead volume is the part you pay for and do not transfer into the reaction. A vial with a conical bottom still leaves a film. A multichannel reservoir leaves a deeper puddle than a single tip. Automated liquid handlers declare a dead volume in their own manuals; if you use one, use that figure rather than a hand-pipette habit. Accurate pipetting is the bench skill. The sourcing note only needs the allowance you will add so the allowance is not a surprise.

Controls are samples in the scientific sense. A no-template control, a positive control, a standard curve with its points, and blank wells all consume the same master mix as the unknowns. Count them. A design that boasts twenty-four samples and hides eight standards has thirty-two wells of consumption before any repeat.

A reasonable repeat is the rerun you already know you might need. One extra set of the master mix for a single failed plate is a planning choice. Three extra sets "just in case" is a different choice, and you should label it as contingency rather than pretend it is the core design. Write the number down either way.

Open-container life is the other constraint. Some reagents are stable for the life of the sealed pack and much less stable after the first puncture, especially if they are frozen, hygroscopic, or light-sensitive. The manufacturer's storage statement sets that limit. If the experiment size is smaller than the pack but the work is spread over months, the pack may still be the wrong container because the tail of it will be outside the open-vial window. Two smaller packs can match the same experiment size with less waste of degraded material. Again, this is a stability comparison, not a price comparison.

A worked qPCR plate

Suppose a relative-expression plate uses 20 microlitres per well, and the master mix is half of that volume, so 10 microlitres of mix per well. You have 20 unknown wells, a standard curve of 6 points in duplicate (12 wells), a no-template control in triplicate (3 wells), and a positive control in triplicate (3 wells). That is 38 wells. Add one reasonable repeat of the same plate because a sealing or cycler problem is a known way to lose a run. The design now wants master mix for 76 wells: 760 microlitres, before dead volume.

The reservoir and the tips will not give you every microlitre. Add an allowance you can defend. A simple bench allowance is to prepare about ten percent extra when you are pipetting by hand from a tube, and to use the handler's stated dead volume when a robot reservoir is involved. Ten percent of 760 is 76, so the experiment size is about 840 microlitres of that mix. Round in the direction of the SOP, and write the rounding down.

Now look at packs. A vial filled to 1 millilitre covers 840 microlitres with a margin, if you can use the vial in one opening and the product allows that. A vial filled to 500 microlitres does not cover the design, even though 500 is more than one plate's 380 microlitres of mix. Someone who compared pack size with a single plate of unknowns only (20 wells, 200 microlitres) would have thought the 500 microlitre vial was lavish. They forgot the curve, the controls, the repeat, and the dead volume.

If the product's instructions say the mix should not sit diluted, you still draw the stock component sizes the same way: scale each component to the experiment size, then see which pack of each component covers that draw. A pack of the polymerase and a pack of the buffer can limit the design independently. The smallest component, not the most impressive box, sets the ceiling.

Check the arithmetic twice. Checking a pipette between calibrations matters because a tired pipette changes what "10 microlitres" means, but it does not change the logic of counting wells. NIST laboratory metrology is the public frame for treating volume as a measurement rather than a label.

Side by side

When the pack is larger, and when it is smaller

If the pack is smaller than the experiment size, the design does not fit in one container. Order the container count that covers the sum, or shrink the plate on purpose and rewrite the design. Do not plan to borrow an unlabelled aliquot from another group. That aliquot has an unknown open date.

If the pack is larger than the experiment size, ask whether the remainder will still be inside its storage class when you need it. A hygroscopic salt in a humid room can cake after a few openings. A frozen enzyme can suffer from repeated thawing. The manufacturer's limit, not optimism, decides whether the remainder is future experiment or future waste. Split the order into the number of containers that matches how you will open them.

Reaction-count packs need extra care. "Fifty reactions" means fifty of the seller's reaction definition. If your well volume is twice their definition, you have twenty-five of your wells, before controls. Write both definitions in the specification so the quotation cannot answer with a naked reaction count.

Failure modes

The first is counting tubes of samples and forgetting wells of master mix. The second is ignoring dead volume because a careful person "can get it all out". They cannot, not every time, and the specification should not depend on heroics. The third is treating a repeat as a moral failing and therefore leaving it out of the sum. Plates fail for dull reasons. One planned repeat is part of a serious design. An open-ended stack of repeats is a different budget conversation, and it still should be a number rather than a mood.

Public method collections such as Addgene's protocols and protocols.io show how often a workflow's volume is the sum of many small additions. Use them as a reminder to add the additions up. Do not copy their volumes into your purchase line unless they are your SOP.

Safety and research limits

Quantity does not change hazard class, but it changes waste and storage. A larger pack of a flammable or toxic reagent is a larger store, and the institution's chemical hygiene rules decide whether that store is allowed. This comparison is for research planning. It is not a diagnostic protocol, and it does not approve a biosafety level. If the material is infectious, the experiment size still includes controls, and the containment decision still belongs to the institution.

Humidity, power, and a repeat you meant

In a humid laboratory, an opened bottle of a hygroscopic reagent is a different product by the next dry day you expected. Count a reasonable repeat, and also count whether the pack can be resealed and stored as the label says. A power cut during a run is one ordinary reason the repeat exists. You do not need a statistic about outages to justify one extra plate you already decided to allow. Write it into the experiment size so the quotation's pack count can be checked against it.

What to put in the enquiry

State pack size and experiment size as separate lines. Pack size: fill, unit, and how many containers. Experiment size: the sum, with a short note that it includes dead volume, controls, and one named repeat. Ask the quotation to confirm the fill and the unit definition, especially when the seller sells "reactions". Send that through laboratory procurement or academic research, using contact or a quotation request. A scope can be discussed. On receipt, check the label against the pack line. Before you open the box onto the bench, check the plate map against the experiment line one more time, in case the design grew while the quotation was in flight.