The planning arithmetic

Total material is uses per week times weeks times amount per use. That gives a mass, which divided by the vial size gives a vial count, which has to be rounded up because a partial vial is not something you can order.

Running the sum for several compounds at once matters because they rarely align. One compound may need three vials for a twelve week programme and another eleven, and the shipping and handling economics of a single order are better than of three staggered ones.

Why a waste allowance is not padding

Every vial loses material that the arithmetic does not see. Liquid stays in the needle and the syringe hub after each withdrawal, some clings to the vial wall above the fill line, and the final withdrawal from a nearly empty vial is difficult to complete cleanly. Across twenty withdrawals these losses commonly amount to about one withdrawal's worth.

A ten percent allowance covers that for a typical vial. Smaller amounts per use, or a needle with a large dead space, justify more; a vial that will be emptied in a handful of large withdrawals justifies less.

• •Zero percent: only defensible if you will not use the last portion of any vial.
• •Five percent: few withdrawals per vial, each of them large.
• •Ten percent: the general case, and the default here.
• •Fifteen to twenty percent: many small withdrawals, or standard high dead space syringes.

The rounding always goes up

Needing 5.1 vials means ordering six. Rounding to five leaves the programme short at the point where it is hardest to fix, which is the end, and a mid-programme reorder introduces a new batch with its own certificate and its own delivery time.

The counterweight is that the surplus is not wasted so long as it stores well. An unopened lyophilised vial kept cold is stable for a long time, so over-ordering by a fraction of a vial costs the price of the vial and nothing else.

What the vial count does not tell you

It says nothing about batch consistency. A programme fed from two batches has a step change in it at whatever point the first batch runs out, and if purity or net peptide content differ between them, that step is real. Ordering the whole quantity in one batch avoids the question; the batch comparison tool addresses it when you cannot.

It also says nothing about shelf life. A count that spans a long programme should be checked against the expiry dates on the vials you actually receive, which the inventory tracker is built for.

How the vial count is calculated

A multiplication per compound, a waste allowance, and a ceiling division. Everything is carried in micrograms internally so that mixed milligram and microgram entries cannot drift.

total uses      = uses per week x weeks
total material  = total uses x amount per use
with allowance  = total material x (1 + waste percent / 100)
vials to order  = ceiling( with allowance / vial size )

1. 1Count the uses. Uses per week times the number of weeks. Entering the programme length in weeks rather than as a date range keeps the arithmetic independent of the calendar.
2. 2Multiply by the amount per use. Gives the total material in micrograms. Working in micrograms avoids a rounding step when amounts per use are small.
3. 3Apply the waste allowance. A percentage uplift covering residual volume in the needle and hub, wall losses and the incomplete final withdrawal from each vial.
4. 4Divide by the vial size and round up. The ceiling is the point of the calculation. Rounding down produces a shortfall at the end of the programme, which is the worst moment to discover one.
5. 5Total across compounds. Each row is rounded independently before summing, because vials are bought per compound and a fractional surplus in one cannot cover a shortfall in another.

What this method cannot tell you

• •It assumes the amount per use stays constant for the whole programme. A plan that changes partway through needs to be entered as two rows.
• •It uses the label mass. For a salt form or a preparation below full purity, the actual peptide per vial is lower, and the net peptide content calculator gives the corrected figure to enter.
• •It does not check expiry. A count spanning many months may exceed the shelf life of the vials supplied.
• •It does not consider batch continuity. Ordering across two batches introduces a discontinuity the arithmetic cannot see.