Peptide Reconstitution Calculator: The Math

Reconstitution arithmetic is one division and one multiplication. It causes more confusion than it should, mostly because the calculation is done after the water has already gone in, at which point it is no longer a calculation but a discovery.

The only formula you need

concentration (mg/mL) = peptide mass (mg) / water volume (mL)

That is the whole thing. A 10 mg vial reconstituted with 2 mL gives 5 mg/mL. The same vial with 5 mL gives 2 mg/mL. Neither answer is more correct than the other. What matters is that the number was chosen on purpose and written on the label.

Rearranged for the case where you know the concentration you want:

water volume (mL) = peptide mass (mg) / target concentration (mg/mL)

A 10 mg vial at a target of 2.5 mg/mL needs 4 mL of water.

A reference table

For the vial sizes research peptides usually ship in:

5 mg vial

• 1 mL gives 5 mg/mL
• 2 mL gives 2.5 mg/mL
• 2.5 mL gives 2 mg/mL
• 5 mL gives 1 mg/mL

10 mg vial

• 1 mL gives 10 mg/mL
• 2 mL gives 5 mg/mL
• 4 mL gives 2.5 mg/mL
• 5 mL gives 2 mg/mL
• 10 mL gives 1 mg/mL

2 mg vial

• 1 mL gives 2 mg/mL
• 2 mL gives 1 mg/mL
• 4 mL gives 0.5 mg/mL

The pattern is the same in every case, which is the point. Nothing about a particular compound changes the division.

Converting a mass to a volume

Once the concentration is known, the volume that contains a given mass follows directly:

volume in mL = mass in mg / concentration in mg/mL

At 2.5 mg/mL, a 0.5 mg measurement occupies 0.2 mL. At 5 mg/mL the same 0.5 mg occupies 0.1 mL.

Notice what happened there. Doubling the concentration halved the volume, and halved the number of graduations available to measure it. That is the tradeoff that should drive the choice of water volume.

Choosing a volume you can actually measure

Two constraints decide the number, and neither is chemistry.

Instrument resolution. A graduated instrument resolves to its smallest marking, and reading between graduations is guesswork. On one marked in 0.01 mL steps, a measurement of 0.04 mL misread by one mark is a 25% error. The same misread on a 0.4 mL measurement is 2.5%. More dilute is easier to measure accurately, and that is usually the deciding argument.

Vial capacity. The vial has a physical ceiling. A 2 mL vial cannot take 5 mL of water no matter what the arithmetic says. When a protocol calls for a dilution the vial cannot hold, reconstitute at a higher concentration and perform a secondary dilution into a separate sterile container, recording both steps rather than pretending it was one.

Between those two, aim for the most dilute preparation the vial will physically accept and the storage space allows.

Worked example

A 10 mg vial. The target concentration is 2 mg/mL.

1. Water volume: 10 mg / 2 mg/mL = 5 mL.
2. Check the vial holds 5 mL. If it is a 10 mL vial, it does.
3. Add 5 mL of bacteriostatic water down the inside wall, following the method in how to reconstitute peptides.
4. A 0.4 mg measurement is 0.4 / 2 = 0.2 mL, so the measurement is 0.2 mL.
5. Label the vial: compound, 2 mg/mL, date reconstituted, date to discard.

Step 5 is not optional. A reconstituted vial with no concentration written on it cannot be recovered by inspection, because the solution looks identical at every concentration. The compound specific version of this arithmetic is worked through in the BPC-157 guide.

Two vials, one concentration

Where a protocol runs across more than one vial of the same compound, reconstitute every vial to the same concentration. It costs nothing and it removes an entire category of error, because a measurement worked out for one vial then transfers to the next without recalculation.

Where concentrations genuinely have to differ, the vials need visibly different labels. Two identical vials at different concentrations, distinguishable only by small print, is a setup that eventually produces a tenfold mistake.

The errors this arithmetic invites

• Adding water first. Once it is in, the concentration is fixed. There is no undo.
• Confusing units with millilitres. A unit is 0.01 mL. Reading a unit as a millilitre is a hundredfold error.
• Assuming the powder adds volume. At these masses the contribution of the solid is negligible, and the convention is to ignore it. Do not try to correct for it.
• Not writing the concentration down. The single most common irrecoverable mistake in the whole workflow.

Where to go next

• Why You Swirl Peptides Instead of Shaking

The preparations in this library use bacteriostatic water with 0.9% benzyl alcohol, supplied for laboratory research in 10 ml and 30 ml vials.