Reconstitution Math

Reconstitution is the process of dissolving a lyophilized (freeze-dried) peptide powder in bacteriostatic water. Once reconstituted, you draw a fraction of the vial for each dose. My Pep Calc handles this calculation — this page explains the math behind it.

The core formula

After reconstitution, the vial contains a known total amount of peptide (in micrograms or milligrams) dissolved in a known volume of bac water (in milliliters). That gives a concentration:

Concentration = Total peptide (mcg) ÷ Bac water volume (mL)

Units to draw = (Target dose (mcg) ÷ Concentration (mcg/mL)) × 100

The ×100 at the end converts milliliters to insulin-syringe units (U100 syringes are marked 1–100, where 100 units = 1 mL).

Worked example

Setup: 5 mg BPC-157 vial, reconstituted with 2 mL bacteriostatic water. Target dose: 250 mcg.

1. Convert vial to mcg: 5 mg × 1,000 = 5,000 mcg total
2. Calculate concentration: 5,000 mcg ÷ 2 mL = 2,500 mcg/mL
3. Units to draw: (250 mcg ÷ 2,500 mcg/mL) × 100 = 10 units

You would draw to the 10-unit mark on a U100 insulin syringe. My Pep Calc shows you this result instantly as you enter the inputs.

A second example — different bac water volume

Same 5 mg BPC-157 vial, but reconstituted with 1 mL bac water. Target dose still 250 mcg.

1. Total: 5,000 mcg
2. Concentration: 5,000 mcg ÷ 1 mL = 5,000 mcg/mL
3. Units to draw: (250 mcg ÷ 5,000 mcg/mL) × 100 = 5 units

With less bac water, the solution is more concentrated — so you draw fewer units for the same dose. This is why the bac water volume matters and is one of the most common sources of reconstitution error.

Common mistakes

Forgetting to convert mg → mcg

Vials are typically listed in milligrams (5 mg, 10 mg). Doses are typically described in micrograms (250 mcg, 500 mcg). 1 mg = 1,000 mcg. If you skip this conversion, your result will be off by a factor of 1,000.

Mixing up the bac water volume

The concentration — and therefore the units to draw — changes depending on how much bac water you added. My Pep Calc stores your bac water volume per vial so you're always calculating from the right number.

Drawing from a partially-used vial

The formula assumes the vial still has its original full amount of peptide dissolved in the original bac water volume. If you've drawn from the vial before, the concentration does not change (you're removing solution, not changing the ratio). You still draw the same number of units for the same dose.

Syringe type mismatch

This formula assumes a U100 insulin syringe (1 mL = 100 units). If you use a U40 syringe or a regular 1 mL syringe without unit markings, the math changes. My Pep Calc defaults to U100; verify your syringe type before drawing.

Half-life and the related concept of active concentration

Once injected, your body clears the peptide over time according to its half-life. Half-life (t½) is the time it takes for half the active compound to be eliminated. The active concentration at time t after injection is:

Concentration(t) = Initial dose × 0.5^(t ÷ t½)

After 1 half-life: 50% remains. After 2: 25%. After 3: 12.5%. After ~3.3 half-lives, about 90% has been cleared.

For BPC-157 with a 4-hour half-life: by 13 hours after injection, 90% has cleared. For semaglutide with a 168-hour half-life: 90% clearance takes about 23 days.

See the Half-Life Chart to visualize these curves.

How My Pep Calc uses this math

The reconstitution calculator takes four inputs you provide:

• Vial size (mg or mcg) — total peptide in the vial
• Bac water volume (mL) — how much you added
• Target dose (mcg) — your intended dose per injection
• Syringe type — defaults to U100

It returns: units to draw, concentration per mL, and doses remaining in the vial. These inputs are saved per compound so you don't re-enter them at every dose.

Questions?

If the math looks wrong, email hello@mypepcalc.com with your inputs and expected output. We'll investigate.

This page is informational only. The formulas are standard and arithmetically correct given correct inputs. Whether the inputs — and the resulting action — are appropriate for you is a decision for you and your licensed healthcare provider. Medical Disclaimer