How a Peptide Calculator Works — The Math Behind Reconstitution

How It Works

The formula is simple.
The variable that matters is one.

Vial size and dose are fixed. The only choice you make is how much bacteriostatic water to add — and that single decision determines every syringe draw you take from that vial.

The Math

Concentration Is Everything

A peptide reconstitution calculator takes three inputs: the vial size in mg or mcg, the bacteriostatic water volume in mL, and your target dose in mcg. From these it derives concentration (mcg per mL), then converts that to syringe units for a U-100 insulin syringe.

The formula has two steps. Concentration equals vial size divided by BAC water volume. Draw units equal your dose divided by concentration, multiplied by 100. A 5mg vial plus 2mL BAC water produces a 2,500 mcg/mL solution. A 250mcg dose in that solution is 10 units on a U-100 syringe. That is the entire calculation.

"One variable. One formula. The rest is arithmetic."

The practical implication: choose your BAC water volume to produce a syringe draw in the 10–20 unit range for your dose. Draws below 5 units introduce measurement error. Draws above 40 units are fine but unnecessary. Protocol's Custom Draw mode lets you specify the draw units you want and calculates the required BAC water amount — the only calculator that works in this direction.

Step by Step

The Reconstitution Sequence

Enter the vial

Your peptide amount in mg. Convert to mcg for the formula: 5mg = 5,000mcg. Vial size is fixed — you can't change it once you've ordered.

Choose the BAC water volume

This is your only decision. It determines concentration. Standard: 2mL for a 5mg vial gives 2,500 mcg/mL. Adding 4mL gives 1,250 mcg/mL and doubles your draw units per dose.

Enter your dose

Your target dose in mcg. This comes from your protocol — 250mcg, 500mcg, 1mg (1,000mcg), or whatever your compound and schedule require.

Read the draw

The calculator returns syringe draw units (for U-100), draw volume in mL, and doses per vial. Fill your syringe to the stated unit marking on every dose.

The full calculation suite — including Custom Draw (which works backwards from draw units to BAC water amount) and a PK half-life plotter for 22 compounds — is available free at reconstitution.health and in the Protocol app.

Common Questions

FAQ

What is bacteriostatic water and why does the calculation need it?

Bacteriostatic water is the diluent — the liquid that converts freeze-dried peptide powder into an injectable solution. The volume of BAC water you add is the only variable in the concentration formula. More BAC water means lower concentration and higher draw units per dose. Less means higher concentration and lower draw units. The calculator needs this input to derive everything else.

What happens if I calculate wrong?

If you add the wrong BAC water volume and use the calculated draw units anyway, every dose will be at the wrong concentration. The vial will look identical — there's no visual indicator of an error. The only check is to recalculate before reconstituting and verify the draw units match expectations. If your draw seems unexpectedly small (under 5 units) or large (over 50 units) for a standard dose, recheck your BAC water entry.

Is a U-100 syringe the standard for peptide injections?

Yes. U-100 insulin syringes (100 units = 1mL) are the standard for subcutaneous peptide injections. They're available in 0.3mL (30 unit), 0.5mL (50 unit), and 1mL (100 unit) sizes. Use the size appropriate for your draw volume. Never use a U-40 syringe (40 units = 1mL) with U-100 calculations — the unit graduation is different and will produce a 2.5× dosing error.