Peptipedia AI active · 2 h ago

Reverse dose calculator

Was there too little in the vial at your last planned injection (the dose came up short) - or, the other way round, did an unexpected remainder stay behind? No drama, measuring a few hundredths of a millilitre by hand goes wrong now and then. Just tell us below what happened; we guide you step by step and work out what you really had per shot. The only thing that matters is that in the end the whole liquid is accounted for - so the vial is empty.

Note: a calculation aid only, not medical advice and not a dosing recommendation. When in doubt, consult a physician.

How the reverse calculation works

The trick: once a vial is empty, you do not need to know the exact water amount. The full active ingredient (e.g. 5 mg tirzepatide) was dissolved in the total volume you drew. On a U-100 syringe, 100 units equal 1 ml, so the sum of all your injection units is the real volume. Divide the milligrams by the total units and you get the real mg per unit - and from there the real dose of each single injection.

This only works when the vial is truly empty (all units accounted for). If there is still solution left and you do not know the real water amount, the maths cannot reconstruct it yet - finish the vial first.

Tip: a free account gives you the injection manager - it logs your vials and injections and offers this back-calculation automatically when a vial runs short. To the injection manager

Frequently asked questions

What is a peptide calculator?

A peptide calculator works out the exact liquid volume you need to draw up to inject a given dose. Because peptides ship as a powder (lyophilisate), you first mix them with bacteriostatic water (BAC). The calculator removes the guesswork from converting milligrams of powder into millilitres on the syringe.

Why is the calculation so important?

Peptides are highly potent, so a wrong dose can reduce effectiveness or increase side effects. The calculation matters because tiny volume differences translate into large dose differences. The calculator removes the risk of human arithmetic errors and keeps the maths reproducible.

What is BAC water?

Bacteriostatic water contains about 0.9 % benzyl alcohol, which inhibits bacterial growth in an opened vial. It matters because it lets you store a reconstituted vial for several uses. Never use tap water or boiled water for injections.

How do I calculate the right peptide dose?

Volume to draw up (ml) = (target dose ÷ vial strength) × BAC water added. Example: 250 mcg target, 5 mg (5000 mcg) vial, 2 ml water → (250 ÷ 5000) × 2 = 0.1 ml. On a U100 syringe, 0.1 ml = 10 units (IU). This formula always converts your target dose into a volume you can read on the syringe.

Units (IU) or ml - what counts on the syringe (U-100, U-40, 50-unit syringe)?

What matters is the syringe's scale (U-100 or U-40), not its size. On the common U-100 insulin syringe: 100 units (IU) = 1 ml. So 50 IU = 0.5 ml, 20 IU = 0.2 ml, 10 IU = 0.1 ml.

Don't confuse the scale with the barrel size: U-100 syringes come in different capacities. The most common for peptides is the "50-unit" (0.5 ml) - it holds at most 50 IU = 0.5 ml. A full millilitre does not fit; 1 ml equals 100 IU, i.e. a full 1-ml/100-unit syringe. Common U-100 sizes:

  • 0.3 ml = 30 IU (U-100)
  • 0.5 ml = 50 IU (U-100) - the most common
  • 1.0 ml = 100 IU (U-100)

Less common is the U-40 scale (40 IU = 1 ml, i.e. 10 IU = 0.25 ml), typical for some veterinary insulins. Always read the "U-100" or "U-40" marking on the syringe and convert accordingly. Our injection calculator gives you the right volume.

How much BAC water should I use?

Choose a practical amount: not too little (dissolves poorly, stings) and not too much (injection volume too large). For standard vials (5-10 mg), 2-3 ml is a proven default. More water lowers the concentration, so you draw up a larger volume for the same dose.

Can I change the concentration?

Yes. More water lowers the concentration, so you draw up a larger volume for the same dose; less water does the opposite. Adjust the calculation accordingly - the calculator does this automatically once you change the water volume.

My vial has less BAC water than planned - how do I recalculate the dose correctly?
Peptides such as tirzepatide (a GLP-1/GIP peptide hormone) and other GLP-1 analogues become more concentrated than planned when less BAC water is in the vial than intended. Every unit on the syringe then contains more mg of active ingredient than your calculator assumed. Recalculate the actual concentration: concentration (mg/ml) = peptide in the vial (mg) ÷ water actually added (ml). From that: volume per dose (ml) = desired dose (mg) ÷ actual concentration; on a U-100 syringe (100 units = 1 ml) multiply by 100 to get insulin units. In practice:
  • Recalculate the actual concentration (mg/ml) using the dose guide
  • Units (IU) × 0.01 ml = ml per dose
  • Remaining doses = leftover volume in vial ÷ volume per dose
  • Use up the remaining vial, then start fresh with a correctly mixed one
Important hindsight: if you dosed by units based on the planned (lower) concentration, you actually drew up more mg per syringe than intended. That is why the vial ran out earlier than expected. For the next vial, plan the water amount with the peptide calculator first: more water gives a larger, easier-to-read volume and reduces measurement error when drawing up.

Work it out: if your vial already came up short, the reverse dose calculator reconstructs how many mg you really had per injection.