Peptide Vial Math

Bacteriostatic water calculator

For a 5 mg vial, 2 mL of bacteriostatic water gives 2.5 mg/mL (1 unit = 0.025 mg) and 1 mL gives 5 mg/mL. The reverse formula: water (mL) = vial mg × target units ÷ (amount mg × 100).

It works in reverse: pick a reading and it returns the water. Confirm with the peptide calculator.

By Peptide Vial Math Editorial Team Reviewed by the team on (how we review) Updated Formulas unit-tested (methodology)

How much water to add?

Unit

Add

2 mL

→ 5 mg/mL; 20 units = 1 mg.

Arithmetic only. Check that the vial physically holds this volume.

The reverse formula

water (mL) = vial (mg) × target units ÷ (amount (mg) × 100)

It's the normal reconstitution formula solved for the water instead of the units. It answers the question people actually ask before mixing: how much should I add so the numbers are easy? A reading of 10, 20, 25 or 50 units lands on a printed major mark on every U-100 barrel size that can hold it.

Water tables by vial size

Water (mL) needed so that each amount reads the target. Dashes mark volumes above 10 mL or below 0.25 mL, which are impractical for typical vials.

Water for a 10-unit reading

mL of water so the amount reads 10 units (U-100)
Vial0.25 mg0.5 mg1 mg2 mg2.5 mg5 mg
2 mg0.80.4————
5 mg210.50.25——
10 mg4210.50.4—
15 mg631.50.750.60.3
20 mg84210.80.4
30 mg—631.51.20.6
40 mg—8421.60.8
60 mg——632.41.2

Water for a 20-unit reading

mL of water so the amount reads 20 units (U-100)
Vial0.25 mg0.5 mg1 mg2 mg2.5 mg5 mg
2 mg1.60.80.4———
5 mg4210.50.4—
10 mg84210.80.4
15 mg—631.51.20.6
20 mg—8421.60.8
30 mg——632.41.2
40 mg——843.21.6
60 mg———64.82.4

Water for a 25-unit reading

mL of water so the amount reads 25 units (U-100)
Vial0.25 mg0.5 mg1 mg2 mg2.5 mg5 mg
2 mg210.50.25——
5 mg52.51.250.630.50.25
10 mg1052.51.2510.5
15 mg—7.53.751.881.50.75
20 mg—1052.521
30 mg——7.53.7531.5
40 mg——10542
60 mg———7.563

Water for a 50-unit reading

mL of water so the amount reads 50 units (U-100)
Vial0.25 mg0.5 mg1 mg2 mg2.5 mg5 mg
2 mg4210.5——
5 mg1052.51.2510.5
10 mg—1052.521
15 mg——7.53.7531.5
20 mg——10542
30 mg———7.563
40 mg———1084
60 mg—————6

What each unit holds at common volumes

Any peptide: mcg per U-100 unit (mg/mL in brackets) by vial size and water added
Vial1 mL2 mL3 mL5 mL
2 mg 20 (2)10 (1)6.67 (0.667)4 (0.4)
5 mg 50 (5)25 (2.5)16.67 (1.67)10 (1)
10 mg 100 (10)50 (5)33.33 (3.33)20 (2)
15 mg 150 (15)75 (7.5)50 (5)30 (3)
20 mg 200 (20)100 (10)66.67 (6.67)40 (4)
30 mg 300 (30)150 (15)100 (10)60 (6)
40 mg 400 (40)200 (20)133.3 (13.33)80 (8)
60 mg 600 (60)300 (30)200 (20)120 (12)

Background on the diluent itself: what bacteriostatic water is.

Frequently asked questions

How much bacteriostatic water do I add to a 10 mg vial?

Choose the reading you want first. For 1 mg to read 10 units, add 1 mL (10 mg/mL). For 1 mg to read 20 units, add 2 mL (5 mg/mL). For 2 mg to read 20 units, add 1 mL. Formula: water mL = vial mg × target units ÷ (amount mg × 100).

How much bac water for a 5 mg peptide vial?

1 mL gives 5 mg/mL (50 mcg per unit), 2 mL gives 2.5 mg/mL (25 mcg per unit), 2.5 mL gives 2 mg/mL (20 mcg per unit). For 250 mcg to read 10 units, add 2 mL.

Is more bacteriostatic water better?

Not inherently. More water makes each unit hold less, which helps small readings but makes large amounts take more volume and may exceed the barrel or the vial. It never changes the total amount in the vial.

What is the formula for how much water to add?

water (mL) = vial (mg) × target units ÷ (amount (mg) × 100) on a U-100 syringe. Example: 10 mg vial, 2 mg should read 40 units → 10 × 40 ÷ (2 × 100) = 2 mL.

Sources

  1. Bacteriostatic Water for Injection, USP (Hospira) — prescribing label — DailyMed, U.S. National Library of Medicine
  2. Preventing Unsafe Injection Practices (multi-dose vial beyond-use dating) — U.S. Centers for Disease Control and Prevention

Links checked 2026-10-19.