Key takeaways
- mcg = units ÷ 100 × mcg per mL on a U-100 syringe. 10 units of a 2.5 mg/mL mix is 250 mcg.
- The shortcut is mcg per unit = mg/mL × 10. At 5 mg/mL each unit holds 50 mcg.
- The same reading means 2.5 times more on a U-40 syringe, because a U-40 unit is 0.025 mL.
- Exact multiples of 10, 100, 1,000 or 2.5 between what you drew and what you meant almost always point to a unit or syringe mix-up.
- No concentration, no answer. Units alone never tell you the dose.
How to use the reverse peptide calculator
A normal peptide calculator starts from a dose and tells you how many units to draw. This one runs the other way. You start with a syringe reading and find out what it contains.
- Units drawn. Read the syringe at the top edge of the plunger's rubber stopper, not the tip of the plunger.
- Syringe type. Choose U-100 or U-40 as printed on the barrel. Unit counts for peptides and compounded GLP-1 drugs are normally written for U-100 syringes, as in the FDA semaglutide alert.
- Concentration. Enter the vial mg and the water you added, or switch to mg/mL known and type the strength from a pharmacy label. You can also enter how many doses you have already taken to see how many are left.
- Dose you meant to draw (optional). Enter it and the calculator shows the difference in mcg and percent, and names the likely cause if the gap is a telltale multiple.
- Read the result. The big number is the dose in mcg. Below it you get mg, mL, the share of the vial and the doses left. Show the math lays out each step.
When to use a reverse calculation
Reverse math is a check. It tells you what is actually in the syringe. Four situations call for it.
Checking a prefilled syringe
Some pharmacies send syringes already drawn up, labeled with a concentration and a number of units or mL. Multiply the volume by the concentration and confirm that it matches the dose on your prescription before you inject.
Starting a new vial
A new vial is where most mixing errors happen. If you added a different volume of water than last time, the same number of units now holds a different dose. Run the reverse check on your first draw. To plan the water volume itself, use the BAC water calculator.
A dose someone else drew
When a partner, parent or caregiver draws the dose, you only see the syringe. Enter the reading and the vial details to confirm it. Check the syringe type too: a U-40 syringe changes everything.
Reading old instructions
Instructions that say only "draw 20 units" are incomplete. Units are a volume. Without the concentration, 20 units could be 200 mcg or 2 mg.
The units-to-dose formula
Units measure volume, not peptide. To get the dose you convert units to mL, then multiply by the concentration.
Skip this step if the label already gives mg/mL. Multiply mg/mL by 1000 to get mcg/mL.
Check: 10 × 5 × 10 ÷ 2 = 250 mcg.
| Term | Meaning | Unit |
|---|---|---|
| Units | The syringe reading | units |
| Vial | Total peptide in the vial, from the label | mg |
| Water | Diluent added to the vial | mL |
| C | Concentration after mixing | mcg/mL or mg/mL |
| V | Volume in the syringe | mL |
| Dose | Peptide in the syringe | mcg or mg |
How many mcg are in 10 or 20 units?
Find your concentration across the top and your reading down the side. Every figure assumes a U-100 syringe. For example, 20 units of a 5 mg/mL solution holds 1,000 mcg, and 10 units of a 2.5 mg/mL solution holds 250 mcg.
| Units | mL | 1 mg/mL | 2 mg/mL | 2.5 mg/mL | 5 mg/mL | 10 mg/mL |
|---|---|---|---|---|---|---|
| 5 units | 0.05 | 50 | 100 | 125 | 250 | 500 |
| 10 units | 0.1 | 100 | 200 | 250 | 500 | 1,000 |
| 20 units | 0.2 | 200 | 400 | 500 | 1,000 | 2,000 |
| 25 units | 0.25 | 250 | 500 | 625 | 1,250 | 2,500 |
| 50 units | 0.5 | 500 | 1,000 | 1,250 | 2,500 | 5,000 |
| 100 units | 1 | 1,000 | 2,000 | 2,500 | 5,000 | 10,000 |
Values in mcg. Divide by 1,000 for mg. The mcg to mg converter handles other conversions.
Units to mg for common vial mixes
If you know the vial size and the water, you do not need to work out mg/mL first. Find the mix below. For a 10 mg vial mixed with 2 mL, 50 units holds 2.5 mg.
| Vial + water | mg/mL | Per unit | 10 units | 25 units | 50 units |
|---|---|---|---|---|---|
| 5 mg + 1 mL | 5 | 50 mcg | 500 mcg | 1.25 mg | 2.5 mg |
| 5 mg + 2 mL | 2.5 | 25 mcg | 250 mcg | 625 mcg | 1.25 mg |
| 5 mg + 2.5 mL | 2 | 20 mcg | 200 mcg | 500 mcg | 1 mg |
| 10 mg + 1 mL | 10 | 100 mcg | 1 mg | 2.5 mg | 5 mg |
| 10 mg + 2 mL | 5 | 50 mcg | 500 mcg | 1.25 mg | 2.5 mg |
| 10 mg + 3 mL | 3.333 | 33.3 mcg | 333.3 mcg | 833.3 mcg | 1.667 mg |
| 15 mg + 3 mL | 5 | 50 mcg | 500 mcg | 1.25 mg | 2.5 mg |
| 20 mg + 2 mL | 10 | 100 mcg | 1 mg | 2.5 mg | 5 mg |
| 30 mg + 3 mL | 10 | 100 mcg | 1 mg | 2.5 mg | 5 mg |
Per unit = mg/mL × 10 mcg. For a mix not listed, use the calculator or the reconstitution calculator.
GLP-1 vials follow the same math. The tirzepatide calculator and semaglutide calculator have presets for common vial sizes, and both show the units for a dose in mg.
U-100 vs U-40 syringes
The number after the U is how many units fit in 1 mL. A U-100 syringe marks 100 units per mL, so one unit is 0.01 mL. A U-40 syringe marks 40 units per mL, so one unit is 0.025 mL, 2.5 times as much. U-40 syringes are made for U-40 insulin, such as the veterinary insulin Vetsulin, and for U-40 human insulin still sold in some countries. The Vetsulin label warns that using any other syringe with it will result in incorrect dosing. The same logic works in reverse: a unit count written for U-100 is wrong on a U-40 syringe.
The table puts numbers on it for a 2.5 mg/mL mix, such as 5 mg in 2 mL.
| Reading | U-100 volume | U-100 dose | U-40 volume | U-40 dose |
|---|---|---|---|---|
| 5 units | 0.05 mL | 125 mcg | 0.125 mL | 312.5 mcg |
| 10 units | 0.1 mL | 250 mcg | 0.25 mL | 625 mcg |
| 20 units | 0.2 mL | 500 mcg | 0.5 mL | 1.25 mg |
| 40 units | 0.4 mL | 1 mg | 1 mL | 2.5 mg |
Many U-40 syringes have a red cap and red markings, while U-100 syringes usually have an orange cap. Always read the label on the barrel or box rather than relying on color.
How to read a syringe accurately
A reverse check is only as good as the reading you put into it. Three habits make it trustworthy.
- Read at the right edge. The dose is measured at the top ring of the rubber stopper, the edge closest to the needle. The far end of the plunger tells you nothing.
- Know your marks. Many 1 mL syringes print a line every 2 units, 0.5 mL syringes every unit, and some 0.3 mL syringes add half-unit lines. Count from the nearest printed number, not from zero.
- Clear the air. An air bubble takes up space on the scale but holds no peptide, so a bubbly draw reads higher than what it contains. Tap the bubble to the top and push it out before you read.
Small draws magnify reading errors. At 5 units, being off by one unit changes the dose by 20%. At 50 units, the same slip is 2%. If your reading is under about 10 units, a smaller syringe or a more dilute mix makes the check more reliable. The mg to units calculator shows which syringe size suits a dose.
Worked examples
Example 1: checking the first draw from a new vial
You mixed a 10 mg vial with 3 mL and drew 30 units on a U-100 syringe. The concentration is 10,000 ÷ 3 = 3,333.3 mcg/mL. 30 units is 0.3 mL, so the syringe holds 0.3 × 3,333.3 = 1,000 mcg (1 mg), or 10% of the vial.
Example 2: a prefilled syringe labeled 2.5 mg/mL
The syringe reads 20 units. That is 0.2 mL × 2.5 mg/mL = 0.5 mg (500 mcg). If the prescription says 0.5 mg, it matches. If it says 0.05 mg, the syringe holds ten times too much.
Example 3: a dose drawn on the wrong syringe
The plan was 250 mcg from a 5 mg vial with 2 mL, which is 10 units on a U-100 syringe. Someone drew 10 units on a U-40 syringe instead. That is 0.25 mL × 2,500 mcg/mL = 625 mcg, 2.5 times the plan. The calculator flags the 2.5× pattern as a syringe mix-up.
Example 4: an FDA-reported error, checked in reverse
The FDA described a provider who intended 0.25 mg of compounded semaglutide (5 units) but prescribed 25 units. Working backwards, 5 units holding 0.25 mg means 5 mg/mL. At that strength, 25 units holds 1.25 mg, 5 times the intended dose. A 5× gap is not a standard mix-up multiple, but the calculator still flags anything more than 10% off.
Example 5: how many doses are left
A 10 mg vial mixed with 2 mL, 50 units per dose, and 2 doses already taken. Each dose is 0.5 mL (2.5 mg), so 1 mL remains: 2 more doses. In practice a little liquid stays in the vial and the needle hub, so the last draw may come up short. The doses per vial calculator turns this into dates on a schedule.
How the calculator spots dosing errors
When you enter the dose you meant to draw, the calculator divides the actual dose by the intended one. A ratio close to 1 is fine. Anything more than 10% off gets a warning. A ratio close to one of four exact multiples gets a specific warning, because each multiple has a well-known cause.
| Multiple | If too high | If too low | Most likely cause |
|---|---|---|---|
| 1,000× | 250 mg | 0.25 mcg | A mg/mcg mix-up (1 mg = 1,000 mcg) |
| 100× | 25 mg | 2.5 mcg | An mL/units mix-up (1 mL = 100 units on a U-100 syringe) |
| 10× | 2.5 mg | 25 mcg | A misplaced decimal, such as 5 units read as 50, or 0.05 mL as 0.5 mL |
| 2.5× | 625 mcg | 100 mcg | A U-40/U-100 syringe mix-up (a U-40 unit holds 2.5 times the volume) |
These are not theoretical. The FDA has reported patients taking five to 20 times their intended dose of compounded semaglutide. In one case described in its alert, patients told to draw 5 units (0.05 mL) drew 50 units instead. In another, a provider prescribed 20 units instead of 2. Both are 10× errors.
Common mistakes
Each of these maps to a warning in the calculator.
- Forgetting the water changed. 20 units from a 1 mL mix holds twice as much as 20 units from a 2 mL mix.
- Using a U-40 syringe with U-100 numbers. The draw holds 2.5 times the intended dose.
- Reading mL as units. 0.1 mL is 10 units on a U-100 syringe. Writing "10" when you mean 0.1 mL is a 100× error.
- Mixing up mg and mcg. Entering a concentration of 2,500 mg/mL when the label says 2,500 mcg/mL is a 1,000× error. The calculator flags very high mg/mL values.
- Counting marks wrong. A 1 mL syringe is often marked every 2 units. Read the numbers, not the lines.
- Reading the wrong edge. Read at the top of the rubber stopper, the edge nearest the needle.
- Drawing more than the vial holds. The calculator warns when the draw is larger than the water in the vial.
For a dose you have not drawn yet, start from the other side with the peptide dosage calculator.
Key terms
- Unit (syringe)
- A volume mark on an insulin syringe. On U-100 syringes one unit is 0.01 mL; on U-40 syringes it is 0.025 mL.
- U-100
- A syringe scale of 100 units per mL, and the scale most peptide and GLP-1 unit counts are written for.
- U-40
- A syringe scale of 40 units per mL, made for U-40 insulin used in veterinary care and, in some countries, for people.
- Concentration
- Peptide per volume of solution, written as mg/mL or mcg/mL.
- mcg per unit
- The peptide in one U-100 unit. It equals mg/mL × 10.
- Reverse calculation
- Working from a syringe reading back to the dose it contains.
Frequently asked questions
How many mcg are in 10 units?
It depends on the concentration. On a U-100 syringe 10 units is 0.1 mL. At 2.5 mg/mL (5 mg in 2 mL) that is 250 mcg. At 5 mg/mL it is 500 mcg, and at 10 mg/mL it is 1,000 mcg. Use the chart above or enter your own numbers in the calculator.
How much peptide is in 20 units?
20 units on a U-100 syringe is 0.2 mL. Multiply by the concentration. From a 5 mg vial mixed with 2 mL, 20 units holds 500 mcg (0.5 mg). From a 10 mg vial mixed with 2 mL, it holds 1 mg.
How do I convert units to mg?
Divide the units by 100 to get mL on a U-100 syringe, then multiply by the concentration in mg/mL. 25 units at 4 mg/mL is 0.25 mL × 4 = 1 mg. To go the other way, from mg to units, use the mg to units calculator.
How many units are in 1 mL?
100 units on a U-100 syringe and 40 units on a U-40 syringe. That is why 1 unit is 0.01 mL on U-100 and 0.025 mL on U-40. The barrel size (0.3, 0.5 or 1 mL) does not change this; it only changes how many units fit.
Does a 0.3 mL syringe hold a different dose per unit than a 1 mL syringe?
No. Every U-100 syringe uses 100 units per mL, so one unit is always 0.01 mL. A 0.3 mL syringe simply holds fewer units and spaces the marks further apart, which makes small doses easier to read. The dose only changes if the scale changes, as it does on a U-40 syringe.
What if I do not know how much water was added?
Then the dose cannot be worked out from the units. The concentration is the missing piece. Check the vial label or ask whoever mixed it. If the vial came from a pharmacy, its label should state mg/mL. Once you know the volume, the reconstitution calculator gives the concentration.
Why is my result exactly 2.5 times what I expected?
A 2.5× gap is the signature of a syringe mix-up. A U-40 unit holds 0.025 mL and a U-100 unit holds 0.01 mL. Drawing a U-100 unit count on a U-40 syringe gives 2.5 times the intended dose; the reverse gives 40% of it.
How many doses are left in my vial?
Enter the vial mg, the water you added and the doses already taken. The calculator subtracts the volume used and divides what remains by the volume per draw. The doses per vial calculator also shows when a vial will run out.
Is this calculator a dosing recommendation?
No. It converts a syringe reading into mcg and mg and checks it against the number you enter. It does not tell you what dose to take, and the figures in its tables only show the arithmetic. Follow your prescription label, your pharmacist and your clinician.
Sources
We check every formula and figure on this page against primary sources. See our editorial policy for how we review content.
- FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products
- Vetsulin (porcine insulin zinc suspension) 40 IU/mL: label and U-40 syringe instructions
- Errors of insulin therapy: real-life experiences from developing world (Chowdhury and Chakraborty, J Family Med Prim Care)
- Preventing unsafe injection practices
Reviewed by the PeptideConvert Editorial Team
Last reviewed . PeptideConvert tools do arithmetic only. They do not recommend a dose, and nothing on this site is medical advice. Always follow the label and your prescriber. Read the full medical disclaimer.