Supplied strictly for laboratory and research use only. Not for human or veterinary use.
You’ve got a vial of lyophilised peptide on the bench, and before you can do anything useful with it, it has to go into solution. The moment it does, the same question comes up every time: what have I actually got now?
That question really breaks into three smaller ones. What concentration has the solution ended up at? How much peptide is sitting in a given volume? And what volume do I need to measure out to get the mass I’m after?
The peptide calculator below answers all three as you type. Under it, we’ve laid out the arithmetic step by step, worked through real examples, and then covered the part most calculators quietly skip: why the “10mg” printed on the vial isn’t quite the number you should be calculating with.
Quick answer: Peptide concentration = mass in the vial ÷ volume of solvent added. A 10mg vial reconstituted with 2ml of solvent gives 5 mg/ml (5,000 mcg/ml). To find the volume that contains a given mass, divide that mass by the concentration: 250mcg at 5 mg/ml sits in 0.05ml.
Reconstitution calculator
Enter the mass stated on the vial and the volume of solvent added.
Laboratory calculation tool. Supplied for research use only. Not for human or veterinary use.
This Applies to Lyophilised Material
The calculation on this page is for peptide supplied as a freeze-dried (lyophilised) powder in a vial, where you add the solvent yourself and therefore choose the concentration.
Pre-filled pens are different. Pen-format material arrives already in solution. There is nothing to reconstitute, no solvent to add and no concentration to work out, because the manufacturer has already fixed it. The relevant figure for a pen is simply the concentration stated on the product, which should be given as mass per millilitre.
We also supply pre-filled pens
As well as lyophilised vials, Premium UK Peptides supplies selected research peptides in pre-filled pen format. Pens arrive already in solution at a stated concentration, so there is nothing to reconstitute and no calculation to run.
If you are not certain which format a product is supplied in, that will be stated on the product page. The practical differences between the two formats (storage, handling and stability) are covered in Peptide Pens vs Vials.
How to Use the Peptide Calculator
There are four fields. Only the first two are required.
- Peptide in vial (mg). Enter the mass stated on the vial or on the Certificate of Analysis. If you're working from a CoA, use the figure for the batch in front of you, not one you remember from a previous order.
- Solvent added (ml). Enter the volume of solvent you've added, or the volume you're thinking of adding. The result updates straight away, so it's worth trying a few volumes before you commit.
- Target mass (optional). If you need to know which volume holds a particular amount of peptide (for a stock dilution, an assay plate or an aliquot), enter it here and choose mcg or mg from the dropdown.
- Net peptide content (optional). If your CoA reports a net peptide content percentage, enter it. You'll get a second, adjusted set of figures based on the peptide that's actually in the vial, rather than the gross weight of the powder.
The results give you concentration in both mg/ml and mcg/ml, the mass in 0.1ml and in 1ml and, if you entered a target, the volume containing it in ml and µl. Ask for more than the vial holds and it will tell you.
Nothing you type is stored or sent anywhere. The whole calculation runs in your browser.
The Three Formulas Behind the Calculator
There's nothing clever going on here, and that's a good thing. Every reconstitution calculation comes back to three relationships. Once you've got them, you can sanity-check any calculator (including ours) on the back of an envelope.
1. Concentration
Divide the mass of peptide in the vial by the volume of solvent you add.
C = m ÷ V
A 10mg vial reconstituted with 2ml gives 10 ÷ 2 = 5 mg/ml.
2. Mass in a given volume
Multiply the concentration by the volume.
m = C × v
At 5 mg/ml, 0.1ml contains 5 × 0.1 = 0.5mg, or 500mcg.
3. Volume containing a given mass
Divide the mass you want by the concentration.
v = m ÷ C
At 5 mg/ml, 250mcg (0.25mg) sits in 0.25 ÷ 5 = 0.05ml, which is 50µl.
That really is the whole of it. The calculator isn't here because the maths is difficult. It's here because unit conversions are where people slip, usually at the end of a long day, and usually by a factor of a thousand.
Units and conversions
| Unit | Equivalent |
|---|---|
| 1 mg | 1,000 mcg (µg) |
| 1 mcg | 0.001 mg |
| 1 ml | 1,000 µl |
| 1 mg/ml | 1,000 mcg/ml |
| 1 mg/ml | 1 mcg/µl |
That last line is worth remembering. A solution at 2 mg/ml holds 2mcg in every microlitre, which makes quick mental checks much easier when you're working at small volumes.
Where the mg-to-mcg mistake creeps in
Research peptides are usually supplied in single-digit or low double-digit milligram quantities, but they're worked with at microgram scale. That gap is where most arithmetic errors live.
The trouble is that a mg/mcg mix-up never gives you an answer that's slightly wrong. Enter 250 into a field that expects milligrams when you meant micrograms and the result is a thousand times out. It's an easy mistake to make and an easy one to miss, because both numbers look perfectly reasonable on a page.
The simplest defence is a habit: write the unit next to every number, every time, even when it feels obvious. The calculator shows both units side by side for exactly this reason.
Worked Examples
10mg vial, 2ml solvent
- Concentration: 10 ÷ 2 = 5 mg/ml
- In 0.1ml: 0.5mg (500mcg)
- Volume containing 250mcg: 0.25 ÷ 5 = 0.05ml
5mg vial, 3ml solvent
- Concentration: 5 ÷ 3 = 1.667 mg/ml
- In 0.1ml: 0.1667mg (166.7mcg)
- Volume containing 500mcg: 0.5 ÷ 1.667 = 0.3ml
2mg vial, 1ml solvent
- Concentration: 2 ÷ 1 = 2 mg/ml
- In 0.1ml: 0.2mg (200mcg)
- Volume containing 100mcg: 0.1 ÷ 2 = 0.05ml
10mg vial, 2ml solvent, 80% net peptide content
- Actual peptide in the vial: 10 × 0.80 = 8mg
- Concentration: 8 ÷ 2 = 4 mg/ml (not 5)
- Volume containing 250mcg: 0.25 ÷ 4 = 0.0625ml (not 0.05ml)
Look at the first and third examples together. The same 0.05ml contains 250mcg in one and 100mcg in the other, because the concentrations are different. A volume on its own tells you nothing. It only means something alongside the concentration it came from. That is why every aliquot or stock tube should be labelled with its concentration, not just its contents.
And look at the last example. The arithmetic hasn't changed at all, but the volume needed for the same mass has gone up by 25%. We'll come back to why that happens.
Quick reference table
| Vial | Solvent | Concentration (mg/ml) | Concentration (mcg/ml) | Mass in 0.1ml |
|---|---|---|---|---|
| 2mg | 1ml | 2 | 2,000 | 0.2mg (200mcg) |
| 5mg | 2ml | 2.5 | 2,500 | 0.25mg (250mcg) |
| 5mg | 3ml | 1.667 | 1,667 | 0.167mg (166.7mcg) |
| 10mg | 2ml | 5 | 5,000 | 0.5mg (500mcg) |
| 10mg | 3ml | 3.333 | 3,333 | 0.333mg (333.3mcg) |
| 10mg | 5ml | 2 | 2,000 | 0.2mg (200mcg) |
All figures are based on the nominal label mass, with no net peptide content adjustment.
How Much Solvent Should You Add?
This is probably the question we're asked most, and the slightly unsatisfying answer is that there isn't one right volume. It's a trade-off.
More solvent means a lower concentration. Each microgram of peptide then corresponds to a larger volume, and larger volumes are easier to measure accurately. Most pipettes are far less precise at the bottom of their range, so measuring 200µl reliably is easier than measuring 20µl.
Less solvent means a higher concentration. You get a compact stock and smaller working volumes. The downside is that small measuring errors now represent a bigger share of the mass you're transferring.
There's also a chemistry point. At very low concentrations, a larger proportion of the peptide can be lost to the walls of the vial or tube through surface adsorption. Very dilute solutions aren't automatically "safer" in terms of accuracy.
Work backwards from what you need to measure
The most reliable way to choose a volume is to start from the end:
- Decide the smallest mass you'll need to measure out.
- Decide the smallest volume you can measure reliably with the equipment you have.
- Divide the first by the second. That's your working concentration.
- Divide the vial mass by that concentration. That's how much solvent to add.
For example: if the smallest mass you'll need is 100mcg and the smallest volume you trust yourself to measure is 50µl, you need 0.1mg ÷ 0.05ml = 2 mg/ml. For a 10mg vial, that means 10 ÷ 2 = 5ml of solvent.
Then check the obvious thing: will 5ml actually fit in the vial? If it won't, reconstitute at a higher concentration and dilute a portion of it, which brings us to dilution.
Diluting a stock solution
When you need a weaker solution than the one in the vial, use the standard dilution relationship:
C₁ × V₁ = C₂ × V₂
C₁ is your stock concentration, V₁ is the volume of stock you'll take, C₂ is the concentration you want, and V₂ is the final volume you want.
Say you have a 5 mg/ml stock and you want 1ml at 0.5 mg/ml. Rearranged, V₁ = (C₂ × V₂) ÷ C₁ = (0.5 × 1) ÷ 5 = 0.1ml. So you take 0.1ml of stock and make it up to 1ml with 0.9ml of the same solvent.
Working this way also means you only expose a portion of the material to handling, while the rest of the stock stays untouched.
What the Vial Label Doesn't Tell You
This is the section other calculators skip, and if your number has to be defensible (in a lab notebook, a method write-up or a paper), it's the one that matters most.
A calculator takes "10mg" at face value. A laboratory shouldn't.
1. Net peptide content
A vial labelled 10mg usually contains 10mg of material, not 10mg of peptide.
Lyophilised peptide isn't pure peptide. The dried mass also includes the counterion left behind by purification, bound water and, in some cases, residual buffer salts. The proportion that's actually peptide is called the net peptide content, and it's typically somewhere around 70–90% of the gross mass, depending on the sequence and how it was made.
So a 10mg vial at 80% net peptide content holds roughly 8mg of peptide. Reconstituted in 2ml, the true concentration is 4 mg/ml, not 5 mg/ml. That's a 20% difference, far bigger than any error the arithmetic itself could introduce.
It's worth separating this from purity, because the two get confused all the time. HPLC purity tells you what proportion of the peptide is the correct sequence rather than related impurities. Net peptide content tells you what proportion of the powder is peptide at all. A batch can be 99% pure and still only be 80% peptide by weight.
The calculator has an optional field for this. If your Certificate of Analysis reports net peptide content, enter it and the output adjusts.
2. The counterion
Peptides purified by reverse-phase HPLC usually come out as a trifluoroacetate (TFA) salt, because TFA is used as an ion-pairing agent in the mobile phase. Some are supplied as acetate salts instead, either because acetate was used during purification or because the material was exchanged afterwards.
This matters for two reasons. First, the salt form changes the net peptide content, because the counterion contributes mass. Second, TFA isn't inert in cell culture. Residual TFA can affect cell viability and assay readouts, which is why acetate salts are often preferred for that kind of work.
The salt form should be stated on the Certificate of Analysis. If it isn't, you don't really know what proportion of the vial is peptide.
3. Displacement volume
The powder takes up space. Add 2ml of solvent to a vial with solid material in it and the final volume ends up slightly more than 2ml, so the real concentration is slightly lower than mass ÷ solvent volume.
At the quantities typical of research peptides (a few milligrams in a few millilitres), this is a fraction of a percent and is usually ignored. It's still worth knowing about, because at higher solid loads it stops being negligible.
If the final volume needs to be exact rather than approximate, make the solution up to volume in a graduated vessel instead of adding a measured volume of solvent to the powder.
4. Fill tolerance
Vials are filled to a target with a tolerance, and manufacturers generally overfill slightly rather than risk underfilling. The label is a nominal figure, not a measured one.
For most work this won't matter. For anything where the absolute quantity has to be known rather than assumed, the material needs to be weighed or quantified, not read off the label.
The short version: the calculator gets the arithmetic exactly right. Whether the answer is accurate depends on whether the mass you put in was real. That's a Certificate of Analysis question, not a calculator question.
We cover what a certificate does and doesn't tell you in Peptide Purity and Contamination, and how to read one line by line in How to Read a Peptide Certificate of Analysis.
Choosing a Solvent
The calculator doesn't care what the solvent is. The arithmetic is identical. The choice still matters a great deal for what happens next.
Bacteriostatic water is water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol suppresses microbial growth, which is why it's used where a solution will be stored and drawn from more than once.
Sterile water for injection contains no preservative. It's appropriate for single-use preparations and for work where benzyl alcohol would interfere, but once opened, a solution made with it has no protection against microbial growth.
Sodium chloride 0.9% is used where an isotonic solution is needed. It doesn't contain a preservative.
Solubility comes first, not second. Not every peptide dissolves readily in water. Hydrophobic sequences may need a small volume of a stronger solvent, such as dilute acetic acid, ammonium bicarbonate or DMSO, before being diluted into the working solvent. The right choice depends on the sequence's overall charge and hydrophobicity, so the supplier's solubility guidance for that specific peptide should always be your starting point.
Don't try to force a peptide into solution with heat or aggressive agitation. Both can degrade it, and a cloudy or partly dissolved solution won't give you a reliable concentration, whatever the calculator says.
Reconstituting Without Damaging the Peptide
Good technique protects both the material and the accuracy of your numbers. A few habits make a real difference:
- Let the vial reach room temperature before opening it. A cold vial opened straight from storage draws moisture from the air, and lyophilised powder is often hygroscopic.
- Make sure the powder is at the bottom. A gentle tap, or a brief spin in a centrifuge, brings any material on the stopper or walls back down.
- Add solvent slowly, down the inside wall of the vial, rather than directly onto the powder.
- Swirl or roll gently. Don't shake hard. Vigorous shaking can cause foaming and encourage aggregation in some sequences.
- Give it time. Some peptides dissolve in seconds, others take several minutes. Patience beats force.
- Check the result. The solution should be clear, with no visible particles. If it isn't, you have a solubility problem, not a calculation problem.
- Label it properly. Peptide name, batch number, concentration, solvent and date. Future you will be grateful.
After Reconstitution
A reconstituted peptide is considerably less stable than the lyophilised powder it came from. In solution it's exposed to hydrolysis, oxidation and aggregation in a way the dry form isn't.
General principles:
- Store cold and away from light
- Repeated freeze–thaw cycles are harder on peptide material than steady storage, so splitting the solution into single-use aliquots avoids cycling the whole batch
- Temperature swings in general are worth avoiding
- A solution that's gone cloudy, or has visible particulate, shouldn't be assumed to be at the concentration it started at
Our full guidance is in How to Store Peptides. Requirements differ between freeze-dried powder and pre-mixed material, and we cover those differences in Peptide Pens vs Vials.
Common Calculation Mistakes
Most errors we see aren't arithmetic errors at all. They're assumptions. These are the ones worth watching for:
- Mixing up mg and mcg. The classic. A factor of 1,000, and usually the biggest single source of error.
- Using the solvent volume when the final volume matters. Fine for routine work, not fine when the concentration has to be exact. See displacement volume above.
- Ignoring net peptide content. This can shift your real concentration by 10–30% without a single number on the page looking wrong.
- Rounding too early. 5 ÷ 3 is 1.6667 mg/ml. Round it to 1.7 halfway through a calculation and you've introduced a 2% error before you've started. Carry the decimals through and round at the end.
- Reusing a concentration from another vial or batch. Different batches can have different net peptide content. Recalculate for each one.
- Assuming everything dissolved. If there's material at the bottom of the vial, the liquid above it is weaker than your calculation says.
Limits of This Calculation
It's worth stating these plainly, because a calculator can look more authoritative than it really is.
It's arithmetic, not measurement. The output is only as good as the mass you put in. See the net peptide content section above.
It assumes complete dissolution. If some material hasn't gone into solution, the concentration in the liquid is lower than calculated, and the rest is sitting at the bottom of the vial.
It assumes a homogeneous solution. Peptides can adsorb to glass and plastic surfaces, particularly at low concentrations. Some of what you put in may not be in the bulk solution at all.
It says nothing about stability. The concentration you calculate on day one isn't necessarily the concentration present on day thirty.
Frequently Asked Questions
What is a peptide reconstitution calculator?
It's a tool that works out the concentration of a peptide solution once a lyophilised (freeze-dried) peptide has been dissolved in a known volume of solvent. It also tells you how much peptide is in a given volume, and what volume contains a given mass.
How do you calculate peptide concentration after reconstitution?
Divide the mass of peptide in the vial by the volume of solvent added. A 10mg vial reconstituted with 2ml gives 10 ÷ 2 = 5 mg/ml.
How do you work out the volume containing a given mass?
Divide the mass you want by the concentration. At 5 mg/ml, 250mcg is 0.25mg, and 0.25 ÷ 5 = 0.05ml.
How many mcg are in a mg?
One milligram is 1,000 micrograms. Most reconstitution errors come from this conversion rather than from the division itself.
How much solvent should I add to a peptide vial?
There's no single correct volume. Work backwards: divide the smallest mass you'll need by the smallest volume you can measure reliably to get a working concentration, then divide the vial mass by that concentration. For a 10mg vial at a working concentration of 2 mg/ml, that's 5ml.
Does a 10mg vial contain 10mg of peptide?
Usually not exactly. The dried mass includes the counterion from purification and bound water, so the actual peptide content (the net peptide content) is typically around 70–90% of the gross mass. It should be stated on the Certificate of Analysis.
What's the difference between peptide purity and net peptide content?
Purity, measured by HPLC, is the proportion of the peptide that is the correct sequence. Net peptide content is the proportion of the total powder that is peptide at all. A batch can be highly pure and still be only around 80% peptide by weight.
What is the difference between bacteriostatic water and sterile water?
Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, which suppresses microbial growth in a solution that will be stored and drawn from repeatedly. Sterile water contains no preservative.
Why does the salt form matter?
Peptides purified by reverse-phase HPLC are usually supplied as trifluoroacetate (TFA) salts. The counterion contributes to the vial's mass, so it affects net peptide content, and residual TFA can affect cell viability in culture work. Acetate salts are preferred for some applications.
Does the powder change the final volume?
Slightly. The solid occupies space, so adding 2ml of solvent gives a final volume marginally above 2ml. At typical research quantities this is a fraction of a percent. Where the final volume must be exact, make the solution up to volume in a graduated vessel.
Does this calculator apply to pre-filled pens?
No. A pen is supplied already in solution, so there is nothing to reconstitute and no concentration to calculate. The concentration is fixed by the manufacturer and stated on the product. This calculator is for lyophilised powder supplied in a vial, where the solvent volume is yours to choose.
Can this calculator be used for anything other than peptides?
The arithmetic applies to any solid reconstituted in a liquid. The material-specific points on this page (net peptide content, counterions, adsorption to surfaces) are particular to peptides.
References
- Reconstitution Calculator, R&D Systems / Tocris: https://www.tocris.com/resources/reconstitution-calculator
All products are supplied strictly for laboratory and research use only. Not for human or veterinary use. Nothing on this page is guidance on administration.
