- Free UK delivery · same-day dispatch before 3pm
- Independent third-party testing by PeptideVerify
- Shop all research peptides
- Learn: guides and research tools
Peptide calculator
The British Peptide Labs peptide calculator turns three numbers — the peptide mass in the vial (mg), the solvent volume (mL) and the aliquot mass (µg or mg) — into the stock concentration in mg/mL and the exact transfer volume to pipette in mL and µL. Free, instant and built for in-vitro laboratory work.
1 mg = 1,000 µg (mcg) · 1 mL = 1,000 µL · 1 mg/mL = 1 µg/µL
For research use only. Not for human or veterinary use. Sold strictly for in-vitro laboratory research; not for diagnosis or treatment.
Stock concentration
The concentration of a stock solution made from the whole vial.
- Stock concentration
- 2.5 mg/mL2.5 µg/µL
Aliquot transfer volume
The volume of stock solution that holds one aliquot.
Taken from the stock concentration above.
- Transfer volume
- 0.1 mL100 µL
- Aliquots per vial
- 20
Dilution (C1V1 = C2V2)
Choose the unknown term and enter the other three.
- Stock volume (V1)
- 0.2 mL200 µL
- Solvent to add
- 0.8 mL800 µL
Stock solutions and aliquots
A stock solution is the concentrated solution made by dissolving the whole contents of a vial in a measured volume of solvent. Its concentration is fixed by two numbers you already know: the peptide mass on the vial label and the volume of solvent added. Every working solution in the experiment is then taken from this one stock, so getting its concentration right is the step everything else depends on.
An aliquot is a measured portion of that stock: the volume that holds a chosen mass of peptide. Splitting a stock into aliquots lets each run start from the same, known amount, and keeps the rest of the vial untouched until it is needed. The calculator gives you the transfer volume for each aliquot, the figure you set on the pipette.
Concentration follows directly from mass and volume. Mass divided by volume gives mg/mL, and because 1 mg/mL is the same as 1 µg/µL, the number also tells you how many micrograms sit in each microlitre. Transfer volume runs the other way: the aliquot mass divided by the concentration gives the volume to pipette.
How the peptide calculator works
What you enter:
- Vial quantity (mg): the peptide mass stated on the vial label.
- Solvent volume (mL): the volume of solvent the whole vial is dissolved in.
- Aliquot mass (µg or mg): the amount of peptide one aliquot should hold.
The formulas:
- Stock concentration (mg/mL) = vial quantity (mg) ÷ solvent volume (mL)
- Transfer volume (mL) = aliquot mass (mg) ÷ stock concentration (mg/mL)
- Transfer volume (µL) = transfer volume (mL) × 1,000
- Aliquots per vial = vial quantity (µg) ÷ aliquot mass (µg), rounded down
Every figure the calculator shows comes from these four lines, to four significant figures. A transfer volume under 10 µL is flagged, because volumes that small are hard to pipette accurately. The dilution panel solves C1 × V1 = C2 × V2 for whichever term you leave out.
Three worked examples, start to finish
One vial, followed from stock concentration to transfer volume to molarity. Enter the same numbers above and the calculator returns the same figures.
- Stock concentration: 10 mg of peptide dissolved in 2.0 mL of solvent gives 10 ÷ 2.0 = 5 mg/mL. In other units that is 5,000 µg/mL, or 5 µg in every microlitre.
- Transfer volume: a 250 µg aliquot is 0.25 mg. Divided by the 5 mg/mL stock, 0.25 ÷ 5 = 0.05 mL, which is 50 µL. The vial holds 10,000 µg, so it gives 40 aliquots of that size.
- Molarity: enter the molecular weight from the certificate of analysis. For a peptide of 1419.5 g/mol, a 1 mg/mL solution is 1 g/L ÷ 1419.5 g/mol = 0.7045 mM, or 704.5 µM. The 5 mg/mL stock above is 3.522 mM.
More worked examples
Each example assumes the whole vial is dissolved in the stated solvent volume.
- Worked example: a 5 mg vial dissolved in 2 mL of solvent. Stock concentration: 5 mg ÷ 2 mL = 2.5 mg/mL (2,500 µg/mL, or 2.5 µg per µL). Aliquot mass: 0.25 mg (250 µg). Transfer volume: 0.25 mg ÷ 2.5 mg/mL = 0.1 mL, which is 100 µL. Aliquots per vial: 5,000 µg ÷ 250 µg = 20.
- A 10 mg vial in 2 mL of solvent: 10 ÷ 2 = 5 mg/mL. A 0.5 mg (500 µg) aliquot is 0.5 ÷ 5 = 0.1 mL, or 100 µL, and the vial gives 20 aliquots.
- A 20 mg vial in 5 mL of solvent: 20 ÷ 5 = 4 mg/mL. A 2 mg (2,000 µg) aliquot is 2 ÷ 4 = 0.5 mL, or 500 µL, and the vial gives 10 aliquots.
- Dilution: to take a 5 mg/mL stock to 1 mg/mL in a final volume of 1 mL, V1 = 1 × 1 ÷ 5 = 0.2 mL of stock, made up with 0.8 mL of solvent.
Stock concentration by vial size and solvent volume
Vial quantity divided by solvent volume, in mg/mL. The same figure is µg per µL.
| Vial quantity | 1 mL | 2 mL | 3 mL | 5 mL |
|---|---|---|---|---|
| 5 mg | 5.00 | 2.50 | 1.67 | 1.00 |
| 10 mg | 10.00 | 5.00 | 3.33 | 2.00 |
| 15 mg | 15.00 | 7.50 | 5.00 | 3.00 |
| 20 mg | 20.00 | 10.00 | 6.67 | 4.00 |
| 30 mg | 30.00 | 15.00 | 10.00 | 6.00 |
| 40 mg | 40.00 | 20.00 | 13.33 | 8.00 |
| 50 mg | 50.00 | 25.00 | 16.67 | 10.00 |
| 80 mg | 80.00 | 40.00 | 26.67 | 16.00 |
| Stock (mg/mL) | 1 | 2 | 2.5 | 5 | 10 | 20 |
|---|---|---|---|---|---|---|
| Transfer volume | 1,000 µL | 500 µL | 400 µL | 200 µL | 100 µL | 50 µL |
Choosing solvent volume
The solvent volume sets the stock concentration, and the stock concentration sets how much you pipette for each aliquot. A 20 mg vial in 1 mL gives 20 mg/mL, so a 0.5 mg aliquot is only 25 µL. The same vial in 4 mL gives 5 mg/mL, and the same aliquot becomes 100 µL, a volume most laboratory pipettes handle comfortably.
Work backwards from the smallest aliquot you need: choose a solvent volume that keeps its transfer volume at 10 µL or more, and check that the total fits the vial.
Solvent choice and handling are left to the receiving laboratory's own procedures.
Significant figures and pipette resolution
The calculator rounds every figure to four significant figures: 0.04987 mL shows as 0.04987 mL, and 49.87 µL as 49.87 µL. That is more precision than a pipette can deliver, so round the transfer volume to the step your pipette actually sets. An adjustable pipette set in 0.1 µL steps turns 49.87 µL into 49.9 µL.
Match the pipette to the volume. Accuracy is best toward the top of a pipette's range, so use the smallest pipette whose range covers the transfer volume: a 20–200 µL pipette for a 50 µL aliquot, rather than a 100–1,000 µL one working near the bottom of its scale.
Keep the input precision honest too. A vial labelled 10 mg and a solvent volume measured to 0.1 mL support a concentration of 5.0 mg/mL; quoting 5.000 mg/mL does not make the stock any more exact. When the transfer volume falls under 10 µL, the calculator flags it: a larger solvent volume gives a larger, easier volume for the same aliquot.
Units and conversions
- 1 g = 1,000 mg and 1 mg = 1,000 µg (mcg).
- 1 L = 1,000 mL and 1 mL = 1,000 µL.
- 1 mg/mL = 1,000 µg/mL = 1 µg/µL, so a stock in mg/mL also tells you how many micrograms each microlitre holds.
- 1 M = 1,000 mM and 1 mM = 1,000 µM. Converting mg/mL to molar needs the molecular weight: 1 mg/mL is 1 g/L, divided by g/mol.
Frequently asked questions
A peptide calculator is a free lab tool that turns three numbers — the peptide mass in the vial (mg), the solvent volume (mL) and the aliquot mass (µg or mg) — into the stock concentration in mg/mL and the transfer volume to pipette in mL and µL.
It divides the vial quantity by the solvent volume to give the stock concentration, then divides the aliquot mass by that concentration to give the transfer volume. A 5 mg vial in 2 mL is 2.5 mg/mL, and a 0.25 mg aliquot of it is 0.1 mL, or 100 µL.
Divide the peptide mass in the vial by the solvent volume. 10 mg in 2 mL is 10 ÷ 2 = 5 mg/mL; 5 mg in 2 mL is 2.5 mg/mL. Because 1 mg/mL equals 1 µg/µL, a 5 mg/mL stock holds 5 µg in every microlitre.
Multiply by 1,000. 1 mg = 1,000 µg (also written mcg), so 0.25 mg is 250 µg and a 10 mg vial holds 10,000 µg. To convert back, divide by 1,000.
Divide the aliquot mass by the stock concentration, with both in milligrams. 0.25 mg from a 2.5 mg/mL stock is 0.25 ÷ 2.5 = 0.1 mL. Multiply by 1,000 for microlitres: 100 µL.
Divide the vial quantity by the aliquot mass, in the same unit, and round down. A 5 mg vial holds 5,000 µg, so 250 µg aliquots give 5,000 ÷ 250 = 20.
More solvent gives a lower concentration and a larger transfer volume for the same aliquot. 10 mg in 1 mL is 10 mg/mL, in 2 mL it is 5 mg/mL, and in 5 mL it is 2 mg/mL. Choose the volume that keeps your smallest aliquot at 10 µL or more.
Diluting a solution leaves the amount of solute unchanged; only the volume grows. C1 and V1 are the stock concentration and the volume of stock taken; C2 and V2 are the final concentration and final volume. Rearranged, V1 = C2 × V2 ÷ C1.
Yes. Divide the mass concentration in g/L by the molecular weight in g/mol: 1 mg/mL is 1 g/L. The molarity calculator does this for M, mM, µM and nM from the molecular weight on the certificate of analysis.
Open the molarity calculatorIt works from the vial mass you enter, taken as the mass of peptide. If your laboratory corrects for net peptide content or counter-ions, apply that correction to the mass before entering it, and every figure updates to match.
Multiply by 1,000. 0.05 mL is 50 µL and 0.1 mL is 100 µL. To go from µL back to mL, divide by 1,000: 250 µL is 0.25 mL.
Yes. Both the mass and the volume shift by a factor of 1,000, so the number stays the same: a 5 mg/mL stock holds 5 µg per µL, and 5,000 µg per mL.
Multiply by 1,000. 0.7045 mM is 704.5 µM, and 1 µM is 1,000 nM. The molarity calculator switches between M, mM, µM and nM for you.
From British Peptide Labs: independent third-party testing by PeptideVerify, ≥99% purity (HPLC), same-day dispatch when you order by 3pm Mon–Sat, and next-day delivery with Royal Mail Tracked 24.