Bench math · runs in your browser
Research Peptide Calculator: Stock Concentration, Molarity, Dilution, Aliquot and Sequence Tools
Five connected calculators for preparing research peptide solutions. Start from the mass on the vial and the volume of diluent, then carry that stock through a molar conversion, a working dilution and an aliquot split. A sequence tool estimates molecular weight, pI and 280 nm absorbance. Every result recalculates as you type, and no input leaves your device.
Step A — Make the stock
Stock Concentration & Draw Volume Calculator
Set the labeled peptide mass and the diluent volume you added. Enter a target mass in µg to see the draw volume that holds it on a graduated µL/mL gauge, plus how many draws one vial yields.
Draw volume
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Research solution, pipette.
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Volume for neighboring masses
Draw volumes here describe pipetting a laboratory solution and nothing else. Swap the labeled mass for the measured figure on the lot’s certificate of analysis whenever you have it. For preparation technique, see the reconstitution lab guide.
Step B — Count the molecules
Mass-to-Molarity Converter (mg ↔ mM)
Turn a milligram figure in a known volume into a molar concentration, or work backward from a target molarity to the mass you need to weigh. 24 built-in compounds use free-peptide molecular weights.
moles = mass (g) ÷ MW (g/mol) · molarity (M) = moles ÷ volume (L)
Prepared vial
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Where this lands · log scale
Step C — Bring it to working strength
Working Dilution & Serial Series (C₁V₁ = C₂V₂)
Give the stock strength, the strength you want and the final volume; the planner returns how much stock to pipette and how much diluent makes up the rest. Serial mode repeats one factor down a row of tubes.
C₁ × V₁ = C₂ × V₂ → V₁ = (C₂ × V₂) ÷ C₁
Cₙ = C₀ ÷ fⁿ · transfer = V ÷ f · diluent = V − V/f · T1 is the starting stock (n = 0)
Transfer
dashed line = stock aliquot
Dilution series
View tube volumes and concentrations
| Tube | Concentration | Transfer in | Diluent added | Volume remaining |
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Step D — Split before storing
Aliquot Split & Cold-Storage Planner
Divide one stock into equal tubes by count or by fixed volume. The planner subtracts the dead volume lost in every tube and lays out typical stability windows from the refrigerator to −80 °C.
mass per aliquot = (peptide ÷ reconstitution volume) × aliquot volume
Aliquot rack
Stock remaining: · Total peptide retained in tube dead volume:
Storage temperature & expected stability
Weeks — shipping window only
Hours — working session only
Months
2–4 weeks in solution
1–2 years
1–3 months as single-use aliquots
Several years
6–12 months as single-use aliquots
Ranges are general handling guidance for research peptides and are not a substitute for the stability data on a specific certificate of analysis. Peptides containing Met, Cys or Trp oxidize faster than the table suggests; protect those from light and headspace oxygen.
Stability windows are typical ranges, not lot data. Thaw each tube once and discard the remainder. More detail is in the peptide storage guide.
Step E — Read the sequence
Sequence MW, pI & Hydropathy Estimator
Enter a one-letter amino acid sequence to estimate average molecular weight, isoelectric point, charge at pH 7, GRAVY score and the 280 nm extinction coefficient. It assumes linear L-residues, free termini and no disulfide bonds.
Residues — total
Composition
Net charge vs pH
x: pH · y: net charge · drag or hover to read the curve
Values are computed from the primary sequence assuming free (unmodified) N- and C-termini, all cysteines reduced, and standard L-amino acids. Acetylation, amidation, cyclisation, non-standard residues and counter-ions all shift the real figures — treat these as a starting point, not as a certificate of analysis.
Calculation methods
MW is the sum of average residue masses plus one water molecule. Charge uses Henderson–Hasselbalch contributions with pKa values: N-terminus 8.6; C-terminus 3.6; K 10.8; R 12.5; H 6.5; D 3.9; E 4.1; C 8.5; Y 10.1. The pI is found by bisection across pH 0–14. GRAVY is the mean Kyte–Doolittle score. Reduced ε₂₈₀ = 5,500 × Trp + 1,490 × Tyr; each assumed cystine pair adds 125.
Reference
Molecular Weights Used by the Converter
Average masses for the free peptide, without counter-ions. Acetate or TFA salt forms weigh more per mole of peptide, which is exactly what the net content input corrects for.
| MW (g/mol) | Compound | Sequence / note |
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Where a compound contains modified or non-standard residues, a note replaces the sequence. For the lot in hand, rely on its certificate of analysis.
Before you pipette
Calculator FAQ
Why are results shown in microliters instead of "units"?
A microliter (µL) is a fixed SI volume: 1,000 µL is exactly 1 mL, and pipettes are calibrated against it. A "unit" is not a volume at all, and its meaning changes with whatever it is marked on. Reporting draw volume in µL or mL keeps every result traceable to a calibrated instrument.
How do I get the net content figure from a certificate of analysis?
Find the measured peptide mass reported for the lot and divide it by the labeled mass, then multiply by 100. A 10 mg vial measured at 10.6 mg is 106%; one measured at 9.4 mg is 94%. Always use the certificate of analysis for the specific lot in front of you, since the figure varies between lots.
How many decimal places should I trust in the results?
No more than your least precise input supports. The calculator rounds concentration to 0.01 mg/mL and volume to 0.1 µL, but if the diluent was measured to ±0.05 mL, the last digit is not meaningful. Most air-displacement pipettes lose accuracy below about 10% of their rated range, so choose a stock concentration that keeps each draw volume well inside it.
Why can two solutions at the same mg/mL have very different molarities?
Molarity counts molecules, and heavier molecules mean fewer of them per milligram. At 2 mg/mL, Epitalon (390.35 g/mol) is about 5.12 mM, while Tesamorelin (5,135.87 g/mol) is about 0.39 mM, roughly a thirteenfold gap at identical mass concentration.
What does dead volume mean in the aliquot planner, and is this page usage guidance?
Dead volume is the liquid left behind on tube walls and in pipette tips that is never recovered; the planner subtracts it so the mass per tube reflects what can actually be transferred. The page is not a guide to using peptides. It is solution arithmetic for laboratory work only: the products are not FDA-approved and are not for human or veterinary use.
Terms on This Page
- Stock solution
- The concentrated solution made directly from the vial, from which every working dilution is drawn.
- Draw volume
- The µL or mL of stock that contains a chosen mass of peptide at the current concentration.
- Net content
- Measured peptide mass as a percentage of the labeled mass, taken from the lot’s certificate of analysis.
- Millimolar (mM)
- One thousandth of a mole per liter. It depends on molecular weight, unlike mg/mL.
- Dilution factor
- Stock concentration divided by working concentration; a factor of 4 turns 2 mg/mL into 0.5 mg/mL.
- Dead volume
- Solution retained by tube walls and pipette tips that can never be transferred out.
- Isoelectric point (pI)
- The pH at which net charge is zero and a peptide is least soluble.
- ε₂₈₀
- How strongly a sequence absorbs 280 nm light, set by its tryptophan and tyrosine residues.
