Peptide Calculator
Reconstitute peptide vials and analyze peptide sequences — dose math + ProtParam-style properties on one page.
Outputs are mathematical results from the values you enter, not medical or dosing advice. Research peptides are sold for research use only — consult a licensed medical professional for any clinical decision.
Amino acid composition
| Residue | Count | % |
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What is peptide calculator?
A peptide calculator answers two questions that come up constantly in peptide work, depending on who’s asking. Research-peptide users want to know how many units to draw on an insulin syringe to deliver a specific dose from a reconstituted vial. Biochemists and mass-spec users want molecular weight, theoretical pI, net charge at buffer pH, extinction coefficient at 280 nm, and a hydrophobicity score for a sequence they’re working with. This tool does both — pick a mode at the top.
Reconstitution math is conceptually simple: vial mg ÷ water mL = mg/mL concentration; dose mg ÷ concentration = volume per dose; volume × syringe units-per-mL = units to draw. The calculator handles the unit conversions (mcg ↔ mg), the syringe scale (U-100 vs U-50), and the doses-per-vial division. Common research peptides — BPC-157, TB-500, semaglutide, tirzepatide, and several others — have presets that autofill typical vial sizes. The “common research dose range” shown next to each preset is a reference, not a prescription; this calculator is a math tool, not medical advice. Always consult a licensed medical professional before using any peptide on a human.
Sequence properties follow the standard ExPASy ProtParam method for the linear free-acid form of the peptide. Average MW sums the natural-isotope-weighted residue masses plus one water; monoisotopic MW does the same with the most-abundant isotope of each element. Net charge at a given pH uses Henderson-Hasselbalch over the standard ionizable groups (N-terminus, K, R, H on the basic side; C-terminus, D, E, C, Y on the acidic side). Theoretical pI is found by bisecting the net-charge function over pH 0–14 to find where the sum equals zero. Extinction coefficient at 280 nm follows the textbook formula 5500·n(Trp) + 1490·n(Tyr) + 125·n(cystine pairs); the toggle lets you compare the reduced and oxidized values directly. GRAVY is the mean Kyte-Doolittle hydropathy index — a quick hydrophobicity screen. Modifications such as disulfide bonds, C-terminal amidation, or cyclization are outside the scope of this tool — they shift MW and pI compared to the linear form shown here.
Privacy. Every calculation runs locally in your browser. There is no server, no API, no analytics on your inputs. Refreshing the page clears the form.
This is not medical or dosing advice. Research peptides are sold for research use only. The dose mode performs arithmetic on values you enter — it does not validate them against any clinical guideline. Talk to a licensed medical professional before any human use.
When to use a peptide calculator
- Reconstituting a research-peptide vial — You have a 5 mg vial of a research peptide and you've just added bacteriostatic water. You want the answer to a single question: how many units on a U-100 insulin syringe equals your target dose? Plug in vial size, water volume, and dose; the calculator returns units to draw, doses per vial, and concentration.
- Checking a peptide's molecular weight before mass spec — Paste a 1-letter amino-acid sequence and get both average and monoisotopic MW. Average MW is what most lab protocols and supplier datasheets use; monoisotopic MW is what your mass spectrometer reports. The two differ by roughly the natural isotope abundance — a few hundredths of a Da per residue.
- Estimating net charge at a buffer pH — Drag the pH slider from 0 to 14 and watch net charge update live. Useful for predicting how a peptide will behave on an ion-exchange column, in a buffer for solubility, or during electrophoresis.
- Computing ε₂₈₀ for an A280 concentration measurement — If your peptide contains tryptophan, tyrosine, or cystine pairs, you can measure its concentration at 280 nm using Beer's law. The calculator gives ε₂₈₀ for both reduced and oxidized cysteines so you can match the value to your sample's redox state.
- Quick GRAVY hydrophobicity check — GRAVY (Grand Average of Hydropathy, Kyte-Doolittle) is a one-number summary of how hydrophobic a peptide is. Negative values are hydrophilic (likely water-soluble); positive values are hydrophobic (may stick to membranes or aggregate). A useful first-pass check before you order synthesis.
How to use the Peptide Calculator
- Pick a mode — Reconstitution & Dose for vial math; Sequence Properties for MW / pI / charge / ε₂₈₀ / GRAVY. Switch any time — the modes are independent.
- Dose mode: choose a preset or enter custom values — The preset dropdown autofills typical vial sizes for common research peptides. You can override the autofilled value or skip the preset entirely. Then enter water volume, pick U-100 or U-50 syringe, and type your desired dose in mcg or mg.
- Dose mode: read the units to draw — The big number near the top is units to draw on your selected syringe — that's the answer most users came for. Volume per dose, concentration, and total doses per vial fill in below.
- Properties mode: paste a sequence — Paste a peptide sequence using 1-letter amino-acid codes (case doesn't matter; whitespace and digits are stripped silently). MW, pI, charge, extinction, and GRAVY appear immediately.
- Properties mode: tweak pH and cysteine state — Drag the pH slider to evaluate net charge at any pH from 0 to 14. Toggle reduced/oxidized cysteines to update ε₂₈₀ — the highlighted value matches your selection.
Worked examples
Reconstitute 10 mg vial in 2 mL water, draw a 250 mcg dose on U-100
Input: Vial 10 mg · Water 2 mL · Syringe U-100 · Dose 250 mcg
Output: Units to draw: 5 · Volume per dose: 0.050 mL · Concentration: 5.00 mg/mL · Doses per vial: 40Same math the search engines pull up: 5 mg/mL, 250 mcg = 0.05 mL = 5 units on U-100.
Oxytocin sequence properties (CYIQNCPLG, linear free-acid)
Input: Sequence: CYIQNCPLG · pH 7.0 · cysteines reduced
Output: Length 9 · MW avg 1010.19 Da · MW mono 1009.44 Da · pI ≈ 5.2 · ε₂₈₀ reduced 1490 M⁻¹·cm⁻¹These are the ProtParam values for the linear sequence. Natural oxytocin is cyclic with a disulfide bond and amidated C-terminus, which subtracts ~3 Da overall — handle those modifications outside this calculator.
Bradykinin charge sweep (RPPGFSPFR)
Input: Sequence: RPPGFSPFR · pH 7.0
Output: Net charge ≈ +2.0 · pI ≈ 12.0 · ε₂₈₀ 0 (no W/Y/C)Drag the pH slider to 13 to watch charge cross zero near the pI.