--- title: Calculating Oligonucleotide Quantity description: calculating oligo quantity --- [Skip to content](https://support.sierrabio.com/calculating-oligonucleotide-quantity#main-content) English Show submenu for translations ![](https://support.sierrabio.com/hs-fs/hubfs/JPG%20file%20-02.jpg?width=2000&height=1125&name=JPG%20file%20-02.jpg) Open main navigation Close main navigation - English Show submenu for translations - [Contact us](mailto:support@sierrabio.com) [Contact us](mailto:support@sierrabio.com) How can we help you? - There are no suggestions because the search field is empty. 1. [Sierra Biosystems Support Center](https://support.sierrabio.com/?hsLang=en) 2. [Oligonucleotide Chemistry & Methods](https://support.sierrabio.com/oligonucleotide-chemistry-methods?hsLang=en) 3. [Quantitation & QC](https://support.sierrabio.com/oligonucleotide-chemistry-methods?hsLang=en#quantitation-qc) January 25, 2026 # Calculating Oligonucleotide Quantity ## CALCULATIONS & CONVERSION FACTORS SPECIFICATIONS SHEET — CALCULATIONS & CONVERSION FACTORS PURPOSE Use these formulas and conversion factors to calculate the quantity and molecular weight of synthetic oligonucleotides. ──────────────────────────────────────────────────────── OD260 MEASUREMENT ──────────────────────────────────────────────────────── • The absorbance at 260 nm (OD260) is measured at TWO different dilutions. • The OD260 value reported on the specification sheet is the AVERAGE of these two readings. ──────────────────────────────────────────────────────── QUANTITY CALCULATIONS (FROM OD260) ──────────────────────────────────────────────────────── CONVERSION FACTOR (SINGLE-STRANDED DNA) • 1 OD260 = 33 µg = 0.033 mg CALCULATIONS 1\) Milligrams (mg)    mg = OD260 × 0.033 2\) Nanomoles (nmol)    nmol = (mg ÷ molecular weight) × 10^6 NOTE • The molecular weight (MW) for each oligonucleotide is listed on that oligo’s specification sheet. ──────────────────────────────────────────────────────── MOLECULAR WEIGHT CALCULATION (SYNTHETIC OLIGONUCLEOTIDES) ──────────────────────────────────────────────────────── FORMULA Molecular Weight (g/mol) =   (251 × nA) +   (245 × nT) +   (267 × nG) +   (230 × nC) +   (61 × (n − 1)) +   (54 × n) +   (17 × (n − 1)) +   2 WHERE • nA = number of dA bases • nT = number of T bases • nG = number of dG bases • nC = number of dC bases • n  = total number of bases in the sequence TERM DEFINITIONS • (61 × (n − 1)) accounts for the molecular weight of the phosphate groups. • (54 × n) accounts for hydration of the DNA (≈ 3 water molecules per nucleotide). • (17 × (n − 1)) accounts for ammonium cations associated with the phosphate groups. ──────────────────────────────────────────────────────── SAMPLE CALCULATION ──────────────────────────────────────────────────────── Example sequence: ATG TAA TGT TTG GTC CGC Base counts: • nA = 3 • nT = 7 • nG = 5 • nC = 3 • n  = 18 Calculated molecular weight: • 6,793 g/mol ──────────────────────────────────────────────────────── MODIFICATIONS ──────────────────────────────────────────────────────── Special modifications (e.g., biotin, 5′ phosphate, inosine, uridine, etc.) add additional mass to the oligonucleotide and must be included separately when calculating molecular weight. - 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