DNA Molecular Weight Calculator
Weighing a Strand of DNA
Molecular weight is the conversion factor between "how much DNA" measured in mass (nanograms, micrograms) and "how much DNA" measured in molecules or moles — essential any time a protocol calls for a specific molar amount of a plasmid, oligo, or PCR product but the lab instrument reports concentration in ng/µL. This calculator handles both double-stranded DNA, calculated from length alone, and single-stranded DNA, calculated from the actual base composition.
The Formula
MW = Base Pairs × 650 g/mol per bp
Single-Stranded DNA (oligo):
MW = (A × 313.21) + (T × 304.20) + (C × 289.18) + (G × 329.21) − 61.96
The 650 g/mol-per-bp figure is a standard average molecular weight for one base pair of double-stranded DNA including its sodium counterion. The single-stranded formula sums each base's individual residue weight and subtracts a standard end-adjustment for the strand's terminal phosphate/hydroxyl groups.
Worked Examples
| Input | Type | Molecular Weight |
|---|---|---|
| 1,000 bp | Double-stranded | 650,000.00 g/mol |
| ATGCGTAC (8-base oligo) | Single-stranded | 2,409.64 g/mol |
Where This Calculation Matters
- Molar ratio calculations for cloning — ligation reactions specify insert-to-vector molar ratios, which require converting each DNA fragment's mass concentration into moles via its molecular weight.
- Primer and oligo ordering — synthesized oligos are often quantified in nanomoles; converting to mass tells you how much material a given synthesis scale actually delivers.
- qPCR standard curve preparation — converting a known mass of a DNA standard into copy number requires its molecular weight as an intermediate step.
How to Use This Calculator
- Choose the mode: "Double-Stranded DNA (bp count)" or "Single-Stranded DNA (sequence)".
- For dsDNA, enter the number of base pairs.
- For ssDNA, enter the actual sequence using only A, T, G, and C.
- Select Calculate to get the molecular weight in g/mol.
Related Calculations
Designing primers for the reaction that generated this fragment? See the PCR Primer Tm Calculator.