DNA Molecular Weight Calculator
Estimate the molecular weight of DNA or RNA from its length.
≈ 650 kDa. Using 650 g/mol per base pair. This is an average estimate; the exact mass depends on base composition.
Compared for the same numeric length (1,000) — dsDNA counts per base pair, while ssDNA and RNA count per nucleotide.
To estimate molecular weight, multiply length by an average per-unit mass: double-stranded DNA is about 650 g/mol per base pair, so a 1000 bp fragment is ≈ 650,000 g/mol (650 kDa). Single-stranded DNA uses ≈ 330 g/mol and RNA ≈ 320 g/mol per nucleotide. These are averages; the exact mass depends on base composition.
How the estimate works
Every base pair or nucleotide contributes roughly the same average mass, so molecular weight scales almost linearly with length. Multiply the number of bases by the appropriate average weight and you get a fast estimate in grams per mole (g/mol), the same as daltons (Da). Divide by 1000 for kilodaltons (kDa), and by one million for megadaltons (MDa) when working with long fragments.
Which average to use
Double-stranded DNA counts both strands, so its per-base-pair weight (≈ 650 g/mol) is about twice the single-strand per-nucleotide weight. Single-stranded DNA uses ≈ 330 g/mol per nucleotide, and RNA is slightly lighter at ≈ 320 g/mol because ribonucleotides differ from deoxyribonucleotides.
dsDNA: bp × 650 · ssDNA: nt × 330 · RNA: nt × 320 (g/mol, average)
Worked example
Estimate the molecular weight of a 1000 bp double-stranded plasmid insert:
- 1 Pick the molecule type. Double-stranded DNA measured in base pairs, so use 650 g/mol per bp.
- 2 Count the length. The insert is 1000 bp long.
- 3 Multiply length by the per-unit weight. 1000 bp × 650 g/mol = 650,000 g/mol.
- 4 Convert to kilodaltons. 650,000 g/mol ÷ 1000 = 650 kDa. For very large fragments, divide by one million for MDa.
Average molecular weights
Common approximations. Multiply by length to estimate molecular weight; actual mass varies with base composition.
| Molecule | Length unit | Average weight | Example (1000 units) |
|---|---|---|---|
| Double-stranded DNA | base pair (bp) | 650 g/mol | 650,000 g/mol (650 kDa) |
| Single-stranded DNA | nucleotide (nt) | 330 g/mol | 330,000 g/mol (330 kDa) |
| RNA (single-stranded) | nucleotide (nt) | 320 g/mol | 320,000 g/mol (320 kDa) |
Accuracy and when to use it
These per-unit weights are population averages across the four bases, so the estimate is best for rough conversions — sizing a gel band, planning a ligation, or converting between nanograms and picomoles. A sequence that is unusually GC-rich or AT-rich will deviate slightly from the average because guanine, cytosine, adenine, thymine, and uracil each have a different mass.
For an exact molecular weight, use a formula that sums the mass of every individual nucleotide and subtracts the water lost at each phosphodiester bond (and, for short oligos, accounts for the terminal phosphate). For most bench calculations, the averages above are close enough.