Converting nucleic-acid mass concentration and molarity
Molar concentration counts molecules, whereas mass concentration depends on each molecule’s molecular weight. For a nucleic-acid polymer, approximate molecular weight can be estimated from base-pair or nucleotide count and an average residue weight.
Exact molecular weight depends on sequence, termini, modifications, counterions, hydration, and whether a DNA molecule is linear or circular. Use a sequence-aware tool or manufacturer value when precision matters.
How to use the DNA/RNA molarity calculator
- Choose nucleic-acid type: Select dsDNA, ssDNA, or ssRNA.
- Choose conversion direction: Convert ng/µL to µM or µM to ng/µL.
- Enter concentration and length: Use base pairs for dsDNA and bases for single strands.
- Calculate and assess precision: Treat the output as an average-weight estimate unless exact sequence mass is available.
Formula and variables
The calculator uses 650 g/mol per dsDNA base pair, 330 per ssDNA base, and 340 per ssRNA base, then converts ng/µL and µM consistently.
MW ≈ length × average residue MW; c = mass concentration/MW- MW — Molecular weight
- Approximate molar mass of one nucleic-acid molecule (g/mol)
- n — Length
- Base pairs for dsDNA or bases for single strands
- c — Molar concentration
- Amount of molecules per solution volume (µM)
Five-hundred-base-pair dsDNA
Convert 100 ng/µL of 500 bp dsDNA to molarity.
- Length
- 500 bp
- Mass concentration
- 100 ng/µL
- MW ≈ 500 × 650 = 325,000 g/mol
- 100 ng/µL = 0.1 g/L
- c = 0.1/325,000 mol/L
Result: Estimated concentration is approximately 0.3077 µM, or 307.7 nM.
Sequence-specific molecular weight and sample purity can shift the true value.
Understanding your results
Understand the average-weight estimate
Longer molecules have lower molarity at the same mass concentration.
- Double-stranded length is entered in base pairs.
- Single-stranded length is entered in nucleotides.
- Contaminants measured by absorbance can inflate apparent mass concentration.
Assumptions
- The sample contains the selected nucleic-acid type and stated length.
- Average residue molecular weights adequately approximate the polymer.
- The entered mass concentration represents nucleic acid of interest.
Limitations
- Does not use exact sequence composition, end groups, modifications, counterions, or topology.
- Does not correct spectrophotometric concentration for purity or contaminants.
- Not a substitute for sequence-specific molecular-weight calculation in quantitative assays.
Common mistakes
- Entering nucleotides instead of base pairs for dsDNA.
- Confusing ng/µL with ng/mL.
- Using plasmid base-pair length but selecting ssDNA.
- Reporting excessive precision from average molecular weights.
Practical use cases
Library and assay planning
Estimate molecule concentration from a measured mass concentration.
Standard preparation
Convert a desired molarity to an approximate mass concentration.
Frequently asked questions
Why does sequence affect exact molecular weight?
A, C, G, T, and U residues have different masses, and termini or modifications also contribute.
Is 650 g/mol per base pair exact?
No. It is a practical average used for estimates.
Can I use this for oligos with modifications?
Use the supplier’s exact molecular weight or a sequence-aware calculation for modified oligonucleotides.
Sources and review
- Molecular Biology of the Cell: DNA and RNA structure — National Center for Biotechnology Information Bookshelf. Accessed 2026-07-13.
Reviewed 2026-07-13.