DNA Concentration Calculator
Quantify nucleic acids. Understand your sample.
UV absorbance · Purity indicators · Molarity & total mass
Sample & Measurement
RESULT
Original sample concentration
50.0 ng/µL50.0 µg/mLdsDNA · Dilution 1× · 1 cm equivalent
Approximate dsDNA molecular weight: 660 g/mol per bp
Purity indicators
Near dsDNA reference
Within common reference range
Ratios are screening indicators, not a guarantee of downstream performance. Blank, pH, ionic strength and low signal affect interpretation.
Your measured absorbance
Three readings only — not a measured UV spectrum.
Inside your sample
Prepare a working dilution
What the numbers mean
Concentration = A260 × factor × dilution
For raw readings, divide by path length in cm. For 1 cm-normalized readings, the effective calculation path is already 1 cm.
Conversion factors (ng/µL per A260 at 1 cm): dsDNA 50, ssDNA 33, RNA 40. These are approximate bulk conversions, not sequence-specific extinction coefficients.
Molarity (nM) = concentration (ng/µL) × 10⁶ / molecular weight (g/mol). Total mass (ng) = concentration × original sample volume (µL).
Molecular-weight assumptions
dsDNA defaults to 660 g/mol per bp, a common approximation. Single strands use the published salt-free average formulas: ssDNA MW = 303.7 × nt + 79; RNA MW = 320.5 × nt + 159 g/mol. Different counterion conventions, sequences and chemical modifications change molecular weight. A known molecular weight overrides these estimates. A fragment distribution is not a single known molecule length.
Purity references and display colors
A260/A280 is commonly near 1.8 for DNA and 2.0 for RNA. This interface uses ±0.1 around those values as a visual comparison guide, not a universal acceptance rule. No universal A260/A280 cutoff is assigned to oligos or custom material. Published A260/A230 guidance commonly spans 1.8–2.2. Amber prompts a review of the measurement, not a confirmed contaminant diagnosis. Green does not certify sample purity or suitability. Missing and zero-denominator readings are not scored.
Frequently asked questions
Why does the reading type matter?
Many nucleic-acid instrument reports already normalize absorbance to a 10 mm (1 cm) path. Dividing that value by the tiny physical pedestal gap again inflates the result. Check the report and instrument module. Use Raw absorbance only for genuinely unnormalized readings.
Can ratios identify a contaminant?
No. Low ratios may reflect reagent carryover or measurement conditions. A high A260/A280 ratio does not by itself establish RNA contamination in a DNA sample. Examine the full spectrum, blank composition, extraction method and instrument limits. These three readings cannot reconstruct a UV spectrum.
When should I use fluorometric quantitation?
Consider a nucleic-acid-specific fluorometric assay when concentration is near the instrument’s lower limit, mixed nucleic acids or contaminants may bias UV readings, or the downstream workflow requires it. Use the assay and range specified by your laboratory or library-preparation workflow. Integrity requires a separate assessment.
What does the dilution factor mean?
Use the total multiplier: 1 for undiluted, 10 for a tenfold (1:10) dilution. Concentration is corrected back to the original stock. The sample-volume field therefore refers to the original stock, not the small volume placed on the instrument. Ratios are not multiplied by dilution.
Are µg/mL and ng/µL numerically equal?
Yes. 1 µg/mL = 1 ng/µL. Molarity also requires molecular weight; mass concentration alone cannot determine molecule count.
How is the working dilution calculated?
Stock volume = target concentration × final volume / original stock concentration. Diluent = final volume − stock volume. A target above the stock concentration cannot be reached by dilution. Check practical pipetting limits and buffer compatibility before preparing a mixture.
Are data saved or sent anywhere?
This calculator does not save sample IDs or readings and does not send them to a server. Copy and Download create a summary only when selected. The summary records entered values, assumptions, timestamp and limitations; it is not a laboratory validation certificate.
What conversion factor should I use?
For A260 values normalized to a 1 cm optical path, use 50 for dsDNA, 40 for RNA, or approximately 33 for ssDNA (µg/mL per A260). What does A260 mean? It is UV absorbance at a wavelength of 260 nm. What is dilution factor? Original sample volume divided by the aliquot volume in the diluted measurement, e.g. a 1:2 dilution gives a factor of 2. Are these results definitive? No; contaminants and instrument/path-length settings affect accuracy. Check A260/A280, A260/A230 and sample quality where relevant.
Sources & References
NEBioCalculator · OD260 conversionNucleic acid conversion factors and oligo quantitation guidance from New England Biolabs.
NanoDrop · Quantitation & purity guidanceGuidance on nucleic acid quantitation and purity using UV absorbance.Molecular-weight conventions · Ratio interpretation and low-concentration limitations. Sources checked October 15, 2026. References do not imply review or endorsement.
Quantify nucleic acids. Understand your sample.
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