DNA Concentration Calculator

Related Calculators

A260 absorbance: UV light absorbance at 260 nm. Nucleic acids absorb strongly here, so this reading is used to estimate concentration.

Dilution factor: How much the sample was diluted before measurement. A direct reading uses 1, while a 1-to-10 dilution uses 10.

Path length: The distance light travels through the sample. A standard cuvette path length is 10 mm, which equals 1 cm.

A260 factor: The standard conversion value used to turn A260 into concentration. This calculator uses 50 for dsDNA, 33 for ssDNA, and 40 for RNA.

A260/A280 ratio: A quick purity check often used to look for protein or phenol carryover. DNA is often near 1.8 and RNA near 2.0 when clean.

A260/A230 ratio: A quick purity check often used to look for salts, phenol, carbohydrates, or chaotropic reagent carryover. Values around 2.0 to 2.2 are often considered cleaner.

ng/uL and ug/mL: Two concentration units with the same numeric value. For example, 25 ng/uL equals 25 ug/mL.

nM: Nanomolar concentration, which counts molecules rather than total mass.

Copies per uL: Estimated number of DNA or RNA molecules in each microliter.

Mistake: Entering a diluted sample but leaving Dilution factor (x) at 1.
Fix: Enter the total dilution multiplier so the reported Concentration (ng/uL) matches the original sample.

Mistake: Changing Path length (mm) even though the instrument already reported a 10 mm equivalent reading.
Fix: Leave Path length (mm) at 10 unless you know the absorbance was not normalized.

Mistake: Choosing Custom factor in Sample type but not entering a valid Custom factor (ug/mL) .
Fix: Enter a positive custom factor or switch Sample type back to Double-stranded DNA, Single-stranded DNA, or RNA.

Mistake: Requesting Concentration (nM) or Copies per uL with the wrong Fragment length (bp) .
Fix: Enter the actual fragment length. For RNA and single-stranded DNA, use nucleotide length even though the label says bp.

Mistake: Treating A260/A280 ratio or A260/A230 ratio as proof that the sample is clean.
Fix: Use What the ratios suggest as a quick screen only, then confirm quality with another method if the sample is important.

Mistake: Typing 0 for A280 absorbance (AU) or A230 absorbance (AU) and expecting a ratio.
Fix: Leave that field blank if you do not have a reading, or enter a value greater than 0 so the ratio can be calculated.

• This tool gives an absorbance-based estimate, so contaminants that also absorb UV light can make concentration look higher or lower than the true value.
• Purity ratios are screening clues, not proof of purity. A good ratio does not guarantee intact or amplifiable nucleic acid.
• The nM and copies/uL outputs depend on the entered Fragment length (bp) and on average molecular weight assumptions, so they are approximate.
• For single-stranded DNA and RNA, the shared fragment-length field is labeled in bp for UI simplicity, but the calculation treats the value as nucleotide length.
• If your instrument already normalizes readings to a 10 mm equivalent path length, changing Path length (mm) will distort the result.
• Very low A260 readings can be noisy, and very high readings can fall outside a reliable linear range unless the sample is diluted first.
• This calculator does not assess fragment integrity, sequence composition effects, fluorometric dye binding, or instrument-specific blanking errors.

How the calculator works

The main concentration comes from the A260 reading, the sample-specific conversion factor, the dilution factor, and the path length correction. UV absorbance at 260 nm is a standard way to estimate nucleic acid concentration [1].

path length in cm = path length in mm / 10

concentration (ug/mL) = (A260 x factor x dilution factor) / path length in cm

concentration (ng/uL) = concentration (ug/mL)

The factor is 50 for double-stranded DNA, 33 for single-stranded DNA, and 40 for RNA. If you choose Custom factor, the calculator uses your entered value instead.

Purity ratios

If you enter A280 or A230, the calculator also computes common UV purity ratios.

A260/A280 = A260 / A280

A260/A230 = A260 / A230

The interpretation note is a quick screen. Lower A260/A280 can suggest protein or phenol carryover, and lower A260/A230 can suggest salts, phenol, carbohydrates, or chaotropic reagent carryover. These are clues, not a final diagnosis.

nM and copies per uL

If you choose an extra conversion, the calculator turns mass concentration into molar concentration using average molecular weight assumptions.

for dsDNA: concentration (nM) = concentration (ng/uL) x 1,000,000 / (660 x fragment length)

for ssDNA or RNA: concentration (nM) = concentration (ng/uL) x 1,000,000 / (330 x fragment length)

copies per uL = concentration (nM) x 6.02214076 x 10^8

For ssDNA and RNA, the entered fragment length is treated as nucleotide count.

Mini example

Suppose you have double-stranded DNA with A260 = 0.25, dilution factor = 1, and path length = 10 mm. The path length is 1 cm, so the concentration is (0.25 x 50 x 1) / 1 = 12.5 ug/mL, which is also 12.5 ng/uL. If the fragment length is 1000 bp and you request nM, the estimate is 12.5 x 1,000,000 / (660 x 1000) = about 18.94 nM.

Assumptions used

This calculator assumes the absorbance reading was properly blanked, the chosen sample type matches the material being measured, and the path length entry matches how the instrument reported the reading. Results can differ from fluorometric methods or from direct counting methods because UV absorbance responds to any molecule in the sample that absorbs near these wavelengths [1][2].