DNA molecular weight calculator
Enter a DNA sequence to calculate molecular weight in g/mol, molar extinction coefficient at 260 nm, and base composition for single-stranded or double-stranded DNA.
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See all toolsHow the DNA molecular weight calculator works
The DNA molecular weight calculator takes a nucleotide sequence and determines its total mass. You paste a DNA sequence into the tool, select single or double-stranded, and the calculator outputs the molecular weight in Daltons. It also provides the molar extinction coefficient, which helps you measure DNA concentration in a lab setting.
The science behind mass and extinction
Every nucleotide in a DNA strand has a specific mass. When DNA polymerases link nucleotides together, they form phosphodiester bonds by removing a water molecule. To calculate the molecular weight of single-stranded DNA, the calculator adds the average mass of each nucleotide monophosphate and subtracts the mass of water for each bond formed. For double-stranded DNA, the calculator doubles the mass because two complementary strands are bound together. The average molecular weight of one DNA base pair is approximately 660 Daltons.
The calculator also outputs the molar extinction coefficient at 260 nanometers. This number measures how strongly the DNA absorbs ultraviolet light at that specific wavelength. Each base absorbs light differently. Guanine absorbs the most light, followed by adenine, cytosine, and thymine. The tool accounts for the specific order of bases in your sequence to give an accurate coefficient. Scientists use this number with the Beer-Lambert law to find the concentration of a DNA sample. If you measure the absorbance of your sample at 260 nanometers with a spectrophotometer, you can divide that value by the extinction coefficient to calculate the exact molarity of your DNA.
Practical uses in the lab
This calculation is highly practical for molecular biology workflows. When you order synthetic oligonucleotides from a manufacturer, they often provide the molecular weight and extinction coefficient on the spec sheet. You can verify those numbers using this tool. In the lab, knowing the exact DNA MW allows you to convert between mass and molarity. Recipes for PCR reactions, ligations, and transfections usually require specific molar ratios of DNA. Without an accurate DNA molecular weight calculator, you cannot determine how many picomoles of your DNA fragment you are adding to the reaction tube.
What the calculator leaves out
This tool provides a theoretical calculation based on standard base masses. It assumes your sequence contains only standard adenine, thymine, guanine, and cytosine bases. It does not account for chemical modifications like methylated bases or fluorescent tags, which add extra mass. It also cannot measure the actual concentration of your specific lab sample, as it only calculates the theoretical constants needed for you to do the math yourself.
Frequently asked questions
How do you calculate the molecular weight of DNA?
You calculate the molecular weight of DNA by adding the mass of every nucleotide in the sequence. Because forming chemical bonds removes a water molecule, you subtract the mass of water for each bond. For double-stranded DNA, you multiply the final sum by two.
What is the molar extinction coefficient of DNA?
The molar extinction coefficient is a number that measures how much ultraviolet light a specific DNA sequence absorbs at 260 nanometers. It depends on the order of the bases because guanine absorbs more light than thymine. Scientists use it to calculate DNA concentration.
How do you convert OD260 to DNA concentration?
You convert OD260 to DNA concentration using the Beer-Lambert law. You divide the absorbance reading from your spectrophotometer by the molar extinction coefficient of your specific sequence. This gives you the molarity. You can then multiply by the molecular weight to find the mass concentration.
What is the molecular weight of a DNA base pair?
The average molecular weight of one DNA base pair is approximately 660 Daltons. This average accounts for the slight mass differences between adenine, thymine, guanine, and cytosine. You can multiply this number by the total number of base pairs in your sequence to estimate the total mass.