This document summarizes the formula for calculating the concentration of double-stranded DNA (dsDNA) based on its absorption at 260nm. The formula, known as Beer's Law, states that absorption at 260nm (A260) equals the extinction coefficient at 260nm (E260) multiplied by the concentration (c) and light path length (l). For dsDNA, the extinction coefficient is 20 for a 1mg/mL solution and 1cm light path length. Two examples are provided to demonstrate calculating DNA concentration in μg/mL from a given A260 value using this formula and extinction coefficient.

1. Calculation of dsDNA
Concentration
DR. S. SIVASANKARA NARAYANI., M.SC., M.PHIL.,PH.D.,MRSB (UK)
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2. Formula
 The formula that describes the relationship between absorption at 260nm
(A260), concentration (c) (in mg/mL), the light path length (l) of the cuvette (in
centimeters), and the extinction coefficient at 260 nm (E260) for a 1 cm light
path is
A 260 = E260 l c
Note: The absorption value of 20 for a 1 mg DNA/mL solution is referred to as
DNA’s extinction coefficient.
This relationship is known as Beer’s Law. Since the light path, l, is 1, this
equation becomes
A260 = E260 c
c = A260/ E260
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3. A DNA solution has an A260 value of 0.5. What is the
DNA concentration in µg DNA/mL?
c = A260/ E260
c = 0.5 / 20 = 0.025 mg DNA/mL
0.025 mg DNA/mL X 1000 µg /mg
= 25 µg DNA/mL
 Therefore, the sample has a concentration of 25 µg DNA/mL.
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4. Try it📝
A DNA solution has an A260 value of 1.0. What is the
DNA concentration in µg DNA/mL?
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5. Reference
 Calculations for molecular biology and Biotechnology – A Guide to
mathematics in the laboratory, Second Edition – Frank. H. Stephenson
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