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Walk-through

How to Use This Calculator

4 steps

1

Choose the sequence type

On the Molecular Weight tab, select whether your sequence is double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), or RNA. Each type has a different average mass per base or base pair, so the estimate depends on getting this right.

2

Enter the sequence length

Enter the length in base pairs for dsDNA, or nucleotides for ssDNA and RNA. The result card updates instantly with the estimated molecular weight in both g/mol and kDa.

3

Convert mass to moles

Switch to the Mass ↔ Moles tab and enter a mass in nanograms (for example, from a spectrophotometer or Qubit reading) to convert it to picomoles using the molecular weight from step 2.

4

Get the copy number

The Copy Number tab uses the same mass and molecular weight to compute the number of individual DNA or RNA molecules present, using Avogadro's number.

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Reference

Formula & Methodology

3 formulas

Molecular weight from length and type

dsDNA: MW = length × 617.96 + 36.04 (bp) ssDNA: MW = length × 303.7 + 79 (nt) RNA: MW = length × 320.5 + 159 (nt)

Each nucleotide (or base pair, for double-stranded DNA) contributes an average mass to the molecule, plus a small fixed adjustment for the 5' and 3' end groups. This gives a sequence-independent estimate — the actual molecular weight of a real sequence varies slightly with its exact base composition, since A, T, G, and C each have a different mass.

Mass to moles (ng to pmol)

pmol = (mass in ng × 1000) / MW (g/mol)

Moles = mass ÷ molecular weight. Working in nanograms and picomoles (rather than grams and moles) keeps the numbers in a convenient range for typical lab quantities, so the calculator applies the ng-to-g and mol-to-pmol unit conversions internally.

Copy number

copies = (pmol / 1e12) × 6.022 × 10^23

Copy number is the number of individual DNA or RNA molecules in a sample. It's computed by converting the amount back to moles and multiplying by Avogadro's number — the number of molecules in one mole of any substance.

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Glossary

Key Terms Explained

7 terms

Molecular weight (MW) The mass of one mole of a molecule, expressed in grams per mole (g/mol) or, for large molecules like DNA, in kilodaltons (kDa). For DNA and RNA it's estimated from the sequence length and the average mass per base or base pair.

Base pair (bp) Two complementary nucleotides (A-T or G-C) joined across the two strands of a double-stranded DNA molecule. Double-stranded DNA length is measured in base pairs; single-stranded DNA and RNA length is measured in nucleotides (nt).

Dalton (Da) A unit of molecular mass equal to one atomic mass unit, numerically equivalent to 1 g/mol. Large biomolecules like DNA are often reported in kilodaltons (kDa), where 1 kDa = 1,000 Da.

ssDNA / dsDNA / RNA The three strandedness types this calculator supports: single-stranded DNA, double-stranded DNA, and RNA. Each has a different average per-unit mass, since RNA's ribose sugar and dsDNA's paired strands change the average mass per base.

Picomole (pmol) One trillionth (10⁻¹²) of a mole — a convenient unit for the tiny molar amounts of DNA or RNA typically handled in a molecular biology lab.

Copy number The number of individual molecules of a given DNA or RNA sequence present in a sample, calculated from the molar amount (moles) using Avogadro's number.

Oligo Short for oligonucleotide — a short, single-stranded synthetic DNA or RNA sequence (typically under 100–200 bases), commonly used as a primer or probe.

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Scenarios

Real-World Examples

3 worked examples

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Estimating a plasmid insert's molecular weight

1,000 bp dsDNA fragment

Type dsDNA Length 1,000 bp

MW = 1,000 × 617.96 + 36.04 = 617,996.04 g/mol, or approximately 618 kDa. This sequence-independent estimate is close enough for most cloning and molar-ratio calculations without needing the exact base composition.

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Converting a Qubit reading to picomoles

100 ng of the same 1,000 bp dsDNA fragment

Mass 100 ng MW 617,996.04 g/mol

pmol = (100 × 1,000) / 617,996.04 ≈ 0.162 pmol. This is the amount you'd use to calculate molar ratios for a ligation or a qPCR standard curve.

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Finding the copy number for a qPCR standard

Same 100 ng, 1,000 bp dsDNA sample

Moles ≈ 0.162 pmol Avogadro's number 6.022 × 10²³

copies = (0.162 / 1e12) × 6.022 × 10²³ ≈ 9.76 × 10¹⁰ copies. This is the number you'd dilute down to build a standard curve for absolute quantification by qPCR.

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Deep Dive

Estimating DNA and RNA Molecular Weight

3 chapters · 4 min read

Knowing the molecular weight of a DNA or RNA sequence is the first step to converting between mass, moles, and molecule copy number — numbers that come up constantly when preparing ligations, qPCR standards, or transfections. This calculator gives a fast, sequence-independent estimate from just the length and strandedness of your sequence, then chains that estimate into mass-to-mole and copy-number conversions.

Why the estimate doesn't need the actual sequence

The exact molecular weight of a DNA or RNA molecule depends on precisely how many of each base (A, T/U, G, C) it contains, since each nucleotide has a slightly different mass. For most practical purposes — cloning, PCR, transfection dosing — that level of precision isn't necessary. Instead, this calculator uses the average mass per base (or base pair, for double-stranded DNA) established by standard molecular biology references, which is accurate to within a fraction of a percent for typical sequences. The small fixed end-group adjustment (36.04 for dsDNA, 79 for ssDNA, 159 for RNA) accounts for the terminal phosphate and hydroxyl groups at the ends of the molecule.

Why dsDNA, ssDNA, and RNA give different answers for the same length

A double-stranded DNA molecule of 1,000 base pairs contains 2,000 individual nucleotides — two complementary strands — so its per-unit mass (617.96 g/mol per bp) reflects both strands combined. A single-stranded DNA or RNA molecule of the same numeric length has only half as many total nucleotides, so its mass is calculated per single nucleotide instead (303.7 g/mol/nt for ssDNA, 320.5 g/mol/nt for RNA). RNA is slightly heavier per nucleotide than ssDNA because its ribose sugar carries an extra oxygen atom compared to DNA's deoxyribose. Always confirm which type your sequence actually is before comparing molecular weights across experiments.

From molecular weight to copy number

Once you know the molecular weight, converting a measured mass (from a spectrophotometer, Qubit, or gel quantification) into a molar amount is a single division: moles = mass ÷ MW. From there, multiplying by Avogadro's number (6.022 × 10²³ molecules per mole) gives the copy number — the actual count of DNA or RNA molecules in your sample. This chain of conversions is exactly what's needed to prepare a dilution series for a qPCR absolute-quantification standard curve, or to calculate molar ratios for a ligation reaction.

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Questions

Frequently Asked Questions

6 questions

What formula does this calculator use for molecular weight?+

It multiplies the sequence length by an average mass per base (or base pair for dsDNA) and adds a small fixed end-group adjustment: dsDNA = length × 617.96 + 36.04, ssDNA = length × 303.7 + 79, RNA = length × 320.5 + 159. These are standard sequence-independent approximations used throughout molecular biology.

Why do dsDNA and ssDNA give different molecular weights for the same length?+

Double-stranded DNA length is measured in base pairs, where each unit represents two complementary nucleotides (one per strand). Single-stranded DNA and RNA length is measured per single nucleotide. That's why the per-unit mass used for dsDNA is roughly double the per-unit mass used for ssDNA.

How do I convert nanograms to picomoles?+

Divide the mass in nanograms by the molecular weight in g/mol, then multiply by 1,000 to convert to picomoles: pmol = (mass_ng × 1000) / MW. The calculator's Mass ↔ Moles tab does this automatically once you've set the sequence type and length.

How is copy number calculated?+

Copy number is the molar amount (in moles) multiplied by Avogadro's number (6.022 × 10²³ molecules per mole). The calculator first converts your picomole amount back to moles, then multiplies by Avogadro's number to give the total molecule count.

Is this molecular weight exact for my specific sequence?+

No — it's a sequence-independent estimate based on the average mass per base. The exact molecular weight depends on the precise number of A, T/U, G, and C bases in your sequence, since each has a slightly different mass. For most lab calculations (ligation ratios, qPCR standards, transfection dosing) this estimate is accurate enough; use dedicated sequence-analysis software if you need the exact value for a known sequence.

Does this work for RNA the same way as DNA?+

Yes — select RNA as the type and enter the length in nucleotides. RNA uses its own average mass per nucleotide (320.5 g/mol, slightly higher than ssDNA's 303.7 g/mol) because ribose sugar is heavier than the deoxyribose used in DNA.

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DNA Molecular Weight Calculator

Sequence

Result

Mass to moles

Result

Copy number

Result

How to Use This Calculator

Choose the sequence type

Enter the sequence length

Convert mass to moles

Get the copy number

Formula & Methodology

Key Terms Explained

Real-World Examples

Estimating a plasmid insert's molecular weight

Converting a Qubit reading to picomoles

Finding the copy number for a qPCR standard

Estimating DNA and RNA Molecular Weight

Why the estimate doesn't need the actual sequence

Why dsDNA, ssDNA, and RNA give different answers for the same length

From molecular weight to copy number

Frequently Asked Questions

Get a branded PDF of your results

DNA Molecular Weight Calculator

Sequence

Result

Mass to moles

Result

Copy number

Result

How to Use This Calculator

Choose the sequence type

Enter the sequence length

Convert mass to moles

Get the copy number

Formula & Methodology

Key Terms Explained

Real-World Examples

Estimating DNA and RNA Molecular Weight

Why the estimate doesn't need the actual sequence

Why dsDNA, ssDNA, and RNA give different answers for the same length

From molecular weight to copy number

Frequently Asked Questions

Get a branded PDF of your results

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