fmol (Femtomole) Calculators

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A femtomole (fmol) is 10⁻¹⁵ moles, representing an extremely small quantity of substance — approximately 602,214 molecules. In molecular biology, femtomoles are the standard unit for measuring nanogram-scale amounts of DNA and RNA, particularly for next-generation sequencing library preparation and quantitative PCR assays. Converting mass (ng) to moles (fmol) requires knowing the molar mass of the molecule, which for nucleic acids depends on the number of base pairs or nucleotides and the average molecular weight per base.

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Converting ng to fmol for Double-Stranded DNA

fmol = (mass in ng × 10⁶) / (bp × 660 g/mol)

The average molecular weight of a base pair in dsDNA is approximately 660 g/mol (330 g/mol per nucleotide × 2 strands). Example: How many fmol in 100 ng of a 3000 bp PCR product? fmol = (100 × 10⁶) / (3000 × 660) = 10⁸ / (1.98 × 10⁶) ≈ 50.5 fmol.

fmol in NGS Library Preparation

Next-generation sequencing platforms (Illumina, Oxford Nanopore) require loading libraries at defined molar concentrations — typically 4–20 pM (picomolar, 10⁻¹² M) or specific fmol quantities. Accurate fmol calculation from ng measurements (by NanoDrop or Bioanalyzer) ensures correct cluster density, avoiding under-clustering (no data) or over-clustering (high duplication rates).

Converting RNA ng to fmol

For single-stranded RNA: fmol = (mass in ng × 10⁶) / (nt × 340 g/mol). The average nucleotide molecular weight for ssRNA is approximately 340 g/mol. For a 2000 nt mRNA: fmol = (100 ng × 10⁶) / (2000 × 340) = 10⁸ / 680,000 ≈ 147 fmol.

fmol in Cloning and Ligation

Optimal ligation ratios use molar equivalents of vector and insert. A common recommendation is 1:3 molar ratio (vector:insert) with 50 ng vector. Calculate vector fmol, then multiply by 3 to get insert fmol, and convert back to ng using the insert size formula.

Glossary

Femtomole (fmol)
A unit of amount equal to 10⁻¹⁵ moles; used in molecular biology to express nanogram-scale quantities of DNA and RNA relative to their molecular weight.
Molar Concentration
Amount of substance per unit volume (mol/L or M); in NGS, libraries are diluted to picomolar (pM) concentrations based on fmol calculations from mass and fragment size data.
Ligation Molar Ratio
The molar ratio of insert to vector DNA used in cloning reactions; typically 3:1 or 5:1 (insert:vector) to maximize ligation efficiency and directional cloning.

Frequently Asked Questions

A femtomole (fmol) is 10⁻¹⁵ moles, or one quadrillionth of a mole. It contains approximately 602,214 molecules (Avogadro's number × 10⁻¹⁵). In molecular biology, femtomoles are used to express amounts of DNA, RNA, and oligonucleotides because these molecules have high molecular weights — a nanogram of a typical PCR product represents only tens to hundreds of femtomoles.

For double-stranded DNA: fmol = (mass in ng × 10⁶) / (length in bp × 660). The factor 660 g/mol is the average molecular weight per base pair. Example: 50 ng of a 500 bp fragment = (50 × 10⁶) / (500 × 660) = 5 × 10⁷ / 330,000 ≈ 151.5 fmol. Online calculators streamline this for different fragment sizes and nucleic acid types.

NGS instruments require libraries loaded at precise molar concentrations (fmol or pM) to achieve optimal cluster density for maximum read quality. Under-loading produces sparse, suboptimal clusters; over-loading causes overlap and high duplication rates. Since library fragment length varies between samples, mass-based measurements (ng) from NanoDrop must be converted to fmol using fragment size data from a Bioanalyzer or TapeStation before diluting for sequencing.

For ligation, calculate vector fmol: (ng vector × 10⁶) / (bp vector × 660). Then multiply by the desired molar ratio (typically 3:1 insert:vector) to get insert fmol. Convert insert fmol to ng: ng insert = (fmol insert × bp insert × 660) / 10⁶. This ensures a molar — not mass — ratio of insert to vector, which is critical for efficient ligation and colony yield.