RNA Molecular Weight Calculator

The RNA molecular weight is the total mass of a ribonucleic acid molecule, expressed in daltons (Da) or grams per mole (g/mol). For a typed sequence it equals the sum of the masses of every ribonucleotide in the chain, minus the water lost when those nucleotides are joined by phosphodiester bonds. Knowing the exact molecular weight is what lets you convert freely between mass (micrograms, nanograms) and amount of substance (nanomoles, picomoles), a conversion that underpins almost every RNA protocol from in vitro transcription to siRNA dosing.

This RNA molecular weight calculator works straight from your sequence. It reads the bases you type, ignores anything that is not A, U, G, or C, counts each base, applies the individual ribonucleotide monophosphate masses, and corrects for the water released during chain assembly. Whether you are designing a guide RNA, normalizing a siRNA duplex, quantifying an mRNA transcript, or preparing an oligoribonucleotide standard, an accurate RNA mass calculation is the difference between a clean experiment and wasted reagents. Alongside the molecular weight the tool reports GC content, the molar extinction coefficient at 260 nm, and ready-to-use mass-per-mole figures so you can pipette with confidence.

The calculator sums the molecular weight of each ribonucleotide monophosphate (NMP) present in the strand, then subtracts one molecule of water (18.015 Da) for every phosphodiester bond formed. A chain of n nucleotides contains n − 1 such bonds, so the total water removed is (n − 1) × 18.015 Da. The four monomer masses used are AMP = 347.22 Da, UMP = 324.18 Da, GMP = 363.22 Da, and CMP = 323.20 Da.

RNA is heavier than DNA per nucleotide because the ribose sugar carries an extra 2'-hydroxyl group and because uracil replaces thymine. A handy rule of thumb is roughly 340 Da per nucleotide for single-stranded RNA, slightly above the ~330 Da figure used for DNA. The exact base-by-base calculation below is far more accurate than that flat average for short oligoribonucleotides, where base composition can shift the true mass by several percent.

Always enter U for uracil rather than T, since the calculator only recognizes RNA bases and silently discards any T it finds. Because each base is weighed individually, the reported molecular weight is reliable even for very short guide RNAs and antisense oligonucleotides.

RNA molecules carry different chemistry at their 5' end, and that chemistry adds mass. This RNA molecular weight calculator lets you pick one of three 5' states and adjusts the molecular weight accordingly. The default, 5'-OH, models a synthetic oligoribonucleotide or a cleaved fragment and adds nothing to the base-and-backbone mass.

Selecting 5'-triphosphate adds approximately 159 Da to represent the two extra phosphate groups found on primary transcripts straight off an RNA polymerase. Selecting a 5' cap (m7G) adds about 541.3 Da to represent the 7-methylguanosine cap structure that protects eukaryotic mRNA and promotes ribosome recruitment. The capped value is the most relevant figure when you are quantifying a fully processed mRNA, such as an in-vitro-transcribed therapeutic transcript.

Choosing the right 5' state matters because a cap can change the apparent molecular weight by several percent for short RNAs and by a negligible fraction for long mRNAs. When the modification is set to anything other than None, the calculator displays both the unmodified molecular weight and the modified molecular weight so you can see exactly how much the 5' chemistry contributes.

Alongside molecular weight, the tool reports the approximate molar extinction coefficient at 260 nm, the wavelength at which nucleic acids absorb most strongly. It is estimated from base composition using per-base contributions of 15,400 M⁻¹cm⁻¹ for A, 9,900 for U, 11,500 for G, and 7,400 for C, summed across the whole sequence. This nearest-neighbor-free estimate is convenient for converting a spectrophotometer reading into concentration.

With the extinction coefficient and the molecular weight together, you can move between the three quantities you measure most often at the bench: absorbance, molar concentration, and mass concentration. Beer's law gives concentration from absorbance (A₂₆₀ = ε × c × path length), the molecular weight converts molar concentration to a mass concentration in nanograms per microliter, and the mass-per-mole outputs convert a known number of moles directly into a weight to dispense.

The calculator also reports the mass for 1 nmol by dividing the molecular weight by 1,000 (micrograms per nanomole) and the mass for 1 pmol by dividing by 1,000,000 (displayed as nanograms). These numbers answer everyday questions instantly, such as how many nanograms of a 20-mer guide RNA you need for a 5 pmol transfection, without reaching for a separate conversion chart.

The calculator also reports GC content, the percentage of bases that are guanine or cytosine, computed as (G + C) divided by total length times 100, plus the complementary AU content. GC content influences how RNA folds and how stable a duplex is: GC pairs form three hydrogen bonds versus two for AU pairs, so GC-rich regions raise melting temperatures and create more rigid secondary structure. Seeing GC content next to molecular weight gives a fuller picture of how an oligo or transcript will behave.

A few accuracy notes are worth remembering. The tool models canonical, unmodified ribonucleotides and the three 5' states described above; it does not account for 2'-O-methyl, 2'-fluoro, phosphorothioate, or other backbone modifications common in therapeutic RNA, nor for a poly-A tail unless you include those adenosines in the typed sequence (each adds about 347 Da). It also does not add the mass of fluorescent dyes or conjugates. For modified constructs, add the manufacturer-supplied mass of each modifier to the calculated value. Finally, the per-base AMP, UMP, GMP, and CMP masses used here are the widely cited monophosphate values, so the result is best understood as the molecular weight of the RNA backbone and bases, which is precisely the quantity needed for routine mass-to-mole conversions in transcription, siRNA, and qPCR workflows.