RNA Molarity Calculator: ng/uL to nM, pmol, Copies

RNA Molarity Calculator

Convert an RNA concentration in ng/µL into molarity (nM or µM), pmol per µL, and copies per µL using the transcript length and average ribonucleotide mass. See molecular weight and the ng needed for a target pmol amount.

🧬Real RNA Presets

📝RNA Sample Inputs

RNA concentration (ng/µL)

Reading from a Nanodrop or Qubit, equal to µg/mL.

Transcript length (nt or bp)

Nucleotides for ssRNA, base pairs for dsRNA.

Strandedness

Sample volume (µL)

Used for total moles, mass, and copies in the tube.

Molarity output unit

Target amount (pmol)

Calculator returns ng and µL needed for this many pmol.

End correction (g/mol)

Decimal places

Molarity 0 nM from ng/µL and MW

Amount per µL 0 pmol/µL 1 nM = 0.001 pmol/µL

Copies per µL 0 molarity × 6.022e17

Molecular weight 0 g/mol (nt × base) + end

🔢Formula Snapshot

320.5g/mol per nt (ss)

+159terminal correction

1e6ng/µL to nM factor

640g/mol per bp (ds)

🧪Average Nucleotide Mass Reference

Ribonucleotide	Symbol	Monophosphate MW	Notes
Adenosine	A (AMP)	329.2 g/mol	Purine, heaviest base
Guanosine	G (GMP)	345.2 g/mol	Purine, highest MW
Cytidine	C (CMP)	305.2 g/mol	Pyrimidine
Uridine	U (UMP)	306.2 g/mol	Replaces T in RNA
Average	N	320.5 g/mol	Used for mixed-base ssRNA

📏Common RNA Lengths

RNA Type	Typical Length	Strand	Approx MW
Mature miRNA	21 to 23 nt	ss	~7,090 g/mol
siRNA duplex	21 bp	ds	~13,599 g/mol
tRNA	76 nt	ss	~24,517 g/mol
sgRNA (CRISPR)	~100 nt	ss	~32,209 g/mol
GFP mRNA	~996 nt	ss	~319,377 g/mol
16S-like rRNA	~1900 nt	ss	~609,109 g/mol

📈ng/µL to nM Examples (ssRNA)

Length	MW (g/mol)	At 50 ng/µL	At 100 ng/µL	At 200 ng/µL
22 nt	7,210	6,935 nM	13,870 nM	27,739 nM
100 nt	32,209	1,552 nM	3,105 nM	6,209 nM
500 nt	160,409	312 nM	623 nM	1,247 nM
1000 nt	320,659	156 nM	312 nM	624 nM
2000 nt	641,159	78 nM	156 nM	312 nM
4000 nt	1,282,159	39 nM	78 nM	156 nM

🗂RNA Molarity Comparison Grid

RNA Type	Length	MW (g/mol)	nM at 100 ng/µL	pmol/µL	Copies/µL at 100 ng/µL
miRNA	22 nt	7,210	13,870 nM	13.87	8.35e12
siRNA duplex	21 bp	13,599	7,353 nM	7.35	4.43e12
tRNA	76 nt	24,517	4,079 nM	4.08	2.46e12
sgRNA	100 nt	32,209	3,105 nM	3.10	1.87e12
IVT transcript	1000 nt	320,659	312 nM	0.31	1.88e11
Luciferase mRNA	1650 nt	528,984	189 nM	0.19	1.14e11
rRNA	1900 nt	609,109	164 nM	0.16	9.89e10
mRNA vaccine	4000 nt	1,282,159	78 nM	0.08	4.70e10

⚙Full Formula Breakdown

Molecular weightssRNA MW = (length_nt × 320.5) + 159. dsRNA MW = (length_bp × 640) + end correction. The +159 approximates the 5′ terminal phosphate.

Unit basis1 ng/µL = 1 µg/mL = 1 mg/L = 0.001 g/L, so grams per liter = ng/µL × 0.001.

Molaritymol/L = (ng/µL × 0.001) ÷ MW. Multiply by 1e9 for nM, giving nM = (ng/µL × 1e6) ÷ MW.

pmol per µL1 nM = 1 nmol/L = 0.001 pmol/µL, so pmol/µL = nM ÷ 1000. Equal to ng/µL ÷ MW × 1000.

Copies per µLcopies/µL = mol/L × 6.022e23 × 1e-6 = mol/L × 6.022e17.

Total in tubeTotal pmol = (pmol/µL) × volume_µL. Total mass ng = ng/µL × volume_µL.

Target amountng for target pmol = target_pmol × MW ÷ 1000. µL needed = target_pmol ÷ (pmol/µL).

📋Strand and Factor Reference

Item	Value	Where It Applies	Effect on Molarity
ss average base	320.5 g/mol	Single-stranded RNA	Lower MW, higher nM
ds average pair	640 g/mol	Double-stranded RNA	Higher MW, lower nM
End correction	+159 g/mol	5′ triphosphate approx	Small drop at short lengths
ng/µL to nM	× 1e6 / MW	Any concentration reading	Direct molarity conversion
Avogadro	6.022e23 /mol	Copy number counting	Sets copies per mole

💡Practical RNA Molarity Tips

Length tip: Molarity depends heavily on transcript length. A 200 ng/µL stock of a 4000 nt mRNA is only about 156 nM, while the same reading for a 22 nt miRNA is near 27,700 nM.

Strand tip: Switch to double-stranded for siRNA and dsRNA reagents. Using the ssRNA base mass on a duplex roughly doubles the reported nM and halves the ng you would weigh out per pmol.

You want to order some siRNA and you sit down to do it and realize that twenty nanograms per microliter isn’t the same thing as twenty nanomolar. This is especially true if your transcripts is all over the map in terms of their lengths. In the end, mass concentration doesn’t tell you how many molecules are in the tube. That is what the difference between ng/uL and nM (nanomolar) represents. It is the difference between routine lab work and precision experimentation.

Downstream applications such as CRISPR guides or reverse transcription particularly care about number of molecules, not just their raw weight… And they’ll let you know. The calculator up top will run numbers for you. It takes into account exact length and strandedness of your RNA molecule to convert those ng/uL figures into something meaningful: molarity units.

Why Molarity Is Better Than Weight for RNA

This brings us to the heart of our problem: transcript length scales directly with its molecular weight. Your 22-nucleotide microRNA is ~7,000 daltons; your four-thousand nucleotide mRNA vaccine construct? More like one million daltons. What does that mean? For every unit mass of total RNA, the molarity (concentration) of your shorter molecule will be orders of magnitude greater than your longer molecule. Ignore this variation and assume the same conversion factor apply between two distinct sample, and you’ll either drown your reaction in surplus template or starve it. It’s a small detail, but it breaks reproducibility faster then any pipette mishap.

An average nucleotide has a mass of around 320.5 grams per mole; however, these numbers need to be doubled for double-stranded molecules and you should of remember to account for the molecule’s ends (a triphosphate or phospho group at either end increases the mass by ~159 daltons). For long transcripts, this adjustment won’t make much difference; however, if you’re dealing with short oligonucleotides, then that additional weight will make up more of total mass. Most people don’t think of this, which lead to small errors during stock prep or primer design.

Knowing the number isn’t enough; you also need to understand what it means. Molar concentration (nanomolarity) is critical if you’re setting up hybridization reactions, because speed of binding depends on the number of molecules present. An intermediate unit, picomoles per microliter. Is helpful in seeing amount of material being dispensed in small volume scenarios. Copies per microliter bridges the gap from spectrophotometer readout to the quantification scale of qPCR, and can helps you relate your measurements directly to the number of copies in a standard curve.

This is laid out clearly in the reference table on the page, which shows rate at which molarity drops with increasing length for common RNA types. Before you go and make any dilutions, you ought to consider these calculations. When you’re creating a ribosome profiling library, for example, you want to be aware that your 500-nucleotide spike-in control isn’t realy at the same molarity as your 21-nucleotide miRNA sample. This ensures your normalization factors won’t be skewed later on.

It compels you to acknowledge the physical reality of your molecules, instead of thinking of all the RNA in your reactions as interchangeable sludge. You make adjustments to your volumes to ensure that each reaction get the amount of functional units you intend them to get. Once you know how to get your inputs right, the math itself isn’t complicated, though taking the time to check length and strandedness each time will save you hours of troubleshooting later on.

Guessing costs a lot, while precision have no penalty. If your control behaves just as you expect and your final data is clean, then chances are that somebody paid attention to the count vs. Weight distinction from the very beginning. What might have been a potential source of error becomes a simple step in a strong workflow, where you don’t need to chase down phantom efficiencies anymore; instead, you can trust your reagents.