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Quick guide
Understand the calculation
Scientific converter across 17 chemistry and biology concentration units, grouped by family: molar (M, mM, µM, nM, pM), particle count (copies/µL, copies/mL), mass (g/L, mg/mL, µg/µL, ng/µL, pg/µL), mass/volume proportions (% w/v, ppm, ppb) and the two defined by a reaction, Normality and U/mL. Crossing between molar and mass units asks for the solute molar mass, with a catalogue of 14 molecules and length-based estimation for nucleic acids.
When to use this tool
Preparing solutions and moving between the label on the vial and the working concentration: proteins in mg/mL to µM for molar ratios, DNA and RNA in ng/µL to nM for ligations and libraries, copies/µL for qPCR standard curves, Normality for acid-base and redox titrations, enzyme activity in U/mL, and ppm or % w/v in quality control and environmental analysis.
How it works
Type the value, pick the value unit and the result unit, and the page shows the general formula for that pair and, below it, the same chain with your number substituted. The 17 units do not belong to a single quantity but to two dimensions, each with its own hub: mol/L for molar units and g/L for mass units. Within one dimension the conversion is a pure ratio and does not depend on the solute —1 mg/mL is 1000 ng/µL whatever is dissolved—; crossing between them needs the molar mass, the value that links moles to grams: g/L = M × M_w. That is why molar mass is not a second operand but part of a solute-data rail, shown only when the visible reading needs it: opening “Show all equivalents” widens the rail, because there are now 17 rows on screen and three of them ask for a value of their own. You can type the molar mass or pick one of the 14 catalogue molecules; for nucleic acids it is estimated from the length, using 650 Da per base pair for double-stranded DNA, 330 Da per nucleotide for single-stranded DNA and 340 Da for RNA. Normality needs equivalents per mole and U/mL needs specific activity in U/mg. The significant-figures selector switches between 6 and 10, and that precision governs the result, the visible substitution and the equivalents table alike.
The input must be a finite decimal, with optional scientific notation: an incomplete form such as 1e or text with letters is rejected whole instead of being trimmed. Molar mass, equivalents, specific activity and length must be numbers greater than zero; when one is not, the converter says so on that field and not on the result. A missing value is not the same as an invalid input: the conversion waits and states what is missing, while the equivalents that do not need it keep showing. A negative value is still converted and raises a warning, because the arithmetic is correct even though that concentration does not exist. Rows that fall outside the floating-point range declare it. On the assumptions: ppm and ppb are read as mass over volume (1 ppm = 1 mg/L), the lab convention for dilute aqueous solutions, which does not match mass/mass ppm when the density departs from 1 kg/L; the molar mass of a nucleic acid computed from length is an average estimate that ignores base composition and chain ends; and the tool converts units, it does not prepare dilutions or compute how much to weigh for a given volume.
Verified examples
- Protein · BSA 2 mg/mL, M_w 66430 g/mol
g/L = 2; M = g/L ÷ 66430; µM = M × 10⁶ = 30.1069 - DNA · 1000 bp plasmid at 50 ng/µL
1000 bp × 650 = 650000 g/mol; 50 ng/µL = 76.9231 nM - Physiological saline · 0.9 % w/v NaCl, M_w 58.44 g/mol
0.9 % w/v = 9 g/L = 154.004 mM - qPCR · 10 pM standard
M = 10 × 10⁻¹²; copies/µL = M × Nₐ × 10⁻⁶ = 6.02214e+6 - Titration · 0.1 M H₂SO₄ with 2 eq/mol
N = 0.1 × 2 = 0.2
Frequently asked questions
Why does it ask for molar mass on some conversions and not on others?
Because a molar unit counts particles and a mass unit weighs grams, and the only thing that links them is how much one mole of the solute weighs: g/L = M × M_w. Going from mg/mL to ng/µL needs no knowledge of what is dissolved, because both measure mass over volume and the conversion is a fixed ratio. Going from mg/mL to µM does, because 1 mg/mL of table salt and 1 mg/mL of an antibody hold nowhere near the same number of molecules.
How is the molar mass of DNA or RNA estimated?
From the length, with the commonly used averages: 650 Da per base pair for double-stranded DNA, 330 Da per nucleotide for single-stranded DNA and 340 Da per nucleotide for RNA. A 3000 bp plasmid therefore gives 1950000 g/mol. It is an average estimate: it does not account for base composition or chain ends, so for short oligos —where those differences weigh proportionally more— use the exact mass reported by the supplier.
Are the ppm in this tool mass over volume or mass over mass?
Mass over volume: 1 ppm equals 1 mg/L and 1 ppb equals 1 µg/L, just as 1 % w/v equals 10 g/L. That is the usual convention for dilute aqueous solutions, where the density is close to 1 kg/L and both readings nearly coincide. In a matrix with a different density —an organic solvent, a brine, a solid— mass/mass ppm is not the same number, and that conversion needs the sample density, which this tool does not handle.
What are Normality and U/mL, and why do they need a value of their own?
Normality counts reactive equivalents per litre rather than moles, so it depends on how many H⁺, e⁻ or OH⁻ each formula unit transfers in the actual reaction: H₂SO₄ is 2 eq/mol as an acid and KMnO₄ is 5 in acidic medium. U/mL measures enzyme activity, not amount of substance, and reaching mass requires the specific activity of the preparation in U/mg. Both are properties of the assay rather than of the unit, which is why the converter asks for them instead of assuming them.