About BioCalc
A set of biochemistry calculators that show their working.
BioCalc exists because the arithmetic in a wet lab is simple but the unit bookkeeping is
not. Mass, amount, volume and concentration are tied together by one molecular weight, yet
the units span eighteen orders of magnitude and several of them hide their relationship —
1 µg/µL, 1 mg/mL and 1 g/L are the same number.
How it works
Every calculator converts its inputs to SI base units, does the arithmetic once, and converts back on the way out. That is why you can mix a molecular weight in kilodaltons with a mass in nanograms and a volume in microlitres and get a sensible answer in picomolar. Each result shows the formula and the intermediate values so you can check it or reproduce it in a notebook.
Everything runs in your browser as plain JavaScript. There is no server, no account and no network request — the pages work with the network disconnected once loaded.
Where the numbers come from
- Physical constants follow the 2019 SI redefinition, with the Avogadro constant fixed at exactly 6.022 140 76 × 10²³ mol⁻¹.
- Extinction coefficients at 280 nm use the tryptophan, tyrosine and cystine contributions from Pace et al., Protein Science 4:2411 (1995).
- Nucleotide masses use the sodium-salt convention: 617.96 g/mol per base pair of double-stranded DNA, 303.7 per ssDNA nucleotide, 320.5 per RNA nucleotide.
- Buffer pKa values and temperature coefficients are the conventional 25 °C literature values.
- Amino-acid residue masses are average (not monoisotopic) masses.
How the calculations are checked
The shared unit engine has a regression suite of 95 numerical tests covering unit round-trips, the core relations, Beer–Lambert, centrifugal force, nucleic-acid and protein conversions, and number formatting edge cases. A second pass verifies every built page: that the HTML parses, that every element the scripts reference exists, that no unit string is invalid, and that internal links resolve. Both run in CI on every change — a failure blocks the deploy.
That catches arithmetic and wiring errors. It cannot catch a wrong literature constant, so if you spot one, please open an issue.
Limitations worth knowing
- Henderson–Hasselbalch uses concentrations where it should use activities. At normal buffer ionic strengths the real pH can sit 0.1–0.3 units from the calculation. Get close here, then titrate with a meter.
- Tm estimates use the Wallace and GC/salt approximations, which are typically ±3 °C against measured values. For primer design, treat them as a starting point.
- OD-to-cell-count factors are strain- and instrument-specific. Calibrate your own.
Results are for planning experiments. Verify anything critical independently.
Source
The site is open source under the MIT licence at github.com/robert-henry-goldstein/biocalc.