Calibration Curve Calculator
A linear calibration curve from your standards — slope, intercept and R² — and the concentration of an unknown from its reading.
Measure a series of standards of known concentration, fit signal against concentration by least squares, and the line turns any later reading into a concentration: c = (signal − intercept) ÷ slope.
How the calibration curve calculator works
Measure a series of standards of known concentration, fit signal against concentration by least squares, and the line turns any later reading into a concentration: c = (signal − intercept) ÷ slope. R² tells you how straight the response is; below about 0.99 the range is probably too wide or a standard is off. Readings outside the range of the standards are extrapolations and should be diluted or re-run.
Formula: y = m·c + b (least squares); c_unknown = (y − b) ÷ m
Worked examples
| Inputs | Unknown concentration | Note |
|---|---|---|
| Five standards, unknown at 0.45 | 4.49 | ≈ 4.48 |
| A perfect line | 2.5 | 2.5 exactly |
| Reading above the top standard | 14 | 14 — extrapolated |
FAQFrequently asked questions
How many standards do I need?
At least three, preferably five to seven spread evenly across the working range, plus a blank. More points improve the fit and show up curvature you would miss with three.
Should the line go through zero?
Usually it should pass close to zero but is not forced through it; a non-zero intercept reveals blank signal or a systematic offset. Forcing through zero hides that.
What R² is acceptable?
Above 0.995 for most analytical methods, and many labs require 0.999 for chromatography. A lower value means a standard is wrong, the range is too wide, or the detector is saturating at the top.
Can I extrapolate beyond the top standard?
Not reliably — detectors saturate and Beer–Lambert bends at high absorbance. Dilute the sample into the range instead. The calculator flags readings outside the calibrated range.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.