Free AI-Assisted

Standard Curve Calculator

Back-calculate concentrations from pasted 96-well absorbance with assay-aware fitting and QC flags—free, no account. Built-in AI agent assistant support.

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Key facts

Key facts about Standard Curve Calculator
FactValue
Assay typesBCA, Bradford, Sandwich ELISA, Competitive ELISA
Input modes96-Well Plate (paste + drag-assign), Data Columns, Manual Equation
Curve modelsLinear, quadratic, 4PL, 4PL inverted — side-by-side R² comparison
QC thresholdsReplicate CV >20%; R² <0.98 (linear/quadratic) or <0.995 (4PL); out-of-range ODs flagged
Minimum standards3 standard points required
ExportCopy as TSV, Download CSV, Download PDF, Print / PDF; decimal places 0–6
Session historyLast 5 successful runs stored in browser
Runs in browserYes — no install
Account requiredNo
Data uploadPaste only — calculations run locally

What it does

Most labs still build standard curves in Excel—writing trendline equations, inverting Y = mX + c by hand, and hoping the dilution factor gets applied. Wrong model choice and forgotten dilution corrections are common, silent errors.

The Standard Curve Calculator is a free browser tool that accepts raw absorbance from a 96-well plate, cuvette strips, or a pre-fitted equation and back-calculates unknown sample concentrations with dilution correction and inline QC.

Select Assay (BCA, Bradford, Sandwich ELISA, or Competitive ELISA); the tool preloads the recommended fit model. Toggle Input Format among 96-Well Plate, Data Columns, or Manual Equation. On Plate Input, paste a tab-separated grid and use Assign buttons to mark Standard, Sample, Blank, or Clear wells. Fill the standards table—use Generate Series for a geometric dilution—or enter Dilution factors per sample ID. Click Run Analysis; review the curve, Model comparison panel, and QC banners on Curve & Fit. Open Results for dilution-corrected concentrations with formula paths; export via Copy as TSV, Download CSV, or Download PDF on Report.

Why researchers use it

  • Skip hand-built Excel trendlines and inversion formulas
  • Apply assay-appropriate curve models automatically
  • Catch forgotten dilution factors before reporting
  • Flag high-CV replicates and out-of-range samples inline
  • Compare linear, quadratic, and 4PL fits with visible R²
  • Export audit-ready PDF reports for lab notebooks

Best for

  • BCA and Bradford protein quantitation
  • Sandwich and competitive ELISA back-calculation
  • Reanalysis after exporting from a plate reader
  • QC review before reporting assay results
  • Supplementary figures or ELN records needing fit parameters
  • Cuvette or strip data via Data Columns mode

When to use this vs alternatives

Choose this tool when you have plate or column absorbance data and need concentrations with blank subtraction, model comparison, dilution correction, and QC flags in one browser session. Use the Serial Dilution Planner when you are planning pipette-ready standard dilutions before the assay runs. Use the Dose-Response Curve Fitter when you need batch IC50 or EC50 from a multi-compound dose-response CSV—not single-plate quantitation. Spreadsheets and GraphPad Prism still fit if you already have a trusted template and do not need plate-layout paste or automated QC.

What makes it different

Excel is universal but forces manual model selection, blank subtraction, and dilution math—with no automated CV or range checks. GraphPad Prism fits 4PL curves well but costs hundreds per year and requires reformatting raw plate data by hand. MyAssays handles online 4PL fitting but offers limited QC, export, and kit-validation trust. Plate-reader software integrates with hardware but locks you to one instrument brand.

This browser standard curve calculator accepts a pasted 96-well grid or column data, applies assay presets, compares linear, quadratic, and 4PL models with visible R², flags replicate CV and range problems, and downloads a PDF report with parameters and excluded wells—no install or account. Researchers switch when Excel formulas silently mis-apply dilution factors or free online fitters lack replicate QC and a reproducibility record.

How to get started

  1. Open the workspace; select Assay (BCA, Bradford, Sandwich ELISA, or Competitive ELISA) and Input Format (96-Well Plate, Data Columns, or Manual Equation).
  2. Set optional Graph title, Measurement (e.g. OD450), and Conc. unit fields.
  3. On Plate Input, paste absorbance data; use Assign buttons to mark Standard, Sample, and Blank wells; fill the Standards table and Dilution factors.
  4. Click Run Analysis; review the curve, Model comparison, and QC banners on Curve & Fit.
  5. Open Results; set Decimal places; click Copy as TSV or Download CSV.
  6. On Report, click Download PDF or Print / PDF for the reproducibility summary.

Frequently asked questions

How do I calculate concentration from a standard curve?
Enter standard concentrations and absorbance (OD), fit a curve, then invert to find each sample concentration. For linear fits: concentration = (OD − intercept) ÷ slope after blank subtraction. ELISA typically uses 4PL because the response is sigmoidal. This tool blank-subtracts, picks the assay-appropriate model, and shows dilution-corrected concentrations with the formula path per sample.
Should I use linear or 4PL for my ELISA standard curve?
Sandwich ELISA data is typically sigmoidal, so 4PL fits better than a straight line. BCA and Bradford are near-linear over a narrow range—linear or quadratic fits work. Select your assay type; the tool recommends a default and shows R² for linear, quadratic, 4PL, and inverted 4PL (competitive ELISA) side by side.
How does dilution factor correction work?
If you diluted a sample before the assay (e.g. 20×), multiply the interpolated concentration by the dilution factor to recover stock concentration. Enter one factor per sample ID in Dilution factors; blank or missing values default to 1× with a warning. Formula paths on the Results tab show the full calculation for verification.
What does a CV above 20% mean for my replicates?
Coefficient of variation (CV%) measures replicate spread: CV = (standard deviation ÷ mean) × 100. Manufacturer guidelines often flag duplicate CV above 20% as unreliable. The tool computes CV across replicate wells per sample and marks high-CV groups inline. Review pipetting technique, well homogeneity, or exclude a clear outlier well—the exclusion is logged in the audit trail and included in the PDF report.
What is the minimum number of standard points required?
At least three standard points with assigned concentrations are required before Run Analysis is enabled. Fewer points cannot support a reliable curve fit. Use Generate Series in the standards table to fill a geometric dilution series from a starting concentration and fold factor, then edit individual levels (e.g. set level 1 to 0 for a blank standard).
When should I use Data Columns instead of the 96-well plate view?
Use Data Columns when your data are not on a full 96-well grid—cuvette readings, partial strips, or a simple list of standard and sample ODs. Paste standards as concentration-and-OD lines and samples as sample-ID-and-OD lines (replicates on consecutive rows). Click Refresh plate view to sync. Use Manual Equation when you already have fit parameters from Prism or another tool and only need to back-calculate sample concentrations.

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