This free LOD and LOQ calculator gives you detection and quantification limits from data you already have. Paste your calibration data or replicate blank results to get the limit of detection and limit of quantification. Choose the ICH Q2 calibration-curve approach (3.3σ/S and 10σ/S), the 3s/10s replicate approach, or the US EPA method detection limit. You also get the full regression statistics and a calibration plot.
How to use the LOD and LOQ Calculator
- Pick a method: calibration curve (ICH Q2), replicate 3s/10s, or EPA MDL.
- Paste your calibration data as concentration and response pairs, or your replicate results.
- Choose the estimate of σ where asked, and set the concentration unit label.
- Press Calculate to see the LOD, the LOQ and the working behind them.
Keep the LOD and LOQ calculator output in your validation file, and confirm the LOQ with spiked samples at that level. For a wider view of detection limits, see the Eurachem method validation guide (The Fitness for Purpose of Analytical Methods).
Formulas used
Which LOD method should I use?
- Calibration curve: quick and accepted by ICH Q2 for pharmaceutical validation. Build the curve at low concentrations near the expected limit. A wide-range curve inflates σ and overstates the LOD.
- Blank replicates: best when blanks give a measurable, variable signal, as in ICP, AAS and ion chromatography. Run blanks through the whole procedure.
- Low-level spikes: use when blanks give no signal at all (common in chromatography). Spike at 1–5× the estimated LOD.
- EPA MDL: required for many US environmental methods. Uses at least 7 spiked samples and the one-sided Student’s t at 99% confidence.
Good practice for ISO/IEC 17025 validation
- Report LOD and LOQ in the same basis as results (for example mg/kg dry weight), including dilution and sample mass.
- Verify the LOQ experimentally: replicate spikes at the LOQ should meet your precision (e.g. RSD ≤ 20%) and recovery criteria.
- Re-check limits after major changes: new instrument, new column or detector, or a new matrix.
Frequently asked questions
What is the difference between LOD and LOQ?
The LOD is the lowest concentration you can reliably tell apart from zero. The LOQ is the lowest concentration you can measure with acceptable precision and accuracy. The LOQ is typically about 3 times the LOD.
Why 3.3 and 10?
With σ as the noise, 3.3σ/S gives a low risk of both false positives and false negatives at the detection limit. 10σ/S corresponds to roughly 10% relative standard deviation, a common quantification criterion.
Should I use sy/x or the SD of the intercept?
Both are allowed by ICH Q2. The residual SD (sy/x) is more common. The intercept SD is usually a bit larger and more conservative. State which one you used in your validation report.
How many replicates do I need for an MDL?
At least 7 spiked samples (and preferably method blanks as well), analysed over at least 3 separate batches, as required by the EPA MDL procedure.
How do you calculate LOD and LOQ from a calibration curve?
Under ICH Q2: LOD = 3.3 × σ ÷ S and LOQ = 10 × σ ÷ S, where S is the slope and σ is the residual standard deviation (sy/x) or the SD of the intercept. With σ = 0.002 absorbance and S = 0.05 per mg/L, LOD = 0.13 mg/L and LOQ = 0.40 mg/L.
What is the EPA MDL?
The EPA method detection limit (40 CFR Part 136, Appendix B, Revision 2) is calculated from at least seven spiked replicates: MDL = t(n−1, 0.99) × s, with t = 3.143 for seven replicates. Revision 2 also requires an MDL from method blanks, and you report the higher of the two.
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Results are estimates for general information. Check critical decisions with a qualified professional.