qPCR Efficiency Calculator

How to use this calculator

1. Choose the value you already know: standard-curve slope, efficiency, or amplification factor.
2. For slope mode, check the axes in your analysis software: Cq must be plotted against log₁₀ starting quantity. Enter the fitted negative slope, including its minus sign.
3. For efficiency mode, choose percent or fraction beside the input. For example, 95 percent and 0.95 fraction describe the same efficiency. Changing the unit preserves that value.
4. Read all three equivalent quantities. The result-unit selector changes efficiency between percent and fraction; it does not change amplification factor or the experimental estimate.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Annealing Temperature and DNA Copy Number.

Efficiency and amplification factor describe different quantities

Amplification factor is the multiplier from one cycle to the next in the exponential model. A factor of 1.95 means the modeled target amount becomes 1.95 times its previous value. Efficiency measures the increase relative to that previous amount: 1.95 − 1 = 0.95, or 95%. A factor of two therefore corresponds to 100% efficiency, not 200%.

For an illustrative starting amount of 100 target copies, a constant factor of 1.95 predicts an expected amount of 195 after one cycle and 380.25 after two. Those fractional values describe a mathematical expectation. They are not literal fractional molecules, and the model is not a prediction that amplification continues unchanged into the plateau phase.

Use the correct standard-curve axes

This relationship uses Cq as the vertical coordinate and the base-10 logarithm of positive starting quantity as the horizontal coordinate. Increasing the starting quantity normally reduces the cycles needed to reach a common threshold, giving a negative slope. The starting quantities can be consistently measured copies, concentrations, or relative amounts.

Changing from copies to thousands of copies shifts every horizontal coordinate by the same constant, leaving the slope unchanged. Switching to a different logarithm base changes the slope. A plot against dilution denominators, such as 1, 10, and 100, also needs careful interpretation: those denominators increase while the amount of template decreases. Do not insert that slope directly into this equation.

What the slope says about tenfold dilution

A tenfold reduction in starting quantity changes log₁₀ quantity by −1. With a slope of −3.35, the fitted Cq therefore rises by 3.35 cycles. At exact doubling, the corresponding separation is log₂(10), approximately 3.321928095 cycles. Consequently, −3.322 is a rounded slope that gives approximately 99.997% efficiency, not exactly 100%.

Reverse mode is useful for checking a reported efficiency or translating a proposed performance target into a slope. It performs algebra on the value entered; it does not reconstruct the standards, fit a line, calculate R², or estimate a confidence interval. Keep the original dilution data and fitted model with the calculation.

Interpret the estimate in its experimental context

The NIHS-hosted biomarker-method white paper recommends an average efficiency of 90–110% in its validation context, alongside other performance assessments. This range is useful context rather than a universal certificate of assay quality. Apply the criteria established for the particular method and intended use.

An apparent value above 100% can arise when the fitted curve is distorted. For example, inhibitors in concentrated standards can delay their Cq values, changing the slope as the sample is diluted. Preparation errors, low-copy sampling, and measurement behavior can also affect the estimate. Inspect the underlying data, replicate variation, controls, and specificity instead of treating this single converted number as proof of reliable quantification.