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For the carbon inventory of a reforestation (ARR) project under Verra VM0047, Gold Standard or the CDM. Enter how much your pilot plots vary and the precision your standard asks for; the count uses the CDM sample-size tool (AR-TOOL03).
44 plots, for ±10% at 90% confidence; install 49.
44plots, for ±10% at 90% confidence
Install 49: IPCC guidance recommends 10% more for plots you can’t find again later.
| If one thing changes | Plots |
|---|---|
| Plots vary 1.5 times as much | 97 |
| Target ±20% at 90% confidence | 14 |
| 95% confidence instead of 90% | 62 |
Bigger plots usually vary less, so count both ways with your pilot data before you choose a size.
It follows the CDM tool for sample plots in reforestation projects (AR-TOOL03, equation 1). For one stratum and a small share of the area sampled, that comes down to:
AR-TOOL03, eq. 2
t is the Student’s t-value for your confidence level (1.645 at 90%), CV is how much plot biomass varies as a percentage of the mean, and E is the allowed error as a percentage of the mean. When the answer is under 30 plots, the tool works it out again with the t-value for that many plots minus one, which adds a few. Equation 1 also holds a finite-population term, which takes a few away when plots cover more than a few percent of the area. With strata, plots go to each stratum by its area times its standard deviation, with at least two in each (our choice: the fewest that can estimate a variance), and never more than fit in it.
When plots cover more than 5% of the area, AR-TOOL03 then applies a further cut (equation 3). Equation 1 already holds that finite-population term, so this calculator leaves equation 3 out: on small sites it can give many more plots than the tool.
A planning estimate, not a sample design your verifier has approved. Your methodology and your validation and verification body decide what is enough.
Work out how many forest inventory plots a reforestation (ARR) carbon project needs for the precision its standard asks, from how much your pilot plots vary, with strata, plot size and spare plots. Uses the CDM sample-size tool’s equation (AR-TOOL03); your standard sets the precision.
Read the guide: Forest inventory plots for ARR projects
Example: input
Target ±10% at 90% confidence · 500 ha · 0.05 ha plots · CV 40%
Example: output
44 plots to measure; install 49 with 10% spare (1.645 × 40 ÷ 10)² = 43.3; AR-TOOL03 equation 1, with its finite-population term, gives 43.1, rounded up At ±20%: 14 plots (under 30, so t comes from n − 1 degrees of freedom)
It depends on how much biomass varies between plots and the precision your standard asks for. The CDM sample-size tool (AR-TOOL03) gives n = (t × CV ÷ E)² for one stratum: with plots that vary by 40% of the mean, ±10% at 90% confidence needs (1.645 × 40 ÷ 10)² = 43.3, rounded up to 44 plots, and ±20% needs 14 once the small-sample t-value is used. IPCC guidance recommends 10% spare plots.
From a pilot: IPCC good practice suggests 10–15 plots in each stratum may be enough, measured in a stand like the one you will monitor, at the age your plots will reach. Divide the standard deviation of plot biomass by its mean. Existing inventory data from the area or a similar one works too.
Excess sampling uncertainty costs credits; the inventory’s other requirements still apply. Under Verra VM0047, uncertainty above 10% of the change in carbon stocks (at 90% confidence) is deducted point for point from cumulative removals, and no credits are issued if it exceeds 100%. Under Gold Standard’s A/R methodology v2.1, error above ±20% is deducted from the mean; its STARR draft would deduct total uncertainty above 10%.
No. The calculation runs in your browser, and the numbers you enter stay there.
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