# Forest inventory plot calculator

Last updated: 2026-11-01

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.

Open the tool: https://restorationscope.com/tools/forest-inventory-plot-calculator (free, no account).

Read more: [Forest inventory plots for ARR projects](https://restorationscope.com/blog/forest-inventory-plots-arr).

## How it works

1. Pick the precision your standard asks for: ±10% at 90% confidence for Verra VM0047 and the CDM tool’s default, ±20% at 90% for projects under Gold Standard’s A/R methodology v2.1 (credits up to 2025 vintage), or your own.
2. Enter the project area, the plot size and how much plot biomass varies (CV). Split the project into strata if planting years or sites grow differently.
3. Read the plots to measure, how many to install with spares, and how the count moves if plots vary more or the target changes.

## Input and output

- Input: A precision target (±10% at 90% confidence, ±20% for Gold Standard credits up to 2025 vintage, or your own), the project area, the plot size and how much plot biomass varies (the coefficient of variation), for the whole project or for each stratum.
- Output: The number of plots to measure, plots per stratum, how many to install with 10% spare, the formula with your numbers, and how the count changes if the variation, target or confidence changes.

## Example

Input:

```text
Target ±10% at 90% confidence · 500 ha · 0.05 ha plots · CV 40%
```

Output:

```text
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)
```

## Limits

- The answer is only as good as the coefficient of variation you enter. IPCC’s example is 10–15 pilot plots in each stratum, in a stand like the one you will monitor.
- It counts plots for one estimate of biomass at one point in time. Under VM0047 the precision that is deducted is on the change since the project started.
- It uses the CDM sample-size tool (AR-TOOL03) but leaves out its equation 3, so where plots cover more than 5% of the area it gives more plots than the tool. Your methodology and your verifier decide the sample design.
- It does not place the plots on a map.

## Privacy

Everything runs in your browser. Your files are never uploaded to our servers.

## Questions

### How many inventory plots does a reforestation project need?

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.

### Where do I get the coefficient of variation?

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.

### What happens if my inventory misses the precision target?

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%.

### Is my data uploaded anywhere?

No. The calculation runs in your browser, and the numbers you enter stay there.