How to Calculate Biodiversity Units (BNG Step-by-Step Guide)

A biodiversity unit is a standardised measure of the ecological value of a habitat. It allows planners to quantify how much biodiversity is present on a site or lost/gained through a development. This is important because UK planning law now requires developments to achieve at least a 10% net gain in biodiversity. Using units provides an objective way to show that a project will leave nature in a better state than before.

The 10% BNG requirement became mandatory for most major development planning applications in England on 12 February 2024. For small development sites, the requirement kicked in on 2 April 2024. Any new applicable planning applications after those dates must include a Biodiversity Gain Plan demonstrating a minimum 10% net gain (unless the project is exempt).

In most cases, yes. While anyone can technically download the metric tool, planning authorities expect biodiversity unit calculations to be done or verified by a “competent person,” such as a qualified ecologist. Ecology professionals have the training to assess habitat types and conditions properly. Using an ecologist not only lends credibility, it also helps avoid mistakes—ensuring your calculations will be accepted by the Local Planning Authority.

The statutory biodiversity metric is the official Defra tool for calculating biodiversity net gain. “Metric 4.0” was a previous version released in 2023; the statutory metric (sometimes informally called Metric 4.1 or later) is now the required version for mandatory BNG. In practical terms, the statutory biodiversity metric builds on the Biodiversity Metric 4.0 calculation tool but has been formally adopted under BNG legislation. Older metric versions (4.0 and before) are not accepted for new planning applications – you must use the latest statutory metric tool.

If after using the metric you find your on-site habitat proposals still fall short of +10% net gain, you have a few options. One is to provide off-site biodiversity units on other land you own – meaning you create or enhance habitat elsewhere and count those gains. Another option is to buy biodiversity units from a registered provider (a landowner or habitat bank that has excess units). As a last resort, you can purchase statutory biodiversity credits from the government. These credits fund conservation projects and essentially offset your shortfall. All these off-site measures will come with legal agreements and costs, and they must be documented in your BNG plan. The preference is always to maximise on-site gains first if possible.

By law, any habitats created or enhanced for BNG must be managed for at least 30 years (and some authorities may encourage longer-term or even permanent arrangements). This is to ensure the net gains are sustained and not just temporary. A Habitat Management and Monitoring Plan is typically required, detailing how those habitats will be looked after over the 30-year period (e.g. specific mowing regimes, invasive species control, etc.). Compliance may be enforced via planning conditions or conservation covenants.

Yes. The UK government provides the statutory biodiversity metric calculation tool as a free resource. It is effectively a Biodiversity Net Gain calculator free to download. There’s a version for general use and a simplified Small Sites Metric for qualifying small projects. These tools are free to use. However, using them correctly requires ecological expertise, so while the software is free, you may still need professional input to get reliable results.

A Biodiversity Gain Plan is a document that summarises how your development will achieve net gain. It’s a mandatory submission for planning (for projects where BNG applies). The plan typically includes: a description of the pre-development habitats and their biodiversity unit value, an explanation of the post-development habitat proposals with their expected unit gains, the calculation showing at least +10% net gain, and maps illustrating the habitats before and after. It will also address any other relevant details (like any irreplaceable habitat on-site, or whether you’re using an earlier baseline date due to habitat degradation concerns).

Not exactly. Biodiversity units are the measurement of habitat value (you generate or lose units through actions on the ground), whereas biodiversity credits usually refer to a financing mechanism to compensate for units you cannot deliver. In England’s scheme, statutory biodiversity credits are sold by the government as a last resort for developers who cannot meet their 10% net gain on-site or via local off-site units. Purchasing a credit funds habitat creation elsewhere. You can think of credits as buying the necessary units from an external source (the government), whereas biodiversity units themselves are the underlying “currency” of habitat value.

Trading rules are principles built into the metric to ensure that losses are compensated with gains of at least equivalent habitat type or value. For example, one rule is that you generally shouldn’t compensate the loss of a very high distinctiveness habitat with many units of a lower distinctiveness habitat – like losing ancient woodland can’t be justified by creating the same number of units of grassland. The metric may require extra units (through multipliers) if you try to substitute unlike habitats. Another rule ensures rivers (watercourse units) can’t be traded off against area habitat units, etc. These trading rules mean that it’s not just about the quantity of units but also quality and type of habitats – encouraging “like for like or better” compensation. The user guide for the metric outlines these rules in detail.

The statutory biodiversity metric was published after the introduction of mandatory BNG to formalise the calculation method in planning. It has since been updated, with the approach building on the earlier Biodiversity Metric 4.0 calculation tool and supported by the statutory biodiversity metric user guide.

The official tool used to calculate biodiversity units is the statutory biodiversity metric, produced by Defra and Natural England. It quantifies how land-use change impacts habitats by comparing their pre- and post-development conditions. This metric applies recognised biodiversity assessment methods to measure area, distinctiveness, and condition for each habitat type. It effectively acts as a biodiversity net gain calculator, showing whether your proposal meets the required 10 percent uplift.

For more detailed or complex projects—such as those involving large mixed-use sites or regional strategies—the statutory metric can also complement environmental impact assessments, ensuring all ecological changes and residual effects are captured within wider planning documentation.

While biodiversity unit calculations focus on quantifying habitat value for Biodiversity Net Gain, they also align with broader sustainability tools such as ecological footprint analysis and ecosystem service valuation. Ecological footprint analysis measures the overall environmental resource demand of a development, while ecosystem service valuation assesses the wider benefits nature provides—like carbon storage, water regulation, or recreation. Integrating these approaches gives a more holistic view of environmental performance.

Our team often combines BNG metric outputs with species distribution modelling and other data-driven methods to evaluate how development and land-use change impacts might affect wildlife connectivity and long-term ecological resilience. Together, these techniques support smarter, more sustainable design decisions and a genuinely positive legacy for biodiversity.