Measurement · England
BNG metric
The BNG metric is Defra's statutory biodiversity metric, the formula and calculation tool that must be used to measure biodiversity net gain in England. It converts each habitat parcel's area, distinctiveness, condition and strategic significance into biodiversity units, discounts created or enhanced habitat for difficulty and time to target condition, and compares the post-development total with the baseline. The Biodiversity Net Gain Hub estimator mirrors this logic in simplified, non-statutory form.
What the metric measures
The statutory metric measures all habitat types inside the development's red line boundary, including grassland, woodland, hedgerows, lakes and watercourses. GOV.UK lists the factors it accounts for: the habitat's size, condition, strategic significance and type. For created or enhanced habitats it also considers the difficulty of creation or enhancement, the time the habitat takes to reach its target condition, and the distance from the habitat that was lost. The output is a count of biodiversity units, split into three separate ledgers: area habitats, hedgerows and watercourses. Each ledger must show a 10% gain independently.
| Factor | Typical range | What drives it |
|---|---|---|
| Area | hectares | Measured parcel size from the habitat survey |
| Distinctiveness | 0 (very low) to 8 (very high) | Habitat type: rarity, species richness, priority status |
| Condition | 1 (poor) to 3 (good) | Condition assessment against published criteria |
| Strategic significance | 1.0 to 1.15 | Whether the location is identified in the Local Nature Recovery Strategy or local plan |
| Difficulty (new habitat) | 0.1 to 1.0 | Technical difficulty of creating or enhancing that habitat |
| Temporal (new habitat) | 3.5% discount per year | Years to reach target condition |
| Spatial risk (off-site) | 0.5 to 1.0 | Distance of the gain site from the development |
Units = area × distinctiveness × condition × strategic significance, with the additional multipliers applied to created, enhanced or off-site habitat. Very-low distinctiveness habitat such as sealed surface scores zero.
Baseline versus post-development
- Baseline (pre-development). A competent ecologist surveys and maps every habitat parcel inside the red line, assesses condition and enters the data. GOV.UK is explicit that everything within the boundary must be included, whether it will be lost, retained or improved.
- Post-development. The same boundary is described as it will be on completion: retained habitat at full value, created and enhanced habitat discounted for difficulty and time, and built footprint at zero.
- Comparison. The metric reports the change and whether the 10% gain objective is met for area habitats, hedgerows and watercourses separately, applies trading rules for medium and higher distinctiveness losses, and shows any shortfall to be met off-site or with statutory credits.
The metric is meant to be run repeatedly as the design evolves, saving iterations as versions and submitting the final one with the biodiversity gain plan. GOV.UK's own example: developing on woodland might cost 20 units plus 2 more for the 10% gain, while developing on modified grassland might cost 8 units plus 0.8.
Version and date notes
- Statutory biodiversity metric — the only version accepted for mandatory BNG. Published by Defra; tools and user guide on GOV.UK (page updated 2 June 2026).
- Small sites metric (SSM) — simplified statutory version for qualifying small developments; fixed condition values; no off-site module (off-site units can still be used).
- Biodiversity Metric 4.0 and earlier — Natural England's voluntary predecessors. Not accepted for new applications; calculations can be copied into the statutory tool.
- Biodiversity Net Gain Hub estimator — non-statutory, version 1.0, 17 September 2026. A teaching model of the area-habitat logic only. Not a substitute for the statutory tool.
Try the logic before you commission a survey
The biodiversity net gain calculator on this hub lets you enter baseline and post-development parcels using the same structure, with every multiplier printed on the page and a reproducible worked example. It is useful for early site selection questions ("how much value is in that grassland?") and for understanding what your ecologist's metric output means. It is not a statutory calculation.
Frequently asked questions
How is biodiversity net gain measured?
Biodiversity net gain is measured in biodiversity units using the statutory biodiversity metric. For each habitat parcel the metric combines size, habitat type (distinctiveness), condition and strategic significance; for created or enhanced habitat it also applies difficulty, time-to-target-condition and off-site distance risk factors. Baseline and post-development totals are compared and the post-development total must be at least 10% higher.
Which version of the biodiversity metric do I have to use?
For mandatory BNG you must use the statutory biodiversity metric calculation tool published by Defra on GOV.UK, or the small sites metric if your development qualifies. Earlier Natural England versions (Biodiversity Metric 4.0 and before) are not accepted in planning applications, although calculations started in 4.0 can be copied across.
What is the small sites metric?
The small sites metric (SSM) is a simplified version of the statutory metric for small ("minor") developments: 1 to 9 dwellings on 1 hectare or less, or commercial floorspace under 1,000 square metres or a site under 1 hectare. It fixes condition values so no condition assessment is needed and can be completed by a non-ecologist, but it cannot be used where priority habitats, protected sites or European protected species are present.
What is a good BNG score?
The statutory minimum is a 10% increase in biodiversity units over the baseline for each of area habitats, hedgerows and watercourses that are present. Anything at or above 10% meets the biodiversity gain objective; some local plans ask for more where justified. Trading rules also require losses of medium and higher distinctiveness habitat to be compensated like-for-like or better, so the headline percentage is not the only test.
Sources
Official guidance this page relies on, with the "last updated" date shown on each source when this page was written. Always check the live GOV.UK page before acting.
- Calculate biodiversity value with the statutory biodiversity metric — GOV.UK (Defra) (updated 14 July 2026)
- Statutory biodiversity metric tools and guides — GOV.UK (updated 2 June 2026)
- Biodiversity net gain — Planning Practice Guidance (MHCLG) (updated 31 July 2026)
- Understanding biodiversity net gain — GOV.UK (Defra) (updated 14 July 2026)