Free planning tool
This free Biodiversity Net Gain calculator helps ecologists, consultants and students estimate habitat units before and after a development. Add your baseline habitats, add what will be retained, enhanced or created, and the tool shows the net change for area habitats, hedgerows and rivers, with a report you can download as CSV or PDF.
Important: this is a planning stage estimate with simplified scores. It is not the official Defra statutory biodiversity metric. For a planning application in England you must submit your calculation using the statutory metric. Use this tool to test options early, compare designs and explain biodiversity net gain to clients and students.
Biodiversity Net Gain Calculator
Planning stage estimate with simplified scores. It is not the Defra statutory biodiversity metric.
| Unit type | Baseline | Post development | Change | Net change | Result |
|---|---|---|---|---|---|
| Press Calculate BNG to see your result. | |||||
How to use the Biodiversity Net Gain calculator
- Add project details. Name the site, the assessor and the date, and set your target gain. The legal minimum in England is 10 percent.
- Record the baseline. Add every area habitat parcel on site with its area in hectares, its habitat type, condition and strategic significance.
- Add linear habitats. Enter hedgerows and rivers by length, because they are counted in their own units.
- Plan the post development site. For each parcel choose whether it is retained, enhanced or newly created, and whether it is on site or off site.
- Calculate and export. Press Calculate BNG to see the net change for each unit type, then download the CSV or save the report as a PDF.
How the numbers are calculated
Every habitat parcel is turned into habitat units with one simple idea: how much habitat there is, multiplied by how valuable and how healthy it is.
Area habitat units = area in hectares × distinctiveness × condition × strategic significance.
New or enhanced habitat is less certain than habitat that already exists, so created habitat is reduced by three risk multipliers: difficulty of creation, the time it takes to reach target condition, and spatial risk when it is delivered away from the site. Retained habitat keeps its full value.
Hedgerow and river units use length instead of area, multiplied by distinctiveness and quality. Area, hedgerow and river units are kept separate, because a loss of one type cannot be paid for with gains in another.
Net gain = (post development units − baseline units) ÷ baseline units × 100.
A worked example
A site has 2 hectares of semi natural grassland with a distinctiveness score of 2, condition scored 2 and low strategic significance (1.0). The baseline is 2 × 2 × 2 × 1.0 = 8 area units.
The development keeps 1 hectare of that grassland (4 units) and creates 1.5 hectares of native woodland with a distinctiveness score of 3 and a target condition of 2. Before risk, that is 1.5 × 3 × 2 = 9 units. With moderate difficulty (0.9) and a moderate delay to maturity (0.85), the created woodland is worth 9 × 0.9 × 0.85 = 6.9 units.
Post development total: 4 + 6.9 = 10.9 units. Net change: (10.9 − 8) ÷ 8 = about 36 percent gain, comfortably above a 10 percent target.
What biodiversity net gain means in England
Under the Environment Act 2021, most new developments in England must leave nature measurably better than before. Mandatory biodiversity net gain started on 12 February 2024 for major developments and on 2 April 2024 for small sites. Developers must deliver at least a 10 percent gain, measured with the statutory biodiversity metric, and the gains must be secured for at least 30 years. Gains can be delivered on site, off site, or as a last resort through statutory biodiversity credits.
Frequently asked questions
Can I submit this calculation with a planning application?
No. Planning authorities in England require the Defra statutory biodiversity metric. This tool is for early estimates, option testing and teaching.
Why are hedgerows and rivers calculated separately?
They are linear habitats measured by length, and each unit type must show its own gain. Planting woodland cannot make up for removing a hedgerow.
Why is created habitat worth less than existing habitat?
New habitat takes years to mature and can fail, so the calculation discounts it for difficulty, time and distance from the site.
Is my data saved?
Everything runs in your browser. Download the CSV or PDF before closing the page if you want to keep your work.
Who is this tool for?
Ecologists, planning consultants, developers and students who want a quick, clear picture of biodiversity net gain before running the full statutory metric.
Erzsebet Frey (Eli Frey) is an ecologist and online entrepreneur with a Master of Science in Ecology from the University of Belgrade. Originally from Serbia, she has lived in Sri Lanka since 2017. Eli has worked internationally in countries like Oman, Brazil, Germany, and Sri Lanka. In 2018, she expanded into SEO and blogging, completing courses from UC Davis and Edinburgh. Eli has founded multiple websites focused on biology, ecology, environmental science, sustainable and simple living, and outdoor activities. She enjoys creating nature and simple living videos on YouTube and participates in speleology, diving, and hiking.
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