When you are applying for registration of a pesticide in Great Britain or Northern Ireland, you must calculate predicted environmental concentrations (PECs) for each environmental compartment. There are several computer models available for applicants to use when calculating PECs. Further information on computer models is available on the relevant environmental compartment pages.
This page lists the Excel calculator tools for calculating PECs for soil and surface water exposure. All the Excel calculators calculate PEC values based on single or multiple applications. Similarly, links to the current versions of the FOCUS groundwater modelling tools currently used in GB and NI are also provided.
This page also includes information on a tool for correcting soil degradation rates for temperature or field moisture.
All calculators are Microsoft Excel files that need Microsoft Excel 2007 or later.
Each Excel file includes instructions on how to use the calculators.
Soil
| Calculator/ tool | Use | Related pages |
|---|---|---|
| PECsoil calculator Version 2.1.1 (July 2025) |
Use this to calculate initial and time weighted average (TWA) PEC values for soil (PECsoil). The spreadsheet also calculates the accumulation of a substance in soil (PECsoil accumulation) | Soil assessment for pesticide registration in Great Britain and Northern Ireland |
| Arrhenius and Walker converter Version 1.0 (2014) |
Use this to calculate degradation rates corrected for temperature or field moisture. The Arrhenius equations are used to convert degradation rates based on temperature and can convert based on Q10 values of 2.2 and 2.58. The Walker equation is used to correct for field moisture content. | Soil assessment for pesticide registration in Great Britain and Northern Ireland |
Surface water
| Calculator/ tool | Use | Related pages |
|---|---|---|
| PECsw/sed spray drift and tier 1 drainflow calculator (.xlsx) Version 2.1.1 (July 2025) | Use this to calculate PECsw and PECsed values from spray drift (both standard and drift reduction technology (DRT) calculations) and drainflow (tier 1) exposure routes. | Spray drift surface water exposure assessments for pesticide registration in Great Britain and Northern Ireland - HSE First tier drainflow calculation for pesticides registration in Great Britain and Northern Ireland - HSE |
| MACRO 5 PECsw drainflow (higher tier MACRO) tool Version 2.2 (May 2025) | Use this to calculate higher tier PECsw values due to contamination from drainflow. Works on a Windows 10 operating system. Do not use this spreadsheet for the first-tier assessment | MACRO higher tier drainflow modelling for pesticides registration in Great Britain and Northern Ireland |
| STP PEC calculator | A simple Excel calculation tool to implement the equations from the ECHA guidance along with information on fate within the sewage treatment plants (STP) derived from the SimpleTreat model. Use this when considering post harvest treatments for PPPs | Post-harvest treatments |
Groundwater
The following table lists the current versions of the FOCUS groundwater modelling tools currently used in GB and NI.
| Calculator/tool | Use | Related pages |
|---|---|---|
| FOCUS_PEARL_5.5.5 | Use this to calculate 1st tier PECgw values | Groundwater assessment for pesticide registration in GB and NI. Please also consult the FOCUS website for model installation instructions. |
| FOCUS_PELMO_6.6.4 | Use this to calculate 1st tier PECgw values | Groundwater assessment for pesticide registration in GB and NI. Please also consult the FOCUS website for model installation instructions. |
| FOCUS_MACRO_5.5.4 | Use this to calculate 1st tier PECgw values | Groundwater assessment for pesticide registration in GB and NI. Please also consult the FOCUS website for model installation instructions. |
| FOCUS_PRZM_4.6.2 | Use this to calculate 1st tier PECgw values | Groundwater assessment for pesticide registration in GB and NI. Please also consult the FOCUS website for model installation instructions. |
Refer to the Groundwater assessment for pesticide registration in GB and NI page for more information on how these models should be used.
The current version of guidance for First Tier groundwater assessment in GB and NI is Generic Guidance for Tier 1 FOCUS GroundWater Assessments” Version 2.3.