Application guide
WindCalc
Wind-loading calculations and project wind parameters.
What this tool does
Wind-loading calculations and project wind parameters. This guide describes the main workflow, key information and stated limits to review when using the tool.
Application version: v1.6.
Build the site wind parameters
- Enter or select the site location and confirm the point on the map. Review the coastline-distance sectors and the selected map provider.
- Enter the verified site altitude, structure height and the height at which pressure is required.
- Choose the applicable season and return period, then review the fundamental basic wind velocity against the displayed map figure.
- Set terrain and directional inputs for each sector. If using town terrain or orography, confirm all displacement heights, upwind samples and assumptions.
- Calculate, inspect the sector results and retain the map overlay alongside the output so the selected values can be checked against the source figure.
What the map and sector tools do
- The site map supports location selection and a georeferenced view of the UK National Annex wind map figure.
- A digitised map interpolation provides a starting estimate; manual override fields let you enter a checked source-figure value.
- The coastline tool samples directional rays against a coastline dataset, making exposure direction easy to compare.
- Twelve directional sectors use the UK National Annex Cdir table at 30° increments, with separate sector calculations where terrain inputs vary.
Verify the inputs
The map extraction and coastline data are screening aids, not a substitute for checking the governing code figure and site exposure. The app notes approximate interpolation tolerance and separate regional approximations. Confirm altitude, terrain category, upwind fetch, orography and the selected edition of the standard before using pressures in design.
In-app help
Basic wind speed and pressure
vb,map (m/s) is the Figure NA.1 wind speed at the selected location, before altitude correction. Use the manual override if the source figure gives a different reading.
A and z (m) are site altitude above mean sea level and each sample height above ground where pressure is assessed. The maximum sample height h1 is the total structure height and limits the town displacement calculation.
calt adjusts the map wind speed for site altitude and assessment height. The displayed site value is shared by all 12 directions.
cseason is the entered seasonal factor. T is the entered return period in years; the app calculates cprob, the probability factor, from T. At the default T = 50 years, cprob = 1.
vb,0 = vb,map × calt is the site wind speed before direction, season and probability factors. For each wind-from direction, vb = vb,0 × cdir × cseason × cprob.
qb (kN/m²) is the basic velocity pressure for that direction, calculated as 0.5 × 1.226 × vb² / 1000. The 1.226 is the air density in kg/m³ used by this app.
Directional table: terrain and exposure
Wind FROM (degrees) is the direction the wind arrives from, measured clockwise from north. Each row represents one 30° sector.
cdir is the UK National Annex Table NA.1 direction factor for that row. It changes the basic wind speed before pressure is calculated.
dshore (km) is the distance upwind to the first mapped ocean coastline, or a distance you type or drag along that ray. It is used to estimate the country exposure factor. The mapped line is not mean high water springs. Move it where an estuary or inland water wider than about 1 km lies within 1 km upwind. A missing coast stays blank.
dtown (km) is the upwind distance through town for that wind-from direction. The app classifies each ray from mapped building coverage. Open at the site is country. An edge is town at that distance. Still built-up at the search limit stays town, and the chart uses at most 20 km. Enter 0 for country. A blank classification has no qp.
have and x (m) are the average adjacent-building height and upwind spacing entered for that sector. With structure height h, they determine hdis on town sectors only. Exposure uses z − hdis, with a minimum of 2 m. Country sectors use hdis = 0. If h is missing, displacement is omitted.
ce is the country exposure factor, estimated from dshore and the effective height. ce,T is the additional town correction estimated from dtown and the effective height; it is 1 for a country sector. Both are fitted approximations to Figures NA.7 and NA.8 and must be checked for a design value.
co is the orography factor. It is applied only when it is above 1.05, using the upwind foot altitude Abase. Clear the box to restore the profile. Enter 1 to reject the crest. If a terrain profile cannot be loaded, the app assumes co = 1 and calculates qp without terrain amplification; retry the profile or enter a checked manual co and Abase to include it.
Reading the pressure results
qp (kN/m²) is the peak velocity pressure for a sector. The base value is qb × ce × ce,T for a town sector, or qb × ce for a country sector. Significant orography then multiplies by [(calt at the upwind foot ÷ calt at the site) × (co + 0.6) / 1.6]². The simplified factor is not used above 50 m, or where the structure is taller than 50 m. Those sectors have no qp and need the detailed NA.4b method.
Governing direction is the row with the highest finite qp. Blank sectors are ignored. Sectors complete counts shoreline distances, town or country classification, checked profiles and finite pressures. It is not a count of rows merely created. Use Retry missing terrain where shown.
Site inputs and wind map
Click the UK map to select a location. Drag the divider between inputs and map to resize the input pane.
Selecting or calculating a site estimates altitude above mean sea level from the online Copernicus DEM GLO-90 via Open-Meteo. The 90 m elevation grid is a screening estimate; check it against surveyed or authoritative site levels and edit the field where needed. If the service is unavailable, enter the altitude manually. The three sample heights define pressure bands; h1 must be the maximum and is used as the structure height h for terrain displacement and the simplified-orography limit. Enter neighbouring building heights separately on the Wind Map tab.
Season, return period and site values are shared by all 12 sectors. The Wind Map applies direction factors independently to each sector.
The map estimate uses a digitised 25 km grid interpolated from Figure NA.1, with an approximate ±0.3 m/s reading tolerance. Northern Ireland and the Channel Islands are approximations and are not used until you enter an override or confirm the reading. Check the estimate against the source figure.
Town-terrain screening
Default have and x values prefill the 12 directional rows. Adjust them separately in any sector to represent shielding in that direction. Each row calculates its own hdis using EN 1991-1-4 Annex A.5 and uses z − hdis for exposure factors, with a minimum of 2 m.
Copernicus EMC-BUILT 2022 estimates each directional town distance from local building coverage. The screening uses 100 m total built-up surface averaged over a 5 × 5 cell window (500 × 500 m in the equal-area grid). The first sustained stretch below the chosen coverage is the estimated terrain transition. These settings are screening assumptions, not UK National Annex rules; parks and other open spaces can end a ray before the town's outer edge.
Rays are sampled every 100 m to 50 km. A distance of ≥50 km means no transition was found within that distance. The current NA.8 approximation uses at most 20 km although the table retains the measured distance. Purple markers are the estimated transitions. Drag a marker along its cardinal ray to move the edge past a park or yard, or type the distance. Distance 0 is country. Clear a manual distance to restore the dataset estimate. Missing data needs a manual distance and has no qp.
Copernicus EMC-BUILT 2022, R2025A · © European Union 2025, CC BY 4.0. WindCalc uses a local UK extract, including Northern Ireland.
Terrain profiles and manual overrides
Terrain screening checks each wind-from direction. For a reliable 12-sector SRTM run, start node 14-WINDCALC/server.cjs from the project folder and open http://127.0.0.1:8765/. If that local route is unavailable, the browser may use Copernicus DEM GLO-90 via Open-Meteo. Copernicus DEM GLO-90 data are credited to the Copernicus programme and Open-Meteo.
The downwind reach is half the selected upwind reach. A missing terrain result withholds qp and is identified beneath the directional table. Edit a sector's co and Abase to override it; clear the boxes to restore the calculated values. Enter co = 1, or use the profile button, to reject the crest. Automatic mode uses the hill or cliff formula suggested by that sector. Hill and cliff in the list force that formula for every sector.
Terrain factors are screening estimates, not a surveyed Annex A assessment. Check the selected crest and feature type; use a project-assessed manual override where appropriate. A sector with significant orography uses the upwind foot altitude. Other sectors keep the site altitude. A rise below 10 m, or a slope only one elevation sample long, is not treated as a feature.
The single-direction diagnostic uses the selected feature mode and upwind reach with the same detector as the 12-sector calculation. Wind FROM bearings include 0° = N, 90° = E, 180° = S, 225° = SW and 270° = W. The app samples an upwind/downwind terrain transect and detects the effective crest, H and Lu.
Choose a stored wind-from direction in the terrain-profile selector to review its cross section. Retry missing terrain rechecks only directions without a profile. Print/PDF uses the governing qp profile.
Shore Distance Method
The entered WGS84 point is compared with every usable segment in the default Natural Earth 10m coastline dataset. Twelve rays at 30° intervals are intersected with the mapped coastline to give the first shoreline crossing in each clockwise direction from north.
Directional Factor
Cdir is taken from Table NA.1 of the UK National Annex to BS EN 1991-1-4:2005+A1:2010. The tabulated directions run from 0° to 330° at 30° intervals, measured clockwise from due north.
Wind Map & Basic Velocity
The supplied Figure NA.1 is displayed as a georeferenced overlay on the interactive map. For Great Britain, the app converts the clicked WGS84 coordinate to the British National Grid and bilinearly interpolates a digitised isopleth surface. Northern Ireland and the Channel Islands use separate approximations because the printed figure contains inset mapping. The result is a map-reading aid: keep the overlay visible and use the manual vb,map override whenever the original code figure indicates a different value.
Orography Method
The terrain screening uses 91 elevation samples: 60 upwind, one at the site and 30 downwind. It tests upwind local crests, rejects a rise below 10 m or a slope shorter than two sample intervals, and selects the candidate inside the Annex A window with the largest co. Automatic mode uses the suggested hill or cliff formula for that sector. Inspect the ground profile before relying on the result.
Engineering Limitations
Natural Earth coastline data is suitable for general screening only and is not a legal shoreline, MHWS or flood-boundary dataset. Confirm the terrain profile visually, verify whether the feature is a hill/ridge or cliff/escarpment, and use project-specific survey or authoritative mapping where the result is sensitive.
Tips and field guidance
Report map inset
Zoom this map to update the radius to approximately half its visible height or width (0.1–2,000 km), or enter a radius manually. Use Show all limits to include the available town and coastline limits in all 12 directions.
Fundamental Basic Wind Velocity
Enter three elevations for your wind-load bands. The maximum, h1, is also the structure height used for terrain displacement and the simplified-orography limit.
Directional Peak Pressure Engine
Governing peak velocity pressure over all 12 wind-from sectors
Directional Peak Pressure Engine
Run the directional engine to show the calculation.
Areas in the app
Use these sections to locate the inputs, checks and results shown in the app:
- Location & Coastline
- Wind Map
- Report