At a glance
| Tool | Type | Price | Best for |
|---|---|---|---|
| UTCI Calculator (utci.org) | Web calculator, with Fortran source code | Free | Checking a single UTCI value against the reference implementation from the index's developers. |
| pythermalcomfort | Python library | Free (MIT licence) | Calculating UTCI and other comfort indices in bulk inside your own scripts or research workflow. |
| CBE Clima Tool | Web app, open source | Free | A free first look at the hours of heat and cold stress at a weather station, in sun or shade and with or without wind. |
| EPW Insights | Web app that runs in the browser | Free (AGPL-3.0) | Exploring UTCI, SET and mean radiant temperature from an EPW file you already have, without uploading it anywhere. |
| C4B | Web app, in the browser | Free plan; paid plans from €59/month (cancellation takes effect after 12 billing cycles) or €599/year, with a 7-day free trial | Seeing when outdoor spaces at any address will be comfortable, by season and hour, without a weather file or a 3D model. |
| Thermal comfort indices derived from ERA5 reanalysis (ERA5-HEAT) | Gridded dataset on the Climate Data Store | Free (Copernicus licence; account required) | Historical, gridded UTCI and mean radiant temperature for research, heat-stress mapping or trends over decades. |
| ClimateStudio | Rhino plugin (Windows) | Price on request; free trial for firms | Rhino users who want UTCI in the same environment as their daylight and energy simulations. |
| Ladybug Tools | Plugins for Grasshopper (Rhino), open source | Free and open source; Rhino needs its own licence | Parametric outdoor comfort studies on your own 3D model, from hourly UTCI at a point to spatial comfort maps. |
| Forma Site Design | Cloud app for site planning and early environmental analysis | Subscription, standalone or in Autodesk's AEC Collection; free trial | Comparing outdoor comfort across massing options on a real site model, early in design. |
| ENVI-met | Desktop 3D microclimate simulation software | Price on request (business, university and student licences) | Detailed simulation of how buildings, trees, surfaces and water change comfort in a street, square or neighbourhood. |
How to choose
Start with the scale of the question. A single UTCI value for one set of conditions needs only a calculator or a code library. Hourly UTCI for a whole year at one place comes from a weather file, which is what climate analysis tools such as CBE Clima Tool, EPW Insights and C4B work from. A map of comfort across a courtyard, street or masterplan needs a 3D model and a simulation of the surfaces, shade and wind around each point, which is what Honeybee's comfort map recipe, Forma Site Design's microclimate analysis and ENVI-met provide.
Mean radiant temperature usually decides the result. UTCI combines air temperature, humidity, wind speed and mean radiant temperature, and outdoors the sun dominates the last of these. Weather-file tools estimate it for a person standing in the open and usually let you switch sun and wind on or off; model-based tools calculate it from the geometry around each point. Check how a tool treats radiation before comparing its numbers with another tool's.
Check the wind input. UTCI is defined with wind speed at 10 m above the ground, the height of a weather station's anemometer, and the operational polynomial applies only between 0.5 and 17 m/s. Ladybug Tools' documentation notes that the 10 m speed is 1.5 times the speed felt at ground level in the Fiala model UTCI was built from, so wind measured or simulated at street level has to be converted first.
Match the tool to the design stage and the team. Free, browser-based tools answer early questions about seasons and hours of heat or cold stress in minutes. Rhino plugins and CFD tools answer design-specific questions later, but need a model, a licence and more time. Also ask where the weather data comes from: a station file describes the station, which can be far from the site, and a reanalysis value describes a grid cell, not the microclimate of a particular street.
UTCI Calculator (utci.org)
Checking a single UTCI value against the reference implementation from the index's developers.
- Enter air temperature, the difference between mean radiant and air temperature, humidity (vapour pressure or relative humidity) and wind speed at 10 m.
- The UTCI Fortran source code and documents are available to download from the same site.
- Home of the index developed by ISB Commission 6 and COST Action 730.
Good to know
- One set of conditions at a time: no weather files, time series or charts.
- The calculator notes that its polynomial approximation applies only to wind speeds between 0.5 and 17 m/s.
pythermalcomfort
Calculating UTCI and other comfort indices in bulk inside your own scripts or research workflow.
- Implements UTCI alongside PET, PMV/PPD, adaptive comfort, SET, Heat Index and other indices.
- The UTCI function accepts single values or arrays and returns the index with its thermal stress category.
- Includes a model of solar gain on the human body, useful for estimating mean radiant temperature in the sun.
- Installs from PyPI with pip and needs Python 3.10 or later.
Good to know
- No graphical interface: you supply the weather or simulation data and write the code.
- By default the UTCI function returns no value outside its valid range, including wind speeds below 0.5 or above 17 m/s.
CBE Clima Tool
A free first look at the hours of heat and cold stress at a weather station, in sun or shade and with or without wind.
- Hourly UTCI for a whole year in four scenarios that combine sun or shade with wind or no wind.
- Uses pythermalcomfort's solar gain model for mean radiant temperature; the no-wind case uses 0.5 m/s, the minimum the UTCI model allows.
- Works with EPW files for more than 27,500 locations from EnergyPlus and Climate.OneBuilding.Org, or with your own EPW.
- The underlying data can be downloaded as CSV.
Good to know
- Results are for the weather file's location, for a person in full sun or in generic shade, with or without wind; the geometry of surrounding buildings and trees is not modelled.
EPW Insights
Exploring UTCI, SET and mean radiant temperature from an EPW file you already have, without uploading it anywhere.
- Outdoor comfort analysis with UTCI, SET and mean radiant temperature, next to psychrometric, solar, wind and material analyses.
- Files are parsed in your browser; nothing is uploaded to a server.
- Future climate morphing to 2030, 2050 and 2080 under four SSP scenarios, and side-by-side comparison of two files.
- Charts export as PNG images.
Good to know
- You bring the EPW file (a map links to the official EnergyPlus weather archive), so results describe that station rather than your site.
C4B
We make C4BSeeing when outdoor spaces at any address will be comfortable, by season and hour, without a weather file or a 3D model.
- UTCI calculated with the Bröde et al. (2012) procedure and a SolarCal correction of mean radiant temperature, shown as heatmaps by season and hour in seven stress bands.
- Available on the Free plan with the nearest weather station's data; paid projects can also use site-specific reanalysis data at the exact coordinates.
- Projects on paid plans add comfort strategies for four sun and wind exposure cases, a directional comfort rose with windbreak guidance, and future-climate overlays.
Good to know
- No microclimate or CFD modelling: results describe the weather station or the reanalysis grid cell, not the effect of nearby buildings or trees.
- There is no CSV or image export; paid plans produce a PDF report.
Thermal comfort indices derived from ERA5 reanalysis (ERA5-HEAT)
Historical, gridded UTCI and mean radiant temperature for research, heat-stress mapping or trends over decades.
- Hourly UTCI and mean radiant temperature computed from the ERA5 reanalysis, from 1940 to near real time.
- Daily, monthly and seasonal UTCI statistics are also available.
- Downloaded through the Climate Data Store's web form or its Python API.
Good to know
- Reanalysis values describe regional conditions on a grid, not the microclimate of a specific street or courtyard.
Visit Thermal comfort indices derived from ERA5 reanalysis (ERA5-HEAT)
ClimateStudio
Rhino users who want UTCI in the same environment as their daylight and energy simulations.
- Hourly heatmaps in the site analysis include UTCI, with filters that remove the effect of wind (wind protection) or direct sun (shading).
- A weather browser with more than 30,000 weather files from EnergyPlus.net and Climate.OneBuilding.Org.
- Also covers daylight, electric lighting, glare and EnergyPlus-based thermal analysis.
Good to know
- Requires a licensed Windows version of Rhino 6, 7 or 8; it does not run on the macOS version of Rhino.
- UTCI in the site analysis is calculated from the weather file, with sun and wind filters rather than a model of the surroundings.
Ladybug Tools
Parametric outdoor comfort studies on your own 3D model, from hourly UTCI at a point to spatial comfort maps.
- The UTCI Comfort component takes air temperature, mean radiant temperature, humidity and 10 m wind speed, with outdoor solar MRT components to account for sun.
- Honeybee's UTCI Comfort Map recipe maps UTCI across sensor grids, combining EnergyPlus surface temperatures with Radiance view factors for longwave radiation, and Radiance ray tracing for sunlight (shortwave).
- Built on validated engines: Radiance, EnergyPlus and OpenStudio, Therm and Window, and OpenFOAM.
Good to know
- Requires Rhino, Grasshopper, EPW weather files and visual-scripting skills.
- In the comfort map recipe, outdoor air temperature, humidity and wind come from the EPW unless you supply wind speeds, for example from a CFD study.
Forma Site Design
Comparing outdoor comfort across massing options on a real site model, early in design.
- The microclimate analysis combines Forma's sun, daylight and wind analyses with local weather to calculate UTCI across the site.
- Historical weather sets typical conditions; you can choose the time of day, air temperature and wind speed, direction and height.
- A comfort view shows the share of time within a comfortable range that you can adjust by season.
- Part of a wider set of analyses including sun hours, daylight potential, wind, noise, solar energy potential and total carbon.
Good to know
- Autodesk positions the microclimate analysis for early-stage design decisions.
ENVI-met
Detailed simulation of how buildings, trees, surfaces and water change comfort in a street, square or neighbourhood.
- A holistic 3D microclimate model covering wind flow, solar radiation and shadow, vegetation, humidity, building physics and pollutant dispersion.
- The thermal comfort module calculates static indices such as PET, SET* and UTCI, and dynamic comfort along virtual walks.
- Simulations run on your own computer; the results viewer produces 2D and 3D maps and animations, and all modules can be scripted in Python.
Good to know
- Business licences are annual subscriptions; student licences last one year and exclude parallel computing.
- Its technical FAQ notes that simulation time depends strongly on the model area and settings.
See your site's climate before design begins
Pin any location and get climate, comfort and preliminary energy insight your whole team can read. Start with a 7-day free trial, or book a 30-minute demo.
Frequently asked questions
What is UTCI?
The Universal Thermal Climate Index is an equivalent temperature for outdoor conditions: the air temperature of a reference environment that would cause the same physiological strain as the actual combination of air temperature, humidity, wind and radiation. Values are grouped into thermal stress categories from extreme cold stress to extreme heat stress.
Is there a free UTCI calculator?
Yes. utci.org, the site of the International Society of Biometeorology's UTCI commission, has a free online calculator and the Fortran source code, and pythermalcomfort is a free Python library. CBE Clima Tool, EPW Insights and C4B's Free plan calculate hourly UTCI for a whole year from weather data.
What inputs does UTCI need?
Air temperature, mean radiant temperature, humidity and wind speed at 10 m above the ground. The operational version is a polynomial approximation valid for wind speeds between 0.5 and 17 m/s, so tools either set calm conditions to 0.5 m/s or return no value outside that range.
Should I use UTCI or PET?
UTCI assumes a person walking, with a metabolic rate of about 2.3 met (135 W/m²), whose clothing adapts to the outdoor temperature. For outdoor situations that do not fit those assumptions, such as people sitting rather than walking, Ladybug Tools' documentation recommends the Physiological Equivalent Temperature (PET). pythermalcomfort, Ladybug Tools and ENVI-met calculate both.
Can I map outdoor comfort across a whole site?
Yes, with tools that model the geometry: Honeybee's UTCI Comfort Map recipe in Ladybug Tools, Forma Site Design's microclimate analysis and ENVI-met. Weather-file tools such as CBE Clima Tool, EPW Insights and C4B describe a single point without a 3D model of its surroundings; EPW Insights can apply a simplified street-canyon setting to its radiation values.
Sources
We make C4B. Information about the other tools comes from their official websites and documentation, checked on 18 September 2026. Products and prices change, so check each vendor's site for the latest details. All product names belong to their respective owners.
- UTCI (ISB Commission 6 / COST Action 730): home page
- UTCI calculator
- Błażejczyk et al. (2013), An introduction to the Universal Thermal Climate Index (UTCI), Geographia Polonica 86(1)
- pythermalcomfort documentation
- pythermalcomfort documentation: models (UTCI, solar gain)
- CBE Clima Tool documentation: overview
- CBE Clima Tool documentation: main features
- CBE Clima Tool documentation: UTCI explained
- EPW Insights
- EPW Insights repository and README
- Copernicus Climate Data Store: thermal comfort indices derived from ERA5
- Solemma: ClimateStudio
- ClimateStudio documentation: hourly heatmap (UTCI)
- ClimateStudio documentation: weather file search
- Solemma: ClimateStudio trial and licensing requirements
- Ladybug Tools
- Ladybug primer: UTCI Comfort component
- Ladybug primer: component index
- Honeybee: UTCI Comfort Map component source
- Rhino pricing, North America
- Autodesk: Forma Site Design microclimate analysis
- Autodesk: What is Forma Site Design? (August 2026)
- ENVI-met: software suite
- ENVI-met: licence types
- ENVI-met: technical FAQ
Related on C4B
Outdoor comfort (UTCI)
UTCI outdoor thermal comfort heatmaps by season and hour, and strategies for sun and wind exposure.
Wind rose
Annual and daily wind roses showing wind direction and speed, by season and time of day.
Future climate scenarios
Future weather files for any year up to 2100 under five SSP scenarios, overlaid on the project's analyses.
C4B vs CBE Clima Tool
Both run in the browser. Clima is free and analyses EPW files; C4B works from any address and adds future climate, energy and PDF reports.
C4B vs Ladybug Tools
Ladybug is a free, flexible Grasshopper toolkit; C4B is a ready-made browser workflow from an address to a climate report.
C4B vs ClimateStudio
ClimateStudio is a Rhino simulation plugin for Windows; C4B is a browser-based site climate analysis for any computer.
C4B vs Forma Site Design
Forma Site Design analyses massing models in the Autodesk cloud; C4B profiles a site's climate from an address, before any model exists.