The **Calculation area** tool allows you to define the area where you want to simulate the fire development.

To activate the **Calculation Area** tool, click the button or use the **Alt + J** hot key.

The calculation area has the shape of a 3D-rectangle. The user determines the length and width of the calculation area at the moment of placing it on the scene.

The calculation area can be placed both on floors within a building and on the master plan.

The height of the calculation area is determined by the elements located on the floor or master plan and considered in the fire dynamics simulation. For such objects the property **Do not consider in case of fire** is turned off. Their contours are inside or intersect with the contour of the calculation area in such a way that all elements are completely included within the area.

The height of the calculation area cannot be less than the height of the floor.

You can draw several calculation areas on one floor. Their sizes and other parameters (cell size, open sides) can be different.

Several calculation areas can be positioned arbitrarily relative to each other: they can intersect, touch each other, or not have any common points at all.

When you create the calculation area on the scene, use recommendations for the optimal location of the calculation area. For more information, see Recommendations For Reducing Fire Dynamics Simulation Time.

When placing a calculation area on the scene, its size and location are automatically aligned in the X and Y coordinate axes, in accordance with the *BaseSize* parameter value in the calculation area settings. Fot details, see Setting Parameters for Fire Development Dynamics Simulation.

For example, if *BaseSize* is equal to 1 meter, then alignment is performed with a 1-meter step. This means that, for example, you are not able to draw the calculation area with a width of 5 meters so that its left corner is located at the point with the coordinate of 0.5 meters.
The dimensions of this calculation area will be automatically adjusted so that the coordinate of the left corner is 0 meters and the width is 6 meters.

When placing several calculation areas on the scene, their position and dimensions can also be aligned along the Z axis. But this happens only if position and dimensions of their cells lead to conflicts in FDS.

**Cell size**

Each calculation area is characterized by the degree of spatial discretization within its boundaries (cell size).

You use one of the following values as the cell size: 0.5, 0.25, 0.125, 0.0625, and 0.03125. Ypu can get all valid values within the specified range by dividing one by powers of two. By default, the cell size for calculation area is 0.25 meters.

When you select the cell size of the calculation area, use the following recommendations: Recommendations For Reducing Fire Dynamics Simulation Time.

If necessary, you can define valid cell size values in the fire dynamics simulation parameters. For more information, see Setting Fire Dynamics Simulation Parameters.

You can use several calculation areas with different cell sizes in the same scenario. The cell size determines how accurately the application will deliver the scene to FDS. The smaller the cell size, the more accurately the scene is transferred to FDS.

It is necessary to consider that reducing the cell size requires more RAM needed for fire dynamics simulation with FDS, as well as more actual simulation time required by the Fenix+ 3 application to perform all related actions.

Reducing the cell size also increases the size of simulation results files.

**Margin conditions**

By default, the margin of the calculation area is considered impermeable, and dangerous fire factors cannot spread beyond the margins of the calculation area. In many cases, this condition is unacceptable, since it is necessary to simulate the development of a fire in rooms that have open windows and/or doors facing outside. In this case, it is necessary to note which margins of the calculation area must be open, i.e., through which dangerous fire factors can spread unhindered. The application allows to determine the state of all margins of the calculation area (lateral, upper and lower).

If there are several calculation areas on the scene that are in contact with each other, it does not matter if their margins are closed or open at the point of contact. In any case, dangerous fire factors spread unhindered in the place of the contact.

Fire dynamics simulation is performed with FDS. Immediately after starting the simulation, The Fenix+ 3 application creates an input file for FDS. In this file, calculation areas are represented with one or more cell groups. FDS has a lot of requirements to the cell groups and their mutual arrangement. There are some restrictions which apply to the range of acceptable values for the cell size and the automatic alignment of the calculation area with the coordinate grid. For more information, see the FDS user guide in the “Computational Cells: The Cell Namelist Group”.