# Volumes

TunnelCAD computes areas and volumes between measured and theoretical tunnel profiles. The volume system lets you define exactly what to measure -- overbreak, underbreak, lining thickness, total excavation, or any custom combination -- using a formula builder with terms and arithmetic operations.

## How volume computation works

Volume computation proceeds in two stages:

1. **Area per section** -- for each cross-section, TunnelCAD evaluates a formula against the section's line data and template lines. The result is an area value in square meters (m2) for that station.
2. **Volume across sections** -- area values are accumulated along the alignment to produce volumes in cubic meters (m3), using one of two computation methods.

## Computation methods

TunnelCAD supports two methods for converting per-section areas into volumes:

- **Applicable Length** -- each section's area is multiplied by its applicable length, which is the chainage interval that the section represents. For the first and last sections, this is half the distance to the adjacent section; for middle sections, it is half the distance between the preceding and following sections.
- **Average Areas** -- for each pair of adjacent sections, the volume is computed as the average of the two areas multiplied by the distance between them: `V = (A1 + A2) / 2 * d`. This method is generally more accurate when the profile varies significantly between stations.

## Volume definitions

Volume definitions are stored per tunnel -- each alignment maintains its own list. A volume definition consists of:

- A **name** and **color** (used for hatch preview on the section canvas)
- One or more **terms**, each referencing measured or template lines
- **Operations** (add or subtract) between terms
- Optional **clipping levels** to restrict the computation region
- An optional **additive constant** applied to every section's result

## Results and export

Computed results are displayed in the **Mass Table** -- a grid showing station, area, and volume for each section, with a cumulative total at the bottom. The mass table can be exported to CSV or Excel (XLSX) format.

## Chapter contents

```{toctree}
:maxdepth: 2

volume-editor
presets
mass-table
clipping
```