Raster calculator — Tool (QGIS)

In QGIS: Raster › Raster Calculator…; in the Processing Toolbox › Raster analysis › Raster calculator.

Realises:

Settings that matter

Raster bands: each band is written "layer name@band", e.g. "B08@1". Double-click a band to put it in the expression.
Expression: arithmetic (+ − * /, ^, sqrt, ln), comparisons (=, <, >=; they give 1 for true and 0 for false), AND, OR, and if(condition, value if true, value if false). NDVI: ("B08@1" - "B04@1") / ("B08@1" + "B04@1").
Output layer: a GeoTIFF file; Output format GeoTIFF.
Spatial extent, Resolution and Output CRS: the grid of the result. Use selected layer extent takes them from one input. Inputs on another grid are resampled to it.

Masking to NoData. There is no NoData setting. Dividing by zero gives NoData, so dividing by a condition removes the cells where it is false: (...) / ("SCL@1" = 4 OR "SCL@1" = 5 OR "SCL@1" = 6). An if(…, …, -9999) instead writes −9999 as a value, which statistics then include, unless you set it as NoData afterwards (Raster › Conversion › Translate, Assign a specified NoData value).
Integers are safe here. The calculator computes in floating point and writes 32-bit floats, so integer bands such as Sentinel-2’s 16-bit values give correct ratios. The GDAL Raster calculator (Raster miscellaneous) does not: with 16-bit unsigned inputs a negative difference wraps around, and negative NDVI (water) comes out as large positive values.
Any input cell with NoData gives NoData in the result.
Check the result’s range (Layer Properties › Information, or the histogram). An index outside its possible range, such as NDVI above 1, means a wrong input (see scale and offset).

Checked in QGIS 4.2.2, November 2026. Names and options can differ in other versions.