QGIS 3.16 different results in volume calculation

Question

I am trying to find ways to calculate a volume with :

1 base DEM resolution 1m
1 elevation DEM resolution 1m

First method:
Using the extension volume calculation tool as follows.
The result seems correct 22484 m3.

Second method:
I wanted to do the calculations by hand from a DEM representing the difference between elevation and base DEM.
So in raster calculator :

then in theory, I have to multiply number_of_pixel by surface_of_pixel by sum_of_pixel_value
With zonal statistics algorithm, I have number of pixels and the sum of their values (?).

Here is the point :  56495 * 1 * 22503,27 = too many m3.
It seems obvious that I do something wrong in the second way but can't figure it out.

Luis Perez · Accepted Answer

I think your result is 22503.27, I explain:
The subtraction of the DEM generates the depths, each one of them is a cube, of area the dimensions of the pixel and of height the depth.
The area is 1*1= 1m2
Therefore, the formula would be:
Sum(depth1 * area + depth2 * area +.....) Which would be the same as area*sum(depth1+depth2+...)
The wrong thing is to multiply by the total number of pixels, that sum is included in the sum of the depths. Your operation would be 1 * 22503,27= 22503,27
Slightly different, which may be due to the optimization option

QGIS 3.16 different results in volume calculation

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