I made long ago the following calculation. You can remove the soldiers (died in battle and in POW camps) to get the civilian figure. These figures agree with official Polish estimates quite well.
In 1939 the population of Poland was 35 million: ethnic Poles 23.1 million, Ukrainians 5.3 million, Jews 3+ million, White Russians 1.6 million, Germans 0.77 million, give or take. After the war there was missing 5.47-5.67 million. That includes 2.8-3 million Jews, there is left about 2.5 million non-Jews.
Partially this 2.5 million is explained by a reduced birth rate. The birth rate was lower than the death rate. This can be seen in the demographics pyramid. There are two sharp reductions, year classes, where exceptionally few children were born and another reduction when there year classes should have had children. Why the birth rate was so low? Young men were in POW camps or dead and many young women were in forced labor. On 950.000 men in Polish army died in the attack 66.000. 660.000 become POWs in Germany or Soviet Union. 140.000 escaped to Romania or Baltic and from there to the Great Britain. After the war there was a baby boom in Poland. I calculate the birth rate of the baby boom as 1.9%. During the war it seems to have been 47% of this figure, so 0.894%. I estimate the average life length in war time to 63 years and it gives death rate 1.587%. The difference between deaths and births in 6.5 years yield 6.5*0.00693*24 million=1.08 million.
In the German attack of 1939 died 66.000 soldiers and 150.000-200.000 civilians. In POW camps in Germany and Soviet Union died 0.25 million. In Warsaw uprising died 0.18 million non-Jewish Poles. Together these are 0.646-0.696 million.
In the German terror after the conquest died 61.000 mostly civilians. In the Soviet sector died 0.1 million ethnical Poles in purges. Thus, terror and purges took 0.161 million.
To concentration camps in Germany were sent 0.4 million ethnic Poles, I estimate that half of them died, thus 0.2 million. Soviet Union moved in the years 1940-41 1.5 million Poles, of them, 63.1% ethnic Poles. Of the transferred 0.35 million died. This implies that of ethnic Poles died 0.23 million. In total in transfers and concentration camps died 0.43 million.
Let us add the losses on ethnic Poles: lower birth rate 1.08 million, war 0.646-0.694 million, purges 0.161 million, transfers 0.43 million. These make about 2.3-2.4 million.
Poland’s population dropped by 5.5 million and Jews made 3 million. As German, Ukrainian and White Russian minorities were not in Poland after the war, their part should not be counted. The missing 2.5 million are ethnic Poles. We found 2.3-2.4 million.
The rest can be explained with German work camps. Germans transferred 2.3 million ethnic Poles to work camps. The conditions were poor, thus there must have died many people.
As a conclusion, 1.1 million of the reduction of ethnic Poles was due to lower birth rate, 1.4 million was due to deaths. Reduction of the population of ethnic Poles does not need the assumption that they were killed in extermination camps. The components listed here explain the whole reduction.