A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 811440 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 23 24 28 30 35 36 40 42 45 46 48 49 56 60 63 69 70 72 80 84 90 92 98 105 112 115 120 126 138 140 144 147 161 168 180 184 196 207 210 230 240 245 252 276 280 294 315 322 336 345 360 368 392 414 420 441 460 483 490 504 552 560 588 630 644 690 720 735 784 805 828 840 882 920 966 980 1,008 1,035 1,104 1,127 1,176 1,260 1,288 1,380 1,449 1,470 1,610 1,656 1,680 1,764 1,840 1,932 1,960 2,070 2,205 2,254 2,352 2,415 2,520 2,576 2,760 2,898 2,940 3,220 3,312 3,381 3,528 3,864 3,920 4,140 4,410 4,508 4,830 5,040 5,520 5,635 5,796 5,880 6,440 6,762 7,056 7,245 7,728 8,280 8,820 9,016 9,660 10,143 11,270 11,592 11,760 12,880 13,524 14,490 16,560 16,905 17,640 18,032 19,320 20,286 22,540 23,184 27,048 28,980 33,810 35,280 38,640 40,572 45,080 50,715 54,096 57,960 67,620 81,144 90,160 101,430 115,920 135,240 162,288 202,860 270,480 405,720 and 811,440, with no remainder.
Since 811,440 cannot be divided by just 1 and 811,440, it is not a prime number.