One of the questions that a lot of folk have about Noah's ark is this: How on earth did he get all those animals aboard? The answer is simple. Noah didn't build any little boat! He built a three storeyed ship that was so huge, all the animals could have fit on one deck!
So make yourself an ark of cypress wood; make rooms in it and coat it with pitch inside and out. This is how you are to build it: The ark is to be 450 feet long, 75 feet wide and 45 feet high. Make a roof for it and finish the ark to within 18 inches of the top. Put a door in the side of the ark and make lower, middle and upper decks. (Genesis 6:14-16)
God told Noah he was going to flood the earth so that everything on it would perish, except for Noah, his wife, his sons and their wives. And a selection of animals.
You are to bring into the ark two of all living creatures, male and female, to keep them alive with you. Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive. You are to take every kind of food that is to be eaten and store it away as food for you and for them." (Genesis 7:19-21)
Noah did as God commanded. Fortunately, he didn't have to go around the world rounding up the birds and animals; they came to him.
Pairs of clean and unclean animals, of birds and of all creatures that move along the ground, male and female, came to Noah and entered the ark, as God had commanded Noah. And after the seven days the floodwaters came on the earth.
Did they fit? Here's the answer: Based on measurements given in the Genesis account, most experts agree that the ark had approximately 1,500,000 cubic feet of free space. Picture a three-level barge the size of one- and-a-half football fields. According to numerous feasibility studies, this was more than enough room to keep representatives of the 16,000 to 25,000 distinct kinds of animals of all sizes with all of their food for an entire year. In fact, assuming that the average size of each animal was that of a sheep, this barge could actually hold up to 125,000 separate creatures. Going one step further, not only was the ark large enough for its cargo, it was designed perfectly for stability and sea-worthiness. Various hydrodynamic tests have confirmed that it was virtually impossible to capsize this barge, even in the most violent waves and winds. (source: Noahs--Ark.com)