The ark was to be big enough to fit “two of every kind… of the birds after their kind, and of the animals after their kind, of every creeping thing of the ground after its kind,” and seven of some kinds? There had to be enough room to store enough food to last Noah and his family (8 in all), plus all of the animals, at least a year (Genesis 7:11; 8:13-18), depending on how long it took for vegetation to grow back.
The dimensions for the ark given in Genesis are 300 cubits long, 50 cubits wide and 30 cubits high (Genesis 6:15). A cubit is an ancient unit of measurement, the length of the forearm from the elbow to the longest finger (the term “cubit” comes from the Latin word “cubitum” which means “elbow.” The Hebrew word for “cubit” is “ammah.” As everybody’s arms are different lengths, this unit may seem a bit ambiguous to some, but it represents somewhere between 17 and 22 inches (43-56 centimeters). The ancient Egyptian cubit is known to have been 21.888 inches. So, doing the math, 300 x 22 inches = 6,600; 50 x 22 inches = 1,100; 30 x 22 inches = 660; 6,600/12 = 550 feet; 1100/12 = 91.7 feet; 660/12 = 55 feet.
The ark was up to 550 feet long, 91.7 feet wide and 55 feet high. 550 x 91.7 x 55 = 2,773,925 cubic feet. (If we take the smallest measurement of a cubit, 17 inches, we end up with 1,278,825 cubic feet). The ark had three levels (Genesis 6:16) and a lot of rooms (Genesis 6:14), the walls would have taken up space, nevertheless, it is calculated that a little more than half (54.75%) of the 2,773,925 cubic feet could store 125,000 sheep-sized animals, leaving over 1.5 million cubic feet of free space.
The number of animals is estimated to have been 8,000 “kinds.” What is a “kind”? The designation of “kind” is thought to be much broader than the designation “species.” Even as there are over 400 dog breeds all belonging to one species (Canis familiaris), so many species can belong to one kind. The designation “genus” may be somewhat close to the biblical “kind.”
Nevertheless, even if we presume that “kind” is synonymous with “species,” “there are not very many species of mammals, birds, amphibians and reptiles. The common number of animals is considered to be 17,600. Allowing for two of each species on the ark, plus seven of the “clean” kinds of animals, plus a reasonable increment for known extinct species, this accounts for no more than 50,000 animals on the ark.
Some have estimated that there were as many as 25,000 kinds of animals represented on the ark. This is a high-end estimation. With two of each kind and seven of some, the number of animals would exceed 50,000, though not by very much, relatively speaking. Regardless, whether there were 16,000 or 25,000 kinds of animals, even with two of each and seven of some, there was plenty of room for all of the animals on the ark, plus food and water with room to spare.
The Bible does not specifically say how long it took Noah to build the ark. When Noah is first mentioned in Genesis 5:32, he is 500 years old. When Noah enters the ark, he is 600 years old. The time it took to build the ark would depend on how much time passed between Genesis 6:14, when God commanded Noah to build the ark; and Genesis 7:1, when God commanded Noah to enter the ark. Some scholars teach that it took Noah 120 years to build the ark, based on Genesis 6:3. Others say that it took 100 years, based on Noah’s age in Genesis 5:32 and his age in Genesis 7:6.
Noah entered the ark in the 600th year of his life, on the 17th day of the 2nd month (Genesis 7:11-13). Noah left the ark on the 27th day of the 2nd month of the following year (Genesis 8:14-15). Therefore, assuming a lunar calendar of 360 days, Noah was on the ark for approximately 370 days.