DF2014:Farm size calculations
|This article is about the current version of DF.|
Dwarves require approximately 2 units of food and 5 units of drink each season. Farming can produce enough crops to satisfy part or all of these requirements. However, be aware that the more and larger your farms are, the more time and effort must be accorded to their maintenance.
 The math
Your dwarves can plant 1 seed per tile on each farm plot, and depending on the skill of the grower, whether the farm plot was fertilized, and random chance, each planted tile will yield a stack of 1-9 plants each harvest cycle. Legendary farmers can consistently produce stacks of 4 - 5 (average of about 4.7) without fertilizer, or 8 - 9 when fertilized.
Brewing always quintuples stack sizes; for example, a stack of cave wheat  is brewed into a barrel of Dwarven beer  at a still. Processing quarry bush plants into quarry bush leaves at a farmer's workshop also quintuples stack size, as does processing sweet pods into Dwarven syrup. (Milling does not increase stack size.) These larger stacks (including booze!) are generally not usable directly as food, but can be cooked into prepared meals. Cave wheat, sweet pods and quarry bushes grow 67% slower than other crops, though they generally provide more valuable ingredients. As long as the mature plants are harvested promptly, dwarves can grow multiple plantings in the same plot each season.
|Plant Name||Active Seasons||Days Per Harvest||Harvests Per Tile Per Season||Harvests Per Tile Per Year||Average Plants Harvested Per Tile Per Year*||Average Processed Food/Drink Per Tile Per Year*|
- *: 5 plants per harvest (mid-level farmer with fertilization or legendary farmer without fertilization)
A beginning fortress has 7 dwarves, each of which consumes 7 urists of food and drink per dwarf per season, or 196 for the whole fortress for the year. The starting group can theoretically be supported by a single farm tile, but in practice a larger farm will be necessary since a young fortress is unable to use farm tiles to their fullest potential.
A properly-managed and fully-utilized 3x3 plot growing plump helmets can produce an average of up to 2700 units of alcohol per year, enough to provide food (through booze-cooking with seeds) and drink for a fortress of 95 dwarves. A similar 5x5 plot can produce up to 7500 units of food and drink per year, enough to support 265 dwarves.
This calculation assumes your planters can consistently produce stacks of 5 plants (with or without fertilization), and that there are enough of them to avoid any labor shortages at planting times. The general limitation, then, is not the size of the farm, but the skill of the growers, the availability of seeds and potash for fertilizer, and careful management of labor, stockpiles, and the food industry to ensure that crops are harvested (rather than left to wither in the field), are properly stored away from vermin, the plants are processed, and seeds are recovered so that the cycle can continue next season. However, since adding farm tiles is practically free, most overseers find it easier to create larger farms than to optimize production on a smaller plot.
In practice, dwarves require booze variety to avoid bad thoughts and desire food variety to create good thoughts. Additionally, if your dwarves are going to grow textiles (and dyes) for clothing they will need more farmland.
At the high end, 50 tiles of farmland should provide all the food your fortress will ever need, even if divided between disparate types of less-efficient crops.
Many fortresses begin with a single large farm (5x5) with plump helmets, or several smaller farms (3x3 or 1x5 are common) with a wider variety of crops. Given the small number of dwarves, only 10 or so tiles are needed for a beginning fortresses. As the growers level up and farm yields increase the farm can be expanded and newer, smaller farms with different crops can be added to enable new growers to practice their skills. The more tiles of farm plot you have, the more growers (and harvesters) you will need. A single legendary farmer can plant up to 100 tiles of farmland if they never harvest themselves and the seed stockpile is only a half-dozen tiles from all farm plot tiles. Consider making the seed stockpile in the middle of a ring of farms to save travel time.
If you plan to fertilize, farm plots should ideally be one less than a multiple of 4 tiles -- ex. (1x3), (1x7), (3x5), (3x9), (5x7), (7x9)