First, prepare for a headache.

Second, Christopher Dyer’s book Standards of Living in the Later Middle Ages will save you an enormous amount of time if you want to see how a specific number of acres planted leads to a specific amount of grain available for food, for drink, and for sale.

Third, remember that medieval farming generally was extremely inefficient by modern standards.

Fourth, remember that unmilled grain needs to be milled before it can be turned into porridge, biscuits, bread, and other types of foods. According to some estimates*, about a third of all manors would have a water-driven mill, and milling was often a lord’s monopoly (i.e., no way around the mill tolls).

Finally, remember that peasants often paid their tithes and tolls with bushels of grain, not with coin.

Then, you need to understand the following three concepts.

Yield ratios: each type of grain had a yield ratio for the amount of grain sowed and the amount harvested. It varied by type of cereal. I’ve seen ranges from 1:3.8 to 1:8 for barley, 1:2.4 to 1:4 for oats, and 1:3.8 to 1:5 for wheat. For rye, I’ve seen a number for a high/ideal yield of 1:7. For peas and beans, I’ve seen 1:6 presented as the expected yield.(Jacques Le Goff, in Medieval Civilization, 400-1500, has a good treatment of agriculture in his “Material Culture” chapter.)

Variable yields: Soil quality and other environment factors affected the yield ratios. Bad weather or depleted soil would significantly reduce yield, while optimal weather and fertilized soil would increase yields. In Gimpel (207)*, surviving records for the Bishopric of Winchester revealed a 3.83 average yield rate over a period from 1209 to 1350 on fifty manors. (See also Le Goff, 211-212.) 

Sowing seed: Each type of cereal crop needed an amount of seed per acre. Christopher Dyer lists 2 bushels** per acre for wheat, 4 bushels for barley, 3 bushels for peas, and 3 bushels for oats in 15 sown acres (operating in the two-field rotation).

Here’s an example from one of Dyer’s best tables. On one manor that was farming in the two-field rotation, from planting 5 acres of wheat at a sowing seed rate of 2 bushels per acre, the harvest yield (after Church tithe) was 4 quarters 4 bushels. After setting aside 1 quarter 2 bushels of seed for next year’s planting, 2 quarters 2 bushels went for food, which left 1 quarter remaining. Then, 1 bushel went to the mill toll, which left 7 bushels available for sale. At a market price of 6 shillings per quarter, the 7 bushels yielded 5 shillings 3 pennies.

Context: Donald Engels, in Alexander the Great and the Logistics of the Macedonian Army, and Gregory Bell in his book about logistics during the First Crusade, as well as many other authors, have worked through some interesting calculations. Engels determined that 3.3 pounds of milled grain was necessary to make 3 pounds of bread, which was the daily bread ration (~3,085 calories) for one soldier. So each soldier consumed the equivalent of about 3.6 pounds of unmilled grain per day. (I’m estimating that 1 bushel of unmilled grain makes the daily bread for about 13 soldiers.) A typical peasant family would consume about 6 quarters 5 bushels of grain (as bread or pottage) per year and another 3 quarters of barley (as ale) per year. (Straw, the stalks of the cereal crops, is a byproduct of threshing.)

*See Jean Gimpel, The Medieval Machine: The Industrial Revolution of the Middle Ages.

**When medieval clerks recorded one (1) bushel of grain, they typical used the measure units of their culture. These units can vary dramatically from modern units. For example, an English clerk probably recorded one bushel of sixty-four (64) pounds (in twelve-ounce Tower pounds), which is about forty-eight (48) modern [U.S.] pounds (of sixteen ounces).