Plan a grain store from the number of people: about 200 kg of grain per person per year, plus at least 50% extra for crop failures, divided by 700-800 kg of wheat per cubic meter. A family of 4 needs about 800 kg (1 m³), and a settlement of 25 people with the 50% reserve needs 7,500 kg, which is 9.4-10.7 m³.
Capacity planning for grain storage is the arithmetic that turns a population and a safety margin into the cubic meters and the structural load a store must handle.
The Reserve Is Part Of The Capacity, Not An Extra
Crop failures happen. A store built to hold exactly one year’s need will be empty in the year after a failure. So the reserve of at least 50% extra is included in the sum from the start, not added if there is time. The sum also has a second half that people skip: a full store is heavy, and the floor and walls must carry it.
What You Need
- The population you must feed
- The planning figures: 200 kg of grain per person per year; 1 m³ of wheat is about 700-800 kg; reserve of at least 50%
- A calculator or pencil
- The depth of grain you plan to store, to find the floor area and load
Method: Size The Store
Step 1: Annual need. People × 200 kg.
Step 2: Add the reserve. Multiply by 1.5.
Step 3: Convert to volume. Divide by 800 kg/m³ for the smaller volume and by 700 kg/m³ for the larger.
Step 4: Divide into stores. Several smaller stores spread risk: a rodent breach, a leak or a failed seal then loses one store, not all of it. See Underground Grain Storage Pits on opening pits in small lots.
Step 5: Check the structure. Find the floor load, as below.
Worked Example
Settlement of 25 people.
- Annual need = 25 × 200 = 5,000 kg.
- With a 50% reserve = 5,000 × 1.5 = 7,500 kg.
- Volume = 7,500 / 800 = 9.4 m³ up to 7,500 / 700 = 10.7 m³.
Weight and floor load. The weight force of 7,500 kg is 7,500 × 9.81 = 73,575 N, about 73.6 kN. If the grain is piled 1.5 m deep at 750 kg/m³ (a mid value of the stated range), the mass on each square meter of floor is 1.5 × 750 = 1,125 kg, a pressure of 1,125 × 9.81 = 11,036 Pa, about 11 kPa. The floor area needed for 10.7 m³ at 1.5 m depth is 10.7 / 1.5 = 7.1 m². The floor and posts must be designed for this, and walls must resist grain pushing sideways. See Live Loads for the live-load treatment of stored grain.
Common Mistakes
- Sizing to exactly one year — one failed harvest leaves the store empty.
- Ignoring the structural load — a floor built for people may fail under grain.
- Forgetting seed grain — the next crop’s seed has to come out of the same stock.
Safety
Overloaded floors
Grain is very heavy: about 11 kPa under 1.5 m of it. A floor, loft or raised platform that has not been designed for it can collapse suddenly. Do not put a full year’s grain on a floor made for light storage.
What’s Next
- Learn next: Above-Ground Granaries — building the store
- Combine with: Inventory Systems — tracking what the store holds