Capacity Calculation
Sizing a community’s food storage before the harvest — not after — is how settlements avoid starvation in a poor harvest year, and this article works through every step of that math. The planning baseline is 2,500 kcal per person per day for a mixed-age rebuilding community, multiplied by 1.25 for a 25% safety reserve; for 50 people that target is 57,031,250 kcal. Wheat stores at 3,400 kcal/kg and a bulk density of 750–800 kg/m³, so the worked example derives roughly 10,065 kg of wheat occupying about 13 m³, with the full 50-person community needing approximately 49 m³ of total storage across grain, legumes, root vegetables, preserved meat, and ferments. A single building 5 × 5 × 2.4 m plus a 4 × 5 m root cellar meets that need, and the article shows how to recalculate every season as population and diet composition change.
Part of Food Storage Infrastructure
Capacity calculation determines how much food storage space a community needs, how much of each food type to store, and what physical dimensions storage facilities must achieve. Building a cellar too small wastes the harvest; building one too large wastes construction labor and materials. Getting this right requires caloric math, knowledge of food densities, and planning for realistic loss rates.
The core principle: storage must sustain the population from one harvest to the next — a minimum of 12 months — plus a safety reserve of at least 25% to buffer against a poor harvest year. Communities that calculate exactly to their minimum need one bad year to face starvation.
Step 1: Daily Caloric Requirements
Baseline consumption per person per day:
| Activity Level | Daily Calories |
|---|---|
| Sedentary (minimal physical work) | 1,800-2,000 kcal |
| Moderate work (farming, craft) | 2,200-2,500 kcal |
| Heavy labor (logging, construction) | 2,800-3,500 kcal |
| Nursing mother | Add 400-500 kcal |
| Child (5-12 years) | 1,400-1,800 kcal |
| Adolescent | 2,000-2,800 kcal |
A community engaged in rebuilding activities will average 2,500-2,800 kcal/person/day across all ages. Use 2,500 kcal/person/day as a planning baseline for mixed-age communities.
Annual caloric need:
- Population x 2,500 kcal/day x 365 days = total annual calories
- Example: 50 people x 2,500 x 365 = 45,625,000 kcal per year
Reserve factor: Multiply by 1.25 for a 25% safety reserve. In the example: 45,625,000 x 1.25 = 57,031,250 kcal storage target.
Step 2: Caloric Density and Storage Volume
Different foods provide different calories per unit of storage volume. This determines how much physical space is required:
| Food | Calories per kg | Bulk Density (kg/m3) | Calories per m3 |
|---|---|---|---|
| Wheat grain | 3,400 | 750-800 | 2,550,000-2,720,000 |
| Corn (maize, whole) | 3,500 | 720-770 | 2,520,000-2,695,000 |
| Rice (husked) | 3,600 | 750-800 | 2,700,000-2,880,000 |
| Dried beans | 3,400 | 800-850 | 2,720,000-2,890,000 |
| Dried lentils | 3,500 | 750-800 | 2,625,000-2,800,000 |
| Dried peas | 3,300 | 750-800 | 2,475,000-2,640,000 |
| Oats (whole) | 3,900 | 450-500 | 1,755,000-1,950,000 |
| Sunflower seeds | 5,800 | 600-650 | 3,480,000-3,770,000 |
| Dried potato slices | 3,400 | 300-400 | 1,020,000-1,360,000 |
| Salt (for preservation) | 0 | 1,200 | — |
Working example continued:
A community planning to meet 60% of calories from grain (wheat) and 15% from dried legumes (beans), with the remainder from fresh food:
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Grain calories needed: 57,031,250 x 0.60 = 34,218,750 kcal
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Wheat required: 34,218,750 / 3,400 = approximately 10,065 kg
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Storage volume for wheat: 10,065 / 775 (avg bulk density) = approximately 13 cubic meters
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Bean calories needed: 57,031,250 x 0.15 = 8,554,688 kcal
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Beans required: 8,554,688 / 3,400 = approximately 2,516 kg
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Storage volume for beans: 2,516 / 825 (avg bulk density) = approximately 3 cubic meters
Total grain+legume storage volume needed: approximately 16 cubic meters, plus additional space for root vegetables, preserved meats, and handling room.
Step 3: Storage Building Dimensions
Convert volume requirements to building dimensions:
Usable volume vs. building volume: Not all of a storage building can be filled with food:
- Structural elements (walls, floor, roof) reduce usable interior volume by 15-20%
- Aisles and working space for access: at minimum 0.6-0.9 m wide paths between storage
- Headroom for working: minimum 1.8-2.0 m ceiling height
- Bins/shelves do not always pack perfectly: expect 70-80% efficiency in converting floor area to stored volume
A building with interior dimensions of 4 x 5 m (20 m2 floor area) x 2.4 m ceiling height = 48 m3 total volume. After accounting for working space (30% reduction), available storage volume is approximately 34 m3.
Example storage building for 50 people:
- Required volume (grain + legumes): 16 m3
- Additional root vegetables (potatoes, root crops): 8-10 m3
- Preserved meats, ferments, other: 3-5 m3
- Total: approximately 27-31 m3 with 20% efficiency buffer = approximately 35 m3 needed
A single building 4 x 5 m x 2.4 m ceiling provides adequate capacity, with some working room.
Step 4: Root Cellar Sizing for Produce
Root vegetables require different calculations because they are not packed as densely as grain and require humidity control:
Common root vegetable densities in typical cellar storage:
| Vegetable | Bulk Density (kg/m3) | Recommended Storage Duration |
|---|---|---|
| Potatoes | 600-650 | 4-6 months |
| Carrots (in sand) | 700-750 | 5-6 months |
| Beets | 700-750 | 4-5 months |
| Turnips/Parsnips | 650-700 | 4-5 months |
| Cabbage | 400-500 | 3-4 months |
| Winter squash | 300-400 | 4-6 months |
| Onions (in crates) | 450-500 | 4-6 months |
Planning per person per day:
- Vegetables: 300-500 g fresh weight per person per day
- For 50 people over 5 months of storage (150 days): 50 x 400g x 150 = 3,000 kg total
- Volume required: 3,000 / 650 (average bulk density) = approximately 4.6 m3
Add 20-30% for wastage and selection losses during storage: approximately 6 m3 needed for root vegetables.
Step 5: Preserved Food Equivalents
For preserved items (canned, pickled, smoked), calculate by weight and expected consumption:
Preserved meat:
- Target: 30-50 g protein per person per day from preserved meat
- Protein content: smoked pork approximately 25% protein by weight
- Daily requirement per person: 50g protein / 0.25 = 200 g smoked pork per day
- Annual requirement per person: 200 g x 365 = 73 kg per person
- For 50 people: 3,650 kg smoked/preserved meat
- Storage volume (hanging, with spacing): approximately 8-10 m3
Preserved produce (pickles, ferments):
- Allow 0.5-1 liter per person per week
- For 50 people over 52 weeks: 50 x 0.75L x 52 = 1,950 liters
- At 1 L per crock, approximately 2,000 crock units (stackable ceramic crocks)
- Volume: approximately 5 m3
Putting It Together: 50-Person Community
| Storage Category | Quantity | Volume Required |
|---|---|---|
| Grain (wheat, corn, rice) | 10,000 kg | 13 m3 |
| Dried legumes | 2,500 kg | 3 m3 |
| Root vegetables | 3,000+ kg | 6 m3 |
| Preserved meat | 3,600 kg | 10 m3 |
| Preserved produce | 2,000 L | 5 m3 |
| Salt | 500 kg | 0.4 m3 |
| Working space and aisles | — | 12 m3 |
| Total building volume needed | 49 m3 |
A 5 x 5 x 2.4 m building (60 m3) with an additional 4 x 5 m root cellar (40 m3) comfortably meets these needs with room for growth.
Adjusting for Actual Yields
Calculate storage requirements before each harvest season based on:
- Expected harvest yield (be conservative — 70-80% of best-case estimate)
- Population size and age distribution
- Expected supplement from hunting, gathering, and trading
- Actual storage life of available containers
Recalculate annually. A community that grows significantly or shrinks, or that shifts its diet composition, must revise its storage capacity accordingly. Do not assume last year’s calculations remain valid.