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Rebuilding civilization

Water and food

Cropland to feed a group

Hectares of a crop needed to cover a group's energy for a year, allowing for losses and seed.

kcal

2100 kcal as a population average; 2500-3500 kcal with hard farm work.

%

100 % if it were the only food; in a mixed diet, its share.

Typical values without modern fertilisers or pesticides. Yield in t/ha, seed in kg/ha and energy per kg of harvested product.

t/ha

Leave it empty to use the crop's typical value (shown in the result). Enter your own if you know the harvests in your area.

kg/ha

Leave it empty to use the crop's typical seed rate. It is subtracted from the harvest of each hectare.

%

Threshing, drying, storage and insects. Cereals: 10-20 %; stored potatoes: 10-30 %.

Result

Cropland area
— ha
Area per person
— m²
Food needed per year
— kg
Gross harvest needed
— kg
Yield used
— t/ha
Seed used
— kg/ha

How it is calculated

Need (kg/year) = people × kcal/day × 365 × share ÷ kcal per kg. Each hectare leaves yield (kg/ha) × (1 − losses) − seed (kg/ha) to eat. Hectares = need ÷ what each hectare leaves. 1 ha = 10 000 m²; 1 t = 1000 kg.

Example. 10 people eating only wheat, 2100 kcal: 10 × 2100 × 365 = 7 665 000 kcal ÷ 3300 = 2323 kg per year. With 1 t/ha, 15 % losses and 130 kg/ha of seed, each hectare leaves 1000 × 0.85 − 130 = 720 kg. 2323 ÷ 720 = 3.23 ha, about 3226 m² per person.

Potato at 15 t/ha. 7 665 000 ÷ 770 = 9955 kg; each hectare leaves 15 000 × 0.85 − 2000 = 10 750 kg; 9955 ÷ 10 750 = 0.93 ha.

Keep in mind

  • Yields without modern inputs vary widely with climate, soil and manure: wheat 0.5-2 t/ha; maize 0.8-2.5; potato 5-15; paddy rice 1.5-3; dry beans 0.4-1 (FAO, traditional and subsistence farming). With organic manure and good varieties they can double. Use local figures if you have them.
  • Approximate seed rates: wheat 100-200 kg/ha (more if broadcast); maize 20-30; potato 1500-2500; rice 40-100 (transplanted or broadcast); beans 60-120.
  • Rice is given as paddy: husking and milling leave 65-70 % of the weight; 2400 kcal/kg of paddy already accounts for this. Wheat is assumed to be eaten as whole grain or wholemeal flour.
  • Fallow and rotation are not included: with a two-field system (half the land rests each year) twice the area is needed. Land for draught animals, firewood and paths is not counted either.
  • A single crop is not a complete diet: combine cereal and pulses, and spread the risk of pests and bad years across several crops.

See also

More tools in “Water and food”