Biogas Digester Calculator — Biogas from Manure & Digester Size
Estimate daily biogas production from cattle, pig, poultry, sheep, goat or horse manure, and size a farm biodigester from the daily load and retention time (FAO method), with energy and LPG equivalents.
How it works
Manure (kg/day) = animals × manure per animal × share collected · Biogas (m³/day) = manure × yield per kg · Daily load (L) = manure × (1 + water ratio) · Liquid volume = daily load × retention time · Total volume = liquid ÷ 0.75 · Energy ≈ 21.5 MJ per m³ (60% methane)
A biodigester turns manure into biogas — about 55–70% methane — and a liquid fertilizer (digestate). The FAO Manual de biogás gives typical fresh-manure output and gas yield per species: a 500 kg bovine produces about 10 kg of manure a day yielding 0.04 m³ of biogas per kg, a 50 kg pig 2.25 kg at 0.06 m³/kg, and a hen 0.18 kg at 0.08 m³/kg. The digester must hold the daily load (manure plus mixing water) for the whole retention time, which is longer in cold climates because bacteria work slowly; about a quarter of the total volume is left for gas storage.
Worked example
Ten 500 kg cattle with all manure collected give 100.0 kg/day, producing 100 × 0.04 = 4.00 m³ of biogas per day. Mixed 1:1 with water the daily load is 200 L/day; with a 40-day retention time the liquid volume is 8.0 m³ and the total digester 10.7 m³. The gas holds about 86 MJ/day (23.9 kWh), equal to 1.87 kg of LPG.
Sources: FAO (2011) — Manual de biogás (Table 3.6, retention times) · FAO Family Farming Knowledge Platform — Manual de biogás
Frequently asked questions
How much biogas does one cow produce?
About 0.4 m³ a day from the roughly 10 kg of fresh manure a 500 kg animal deposits, according to FAO figures — enough for about one hour of cooking on a household burner. Only the manure you can collect counts, so grazing cattle produce less usable gas.
How big a biodigester do I need?
Multiply the daily load (manure plus water) by the retention time, then add about a third for gas space. In the tropics, a family digester for 3–5 cows is typically 6–10 m³.
What retention time should I use?
FAO suggests 30–40 days in tropical regions, 40–60 days in warm regions with short winters and 60–90 days in temperate climates with cold winters. Too short a time washes out the bacteria and reduces gas.
How much energy is in biogas?
Biogas with 60% methane contains about 21.5 MJ per m³ (around 6 kWh of heat). One cubic metre replaces about 0.45 kg of LPG for cooking, or produces roughly 1.5–2 kWh of electricity in a generator.
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This calculator is for educational purposes. Double-check important results before acting on them.