Silage Tonnage Calculator: Bunker Silos and Hay Bales

Calculate bunker silo volume, round/square bale tonnage in Dry Matter (DM), and estimate livestock feeding autonomy.

Silage clamp and bales: dry matter tonnage, feed autonomy and face advance

For a bunker silo, the tool calculates volume = length × width × height × 0.95, then dry matter tonnage = volume × density (225 kg DM/m³ for maize silage, 190 for grass). A 35 × 8 × 3.2 m clamp therefore holds about 851 m³, 191.5 t DM and 580 t fresh at 33% DM, i.e. 239 days of feed for a herd eating 800 kg DM a day.

The three storage modes

  • Bunker silo: volume = L × W × H × 0.95; DM = volume × 225 kg/m³ (maize) or 190 kg/m³ (grass); fresh weight = DM ÷ DM content.
  • Round bales: volume of one bale = π × (diameter ÷ 2)² × width; DM = total volume × 150 kg/m³; fresh weight = DM ÷ DM content (85% by default for hay).
  • High-density square bales: fresh weight = number × bale weight; DM = fresh × DM content; indicative volume of 1.2 × 0.9 × 2.4 m per bale.
  • Autonomy = stored DM ÷ the herd's daily need, in days and in 30.5-day months.

Worked examples

Maize clamp of 35 m × 8 m × 3.2 m: 851 m³, 191.5 t DM, 580.4 t fresh at 33%. At 800 kg DM a day: 239 days, or 7.8 months. 250 round bales of 1.3 × 1.2 m: 398 m³, 59.7 t DM, 70.3 t fresh at 85%, 75 days of feed. 250 square bales of 380 kg: 95 t fresh, 80.8 t DM.

Calculating the face advance (Vorschub)

Face advance (m per day) = dry matter taken from the silo each day ÷ (density in kg DM/m³ × face section), where the section is width × height × 0.95, as for the volume. Multiply by 7 for a week. Enter the dry matter that comes out of this silo, not the whole ration if the herd also eats from other silos. With the default values (800 kg DM a day, maize at 225 kg DM/m³, an 8 × 3.2 m face, i.e. 24.3 m²), the face moves 0.15 m a day and 1.02 m a week, about half the planning value. It is also length ÷ autonomy: 35 m in 239 days. To reach 2 m a week at a height of 3.2 m, the face should be no wider than 4.1 m: a 4 m wide clamp advances 2.05 m a week.

Planning value: the Lower Saxony Chamber of Agriculture sizes bunker silos for a face advance of about 2 m per week, and more in summer (LWK Niedersachsen, "Fahrsiloanlagen planen und bauen", 1 December 2024, read on 27/10/2026).

Limits

Densities are averages: a poorly packed clamp or wet grass silage can differ markedly. The 0.95 factor corrects for the heap shape but not for storage losses (fermentation, feed face, edges), which you should deduct from the autonomy. DM content switches to 85% when you choose bales; adjust it for wrapped haylage. The feed ration calculator estimates the daily need per animal to enter here (for example 60 cows × 19.8 kg DM = 1,188 kg DM a day).

Sources and dates: packing densities (225, 190 and 150 kg DM/m³), the 0.95 shape factor and bale dimensions are indicative values of the tool, not verified (checked 14/10/2026).

Frequently Asked Questions

How many tonnes of dry matter are in one m³ of maize silage?

The tool uses 225 kg of DM per m³ for packed maize silage and 190 kg for grass silage. At 33% DM, one m³ of maize therefore weighs about 680 kg fresh.

How do I calculate the volume of a bunker silo?

Multiply length × width × height, then apply 0.95 to allow for the heap shape, as the tool does. A 35 × 8 × 3.2 m clamp gives 851 m³.

How many days of forage do I have left?

Divide the stored dry matter by the herd's daily intake: 191.5 t DM at 800 kg DM a day gives 239 days, or 7.8 months. Deduct storage losses, which the tool does not count.

How much dry matter is in a round bale of hay?

With a density of 150 kg DM/m³, a bale 1.3 m in diameter and 1.2 m wide (1.59 m³) holds about 239 kg of DM, or 281 kg fresh at 85% DM.

How do I calculate the face advance of a bunker silo?

Divide the dry matter taken out each day by the density and by the face section. With 800 kg DM a day, maize at 225 kg DM/m³ and an 8 × 3.2 m face (24.3 m² after the 0.95 shape factor), the face advances 0.15 m a day, or 1.02 m a week. The Lower Saxony Chamber of Agriculture plans for about 2 m a week: at 3.2 m high, the face should be no more than 4.1 m wide.

Is this tool really free?

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