Knowing how much silage is actually in the clamp is the difference between running short in February and wasting forage. This calculator estimates the fresh and dry-matter tonnes in a bunker, wedge, or pile from its dimensions and a bulk density, so you can inventory feed and plan the winter accurately.
Shape, density, moisture: the whole calculation
The volume formula depends on the shape, and density and moisture convert that volume into feed tonnes:
bunker volume = length × width × height
wedge volume = length × width × (tall + short height) / 2
pile (cone) = (1/3) × π × radius² × height
fresh tonnes = volume (m³) × density (kg/m³) / 1000
DM tonnes = fresh tonnes × (1 − moisture% / 100)Because settled silage is denser than fresh-cut crop, use the as-fed bulk density of consolidated silage rather than the chopped crop, and measure it if you can — it is the single biggest driver of the answer.
Worked example
A bunker 30 m long, 8 m wide, and 2.2 m of settled silage holds 528 m³. At a bulk density of 720 kg/m³ that is about 380 fresh tonnes. If a dry-matter sample reads 30 percent, the dry-matter content is 380 × 0.30 ≈ 114 tonnes of dry matter.
A herd of 150 dairy cows eating roughly 12 kg DM per head per day consumes about 1.8 tonnes DM per day. That 114 t DM clamp would last approximately 63 days — just over 9 weeks — before accounting for losses.
Why bulk density varies so much
Bulk density is the most uncertain input and the single biggest driver of the result. Factors that affect it include:
- Crop type — Maize silage typically compacts more densely than grass silage at the same moisture, because its physical structure compresses more readily.
- Dry matter content — Wetter silage (lower DM) tends to be denser as-fed, though it feeds less dry matter per tonne. This is why both fresh weight and DM need to be tracked separately.
- Packing effort — Heavily trafficked bunkers with multiple passes of heavy machinery, sheeted immediately, compact to higher densities than lightly packed piles. A well-packed bunker might reach 750–800 kg/m³; a loosely packed pile can fall below 550 kg/m³.
- Age and settling — Silage continues to settle in the clamp for weeks after filling, so measuring the height immediately after filling gives a higher volume than measuring after settlement.
If you do not have a measured density, use 650–700 kg/m³ as a conservative estimate for mixed-crop grass silage and 700–750 for maize silage, then confirm with a weigh- bridge sample where accuracy matters.
Estimating spoilage and working out days’ supply
The gross inventory figure this tool gives is the theoretical maximum. In practice, subtract estimated losses:
- Surface and shoulder spoilage — poorly sealed edges and exposed areas can lose 5–15% of total DM in aerobic deterioration.
- Feed-out face losses — maintaining a tight, well-managed face minimises further aerobic losses, but every day’s exposure adds some.
A realistic planning figure is often 10–15% below gross inventory for well-managed bunkers, more for loosely sheeted piles or clamps with significant shoulders.
To estimate days’ supply: divide your net DM tonnes by daily DM consumption.
Checking your measurement
Re-measure the clamp face height as you feed out rather than trusting the opening measurement throughout the winter. A face that has settled by 10 percent changes your DM inventory by 10 percent too. Taking two or three height readings at different points across the clamp gives a more reliable average than a single measurement at the centre.
Turning the inventory into a winter feed plan
The tonnage figure earns its keep in a feed budget. The working method:
- Net inventory = gross DM tonnes × (1 − expected loss fraction).
- Daily demand = head count × DM intake per head per day, summed across stock classes (dry cows, milkers and youngstock all eat differently).
- Days of feed = net inventory ÷ daily demand — compare against the days from housing to expected turnout, plus a margin for a late spring.
Running this in autumn, while alternatives (buying forage, adjusting stocking, extending grazing) are still cheap, is the entire point of measuring the clamp. Discovering a shortfall in February leaves only expensive options. Advisory bodies publish worked feed-budget methods — see Teagasc’s winter feed budgeting guidance and the forage-inventory resources from Penn State Extension, which also cover measuring bunker density directly by coring the face — the single best upgrade to this calculation’s accuracy.