Bunker silo capacity calculator: from dimensions to dry matter tons

643.64 US short tons of dry matter fit in a 3-wall bunk model measuring 40 feet wide, 200 feet long and 12 feet high, with the front end open at a 3:1 slope and density at 15 lb DM/ft³. That is the starting example in the SilagePlan capacity calculator. It is a calculation with stated inputs, not a claim about a particular farm. Change the density or filled dimensions and the answer changes immediately.

The useful question is whether those inputs describe the feed you actually have. Concrete wall dimensions, last autumn’s moisture sample and a familiar density preset can produce a precise-looking answer while describing three different things. Put a date beside each assumption before you use the estimate for purchasing, ration planning or a lender discussion.

Start with the filled cross-section

The 3-Wall Bunk choice has one open side; the 2-Wall Bunk has two opposite open sides. At the starting dimensions and front-end 3:1 slope, the 3-wall model contains 85,818 cubic feet. Both choices use the selected slope and orientation. They do not assume the full rectangular space is filled.

Measure the width across the filled footprint and enter a representative requested height. There is one width input. For 2-wall and 3-wall bunks, choose which sides are open and inspect the preview. A height notice appears when the selected footprint and slope limit the effective height. Use that effective height when checking the calculated result against your records.

University of Wisconsin Extension engineer Brian Holmes describes the volume-times-density approach in Making a Feed Inventory (Holmes, 2012). Keep the volume calculation visible. It gives another person something concrete to check before debating how many tons the storage holds.

Convert cubic feet into the right tons

Density in this calculator means pounds of dry matter per cubic foot. At 15 lb DM/ft³, the three-wall example contains 85,818 × 15 = 1,287,270 pounds of dry matter. A US short ton contains 2,000 pounds, so divide by 2,000 to get 643.635 dry matter tons. Do not divide by 2,204.62 unless you deliberately want metric tonnes and label them accordingly.

As-fed weight includes water. With dry matter at 35%, divide 643.635 by 0.35 to get about 1,838.96 as-fed tons. At 40% dry matter, the same assumed dry matter inventory corresponds to about 1,609.09 as-fed tons. The difference does not represent a change in stored nutrients within this example. It reflects a different water assumption. This is why a wet-weight comparison needs a moisture basis.

The dimension selector accepts feet or meters and converts entered measurements when you switch. Every result includes both cubic feet and cubic meters. The conversion is exactly 0.028316846592 m³ per ft³, making the example volume 2,430.10 m³ after rounding. Density stays in lb DM/ft³ and both weight outputs stay in US short tons. Record all three unit choices with the result.

Choose a density you can explain

A preset makes it easy to explore a question; it cannot answer how tightly your forage was packed. The calculator offers 14, 15, 16, 18 and 22 lb DM/ft³, plus a custom entry. For the 85,818-cubic-foot example, each additional pound of dry matter density adds about 42.91 dry matter tons to the estimate. A change from 15 to 18 therefore adds about 128.73 tons without changing the dimensions.

Muck and Holmes examined density and packing practices across 175 Wisconsin bunker silos in Factors Affecting Bunker Silo Densities (Muck and Holmes, 2000). Their measured densities varied widely. Treat that as a reason to verify your chosen number, not a reason to select whichever preset creates the most comfortable feed balance.

A useful worksheet has a density value, its basis and the date it was assigned. Distinguish an assumption from a value calculated using known removed weight and corresponding volume. If separate bunkers contain different crops or were packed differently, retain separate density assumptions. An average across the whole feed center can hide the very shortage you are trying to locate.

Handle ramps and irregular sections separately

The 2-wall and 3-wall choices already include their selected open-end slopes. Do not add another ramp estimate for the same space. If the real storage has additional sections or irregular boundaries, keep those sections distinct and record where they meet. This prevents counting the same feed twice.

The calculator covers flat rectangular footprints and the selected wall arrangement. Uneven ramps, rounded shoulders on bunks and sloping floors may need a measured inventory. The pile volume article explains how to compare the two pile choices without changing the density assumption to compensate for the shape.

Do not enter an unstable face or climb onto stored silage to improve an estimate. Use existing plans, measurements obtained from a safe location or appropriate surveying methods. A calculator can expose uncertainty without requiring someone to stand beside the feed face. An honest range is more useful than an apparently exact number collected unsafely.

Turn stored tons into a feeding horizon

Consider a labeled planning example for a 500-cow herd allocated 20 lb of corn silage dry matter per cow each day. Multiplication gives 10,000 lb daily, or 5 dry matter tons. Dividing the example inventory of 643.635 tons by that rate gives about 129 days before losses or any reserve. The allocation is an arithmetic input here, not a feeding recommendation.

Holmes’s Bunker Silo Sizing Spreadsheet documentation connects storage planning with daily herd requirements, feed removal and losses (Holmes, 2015). A volume result is only one piece of that planning. Ask your nutritionist for the intended allocation and make the reserve or loss allowance explicit in your own feed budget.

Work on a common basis. Dry matter inventory divided by dry matter use gives days. As-fed inventory divided by as-fed use also gives days if moisture assumptions agree. Mixing dry matter tons with an as-fed daily use figure produces a wrong run-out estimate even though the division itself works. Write the units beside the numerator and denominator before using the result.

Put a value on uncertainty without inventing savings

Use your actual feed valuation for budgeting. For illustration only, 5 dry matter tons per day valued at $200 per dry matter ton represent $1,000 daily. Across the hypothetical 500 cows, that is $2/cow/day for this silage component. It is not a full ration cost or a current market price. The unit label $/cow/day makes the assumption easy to compare with your own budget.

If those example cows shipped 400 hundredweights of milk daily, the silage component would equal $2.50/cwt. Neither milk production nor feed price comes from the calculator. Keep these figures outside the capacity result so a useful storage estimate does not become an unsupported profit forecast. A different valuation should change the budget, not your measured cubic feet.

Save a repeatable record

Before leaving the worksheet, save the storage name, measurement date, filled dimensions, chosen shape, density basis and dry matter sample date. Mark whether end ramps were included or excluded. The emailed PDF preserves the calculator inputs and results; your farm records should preserve where those inputs came from. Compare the next estimate using the same section boundaries.

If the estimate implies buying more crop, connect it with the corn silage tons per acre worksheet before committing acreage. Keep a second scenario with the least certain input changed, and note whether the purchase decision changes with it. That tells you which measurement deserves attention first.

Open the bunker silo capacity calculator, enter your filled dimensions and supported density, then email the PDF for your next feed planning meeting.