Supplier Production Capacity: How to Verify a Claim

You verify a glass supplier's capacity claim by reducing it to the one step that limits your bottle, getting that step's monthly allocation in writing, and testing the figure against the plant's own shipping records. A headline tonnage says how big the plant is. It does not say how much melt, machine time, decoration, assembly and packing will be set aside for your bottle, in your colour, in the month you need it. All of this can be done from a desk with documents and arithmetic, before anything is signed.

The three figures a plant may be quoting

When a supplier says "capacity", it can mean any of three things. Buyers get into trouble when they hear one and assume another.

• Furnace day-melt. The tons of glass melted in twenty-four hours. This is the brochure number. Every bottle and jar in the plant draws on the same melt, and the figure cannot be raised for a new customer: a container furnace runs without stopping for a campaign that often lasts years, and only a rebuild changes it.
• Forming. The individual section machines, and more exactly the sections able to carry your mould. A section forms one gob at a time, at a cycle speed dictated by the bottle's weight, height, width and wall thickness. Heavy ware runs slower than light ware, tall narrow shapes slower than squat ones, and decorated bottles sometimes need a slower cycle to protect the surface. So forming is counted in pieces per hour per section, times the sections the plant will give you, not in tons.
• Downstream. Everything after forming: annealing, decoration (screen printing, hot stamping, sleeving, coating), assembly of inserts, droppers, closures and pumps, packing, palletising, warehousing and container loading. Each has its own throughput, shift pattern and queue.

Your order is governed by the smallest of the three. A plant with a generous furnace and a single oversubscribed decorating line is a small supplier for a decorated bottle, whatever its tonnage.

The limiting step also depends on the product. Heavy amber spirits bottles are usually limited by the furnace, printed cosmetic jars by decoration, and small dropper bottles with a fitted dropper and inner seal by assembly. One question finds it quickly: if this volume had to double next quarter, which step would break first? The step named in the answer is the binding constraint, and it is the figure that belongs in a contract.

Documents to request before comparing suppliers

A capacity figure that lives in a sales email disappears when the salesperson changes, cannot be set line by line against a second supplier, and gives you nothing to point to when a month slips. Ask for the facts below on paper. None of them requires the supplier to reveal customer names or prices.

• Furnaces: how many, the day-melt tonnage of each, the colours currently in campaign and how often the colour set changes.
• Forming: machine count, total sections, the sections that would be assigned to your bottle, and whether a mould for your shape exists or must be cut.
• Moulds: the sets held for your shape, their condition, their owner, and how many are mounted now.
• Decoration: number of lines, processes offered, colours per pass, and whether the work is done on site or sent out.
• Assembly: whether closures, inserts, droppers and pumps are fitted on site or shipped loose, and the equipment used.
• Packing and palletising: automatic or manual, the pack formats available, and the bottle weight the line handles comfortably.
• Warehouse and loading: bays available, whether goods can wait there before booking, and containers loaded per working day.
• Committed allocation: the share of each item above already promised to other customers, and the share open to a new programme.
• Season profile: the months when the plant is normally full and the months with slack.
• Yield: typical ware yield after rejects and breakage, so that tons can be turned into shippable pieces.
• Outsourcing: which processes, if any, leave the site.
• Evidence: a machine list, a plant layout diagram, delivery records covering the last twelve months, and two or three references from customers in a comparable category.

These are planning-sheet facts, not trade secrets. A supplier unable to supply them is telling you the number came from the sales side. Read any capacity printed in a catalogue or on a website as an opening range to be confirmed by enquiry for your project, never as a commitment.

What each metric proves and what it leaves open

Use the sheet below to keep one metric from standing in for another. Most capacity disputes are not about a false number. They are about a number being asked to describe something it was never meant to cover, which is why the last column deserves the most attention.

Complete one sheet per supplier in identical units, so a tonnage from one plant is never weighed against a section count from another. Mark the row you think binds your bottle and ask each supplier to agree or disagree. Then file it. If a run slips half a year on, the row that proved wrong shows what to renegotiate.

From installed capacity to the share you can book

Installed capacity is a ceiling reached only with the right mould mounted, a steady colour campaign, no rival orders and no stoppages. What you can actually book is what remains after five deductions:

1. Committed allocation. Customers on annual contracts hold slots, and those slots are not released for a new enquiry.
2. Changeovers. Each colour or mould change halts the section, and the machine turns out off-specification glass until gob weight and parison settle. Time and ware are both lost.
3. Yield. Rejects, checks, cracks and breakage mean fewer pieces ship than are formed, and fewer are formed than the melt would suggest.
4. Trials and sampling. A new mould is commissioned, sampled and adjusted on production machines before it joins the schedule.
5. Planned downtime. Furnace repairs, machine overhauls, holiday closures and maintenance campaigns take weeks out of the year.

A buyer with no signed forecast sits in the outer, flexible layer of the schedule. When an established account raises its order, that layer gives way. This is how a plant protects its continuity, not a sign of bad faith, and the remedy is a written allocation: a band of pieces per month for a named bottle, in a named colour, across named months. An annual total with no monthly breakdown describes the plant, not your order.

Expect a careful supplier to reply with a range, and take that as a good sign. A monthly band that narrows in the two months when the decoration line is full tells you more than one confident number that ignores the squeeze. We work the same way when we handle a project: figures are confirmed per enquiry from the bottle, colour, decoration scope and requested months, and a general statement never replaces that step.

Peak months and slack months

Ask the same plant about the same bottle in June and in November and you may get two different answers. Peak load is several cycles piling up at once: retail and holiday programmes pull production forward, beverage and food seasons bunch demand, launches cluster on one retail calendar, and the plant adds its own colour campaigns, maintenance and holiday closures. Container and vessel space tighten in the same weeks, so a capacity shortfall often reaches the buyer looking like a freight problem.

For a seasonal product, then, annual capacity is the wrong measure. Two plants with equal yearly output can behave quite differently in the month that matters. Request a twelve-month allocation profile and the months in which new work is generally refused. A plant that can name its busy months is working from a schedule. One that says it is equally free all year is reading from a brochure.

A launch tied to a peak month leaves three choices: book earlier, build a bridge stock in a slack month, or accept a longer window. Where timing can flex, shifting volume into a shoulder month tends to win more attention on date and cost. Treat that as something to discuss, though, because no plant can hold back capacity it has already sold.

Checking the claim against delivery records

Statements about next year are hard to test. Last year's records are easy, and two sums cover most of the ground.

The load-count check

Get the containers shipped in each of the last twelve months, plus a typical order quantity. Multiply each month's container count by the number of your bottles that fit the container type in use. That gives a rough monthly shipped volume to hold against the monthly capacity claimed. A plant claiming very large output while shipping few containers owes you an explanation, and the explanation generally turns out to be the true figure.

The melt-to-ware check

Turn the claimed day-melt tonnage into pieces at your bottle weight, apply a realistic ware yield, and multiply by a sensible number of working days. If the result is far above what the shipping record shows, the surplus is either promised to someone else or does not exist in a form you can use. Either finding is worth having.

References and on-time figures

When you call a reference, skip the question of satisfaction and ask about a surge: what happened to delivery dates in the supplier's busiest quarter, and how a slip was communicated. Check whether the reference buys your shape or another, since capacity for a heavy bottle does not transfer to a light one.

An on-time delivery percentage means nothing until it is defined. Measured from loading, from sailing or from arrival, it yields three different numbers, and a supplier with a modest score may simply be reporting the strictest one. Ask for the definition and the period covered. A weaker figure with a clear definition beats a strong one with none.

Reconciling the capacity figure with the quoted lead time

A supplier can tell you your volume is comfortably within its capacity and, separately, quote a delivery period. Each sounds reasonable alone. They may not both hold in the same month.

Test them together. Multiply the claimed daily output of your bottle by the working days inside the quoted lead time. The product ought to exceed your order quantity by a wide margin. If it does not, the capacity figure is inflated or your order is waiting behind other work. Queueing is no fault, but it is the honest explanation and should be priced and planned for.

Next, establish when the clock starts: at purchase order, at confirmation of a production slot, or at sample approval. A supplier quoting from the order while planning from the slot is running two calendars under one name, and the gap can reach several weeks.

One combined question settles both matters. For our bottle in our colour, which step binds, how many pieces per month does it permit in our delivery month, and what is the earliest run date you can confirm? An answer with a constraint and a date gives you capacity and timing from the same plant. How the calendar itself is built, including scheduling windows, colour campaigns, the booking leg and the limits of expediting, is covered in our guide to glass bottle lead time.

Stress-testing before you grow or split the volume

A stress test is a documented search for the ceiling, carried out before your business rests on it. It needs neither a plant visit nor a purchase commitment.

1. Send a profile, not a total. Lay out twelve months by month, quantity, colour, decoration and pack format. Ask the supplier to confirm or amend every row in writing and to name the constraint wherever it cannot confirm. The hedged months carry your risk, and the pattern of hedging reveals more than any summary.
2. Ask the doubling question for your peak month. Which step fails first, and what would the ramp look like? A reply naming a step, a month and a condition describes a plant. "Anything is possible" describes an intention.
3. Place a trial order of realistic size. A sample carton proves little. A single container in your real bottle, colour and pack is the cheapest capacity test there is. Compare the run time with the production window quoted, the packing with the approved sample, the share of goods outside agreed tolerances, and the shipping month with the one promised.
4. Write down any split before it starts. If two plants will share the volume, record each one's monthly quantity, months and colour. A spoken promise is only the memory of a conversation, and it is the first thing to vanish when a plant fills up.

Options when the capacity falls short

Finding the ceiling is not a failure. Finding it after a retail date is fixed is. Once the constraint is known, the responses are few, and each carries a cost:

• Move the launch and accept a longer window. This costs nothing in cash and a good deal in market timing.
• Phase the launch, beginning with one size or one market.
• Shift part of the volume to a stock shape already in production, accepting a bottle not designed for your brand.
• Cut decoration colours or pack variants, which takes work off the constrained operation.
• Divide the volume between two plants.
• Buy bridge stock in a slack month and store it, trading a capacity problem for a warehousing and cash-flow one.

Choose before the date is committed to a retailer, because every option becomes more expensive afterwards. Keep two risks apart as well. A supplier that is temporarily full is a scheduling matter. A supplier whose capacity was never as stated is a sourcing matter.

Adding a second plant is a project of its own, raising specification alignment, tolerance drift, mould ownership and change control across two sites. Our page on setting up a second source for glass bottles deals with that work.

What a desk assessment does not cover

Capacity verification assumes you already know which bottle you are buying. Choosing a category, a size in millilitres, a neck finish or a closure comes first, and that selection starts from the wholesale glass bottle collections and the category pages beneath them.

It also stops at the plant gate. What to look at on the floor, how a running line differs from a display line, what to ask operators and what to photograph are set out in our guide to visiting a glass bottle supplier. The two halves work together: the arithmetic narrows what you need to inspect, and the visit shows whether the arithmetic described a real plant.

Information to send for a capacity risk review

We build a verification checklist from your own numbers, so a short, specific enquiry is enough. Approximate figures are fine.

• Annual volume with a monthly profile, flagging any month that cannot move.
• Bottle specification: capacity, weight if known, glass colour, and whether the mould exists or needs cutting.
• Decoration and assembly scope: print, sleeve, coating, insert, dropper, closure fitment.
• Pack format and pallet preference, if settled.
• Destination and the date the goods must be available to you.
• Your current supply arrangement, the supplier or suppliers under assessment, and why you are testing capacity now.
• The constraint that worries you most, such as a peak-month run, a decoration queue or an assembly step.

In return you receive the list of evidence to request, the arithmetic checks to apply to the plant's figures, the questions most likely to surface the binding constraint, and our judgement of where your volume is most likely to fail first. It is the same instrument we use when confirming capacity for a project of our own, and its purpose is to put suppliers on one sheet instead of comparing brochures.

Frequently asked questions

How does a capacity assessment differ from a factory tour?

An assessment is desk work. It turns a claim into checkable figures, such as day-melt tonnage, forming sections, decoration and assembly throughput and monthly allocation, then tests them with shipping records and arithmetic. A tour records what a plant looked like on one day. Each supports the other, but a tour without the sums tends to confirm the impression the supplier set out to give.

Can a claim be verified without going to the plant?

Yes. Request a machine list, a mould register for your shape, the campaign colour sequence, the months the plant says it is full, and twelve months of container counts. Then run the melt-to-ware comparison against the shipping record and the daily-output comparison against your order quantity. Treat every mismatch as a question to put to the supplier.

How much capacity should a plant reserve for a new buyer?

No standard share exists, and a supplier quoting a fixed percentage is simplifying. The useful test is whether it will commit an allocation to paper with the binding constraint stated. Without a signed forecast you should assume a place in the flexible layer and plan a buffer.

Can a single plant cover my volume in peak season?

Perhaps, but only a month-by-month answer counts. Spare annual capacity does not stop a plant being full in your weeks, because decoration queues, colour campaigns and container supply all tighten together. If the launch sits in a peak month, book earlier or build bridge stock in a quieter period.

How does capacity affect the lead time quoted?

Lead time is the calendar, capacity is the volume, and the two meet in the queue. When daily output times the working days in the quoted period comes out close to your order quantity, your order is competing for the same hours as everyone else's and the quoted period already includes waiting.

When does splitting an order between two suppliers make sense?

When one plant cannot cover your peak month with a documented margin, or when relying on one site is a concentration risk you will not carry. Two sites bring alignment, tolerance and mould questions, so agree the allocation in writing first. A split works as a planned structure and poorly as a rescue once a plant is already full.