Buying Guide
How to Calculate Your Plastic Packaging Requirements
How to work out how much packaging to order: production forecast, wastage, lead time, safety stock and the trade-off between order size and unit cost.
Ordering packaging is a forecasting problem disguised as a purchasing problem. Order too little and you stop production. Order too much and you have tied up cash in bottles that are sitting in a warehouse, possibly in a specification you are about to change.
Here is how to work out the number properly.
Step 1: Start with a production forecast, not a sales forecast
The quantity you need is driven by how much you will fill, not how much you will sell. Those diverge whenever finished goods stock is building up or being drawn down.
Take your planned production volume for the period, in units, per SKU. If you run several pack sizes, do this separately for each — aggregating them hides the fact that one size will run out well before another.
Step 2: Add wastage
Some packaging is lost. Bottles get damaged in handling, rejected at inspection, consumed in line setup, or scrapped during changeovers.
The allowance varies enormously by operation. A well-run line with experienced operators loses very little; a manual filling operation with a new product loses noticeably more. Setup losses are particularly easy to underestimate if you run short batches with frequent changeovers.
The right approach is to measure it: track a full production run, compare packaging consumed against units produced, and use your own number. If you have never measured it, do that before your next order rather than adopting a figure from an article.
Step 3: Work out your reorder point from lead time
This is where most stockouts originate.
Your reorder point is the stock level at which you must place the next order to avoid running out. It is:
Reorder point = (average weekly consumption × lead time in weeks) + safety stock
Two details matter here.
Lead time means total lead time, not the manufacturer’s production time. It includes the time you spend approving the quotation, the supplier’s production time, transport, and any inspection at your end. Businesses routinely underestimate this because they count only the middle part.
Safety stock covers variability, not the average case. If your lead time is usually four weeks but occasionally six, holding exactly four weeks of stock guarantees an occasional stockout. Size the buffer against how much your lead time has actually varied, not against how much you would like it to vary.
Step 4: Balance order size against unit cost
Larger orders bring down unit cost. They also tie up working capital, consume storage space, and expose you to obsolescence if you change the pack.
That last risk is underrated. A year of bottles is a genuine liability if you rebrand, change a closure, or discover a specification issue six months in.
A practical middle path is to agree an annual volume with your supplier for pricing purposes, then take delivery in scheduled tranches. You capture most of the volume discount without carrying twelve months of stock or committing to a specification you may want to revise.
Step 5: Do not forget the closures
Caps, liners and dispensing components are ordered as separate line items and are consumed at slightly different rates from bottles — closures tend to have their own damage and setup losses.
Running out of caps stops the line exactly as effectively as running out of bottles. Calculate them alongside the container, with their own wastage allowance, and check whether their lead time differs; imported dispensing components frequently take longer than the bottle.
A worked structure
For a product filling 20,000 units a month:
- Monthly production requirement: 20,000 units
- Add measured wastage: if you measure 2%, that is 20,400
- Weekly consumption: roughly 4,700
- Total lead time: say 5 weeks including approval and transport
- Lead time cover: 4,700 × 5 = 23,500
- Safety stock: based on your observed lead-time variability — if it has stretched by 2 weeks before, that is another 9,400
- Reorder point: approximately 32,900 units
So you place your next order when stock falls to roughly 33,000 units. The order quantity itself is then a separate decision, driven by the pricing breaks your supplier offers and how much stock you are willing to carry.
What to send your supplier
When you request a quote, include your monthly quantity and whether it repeats. The difference matters: a one-off order of 20,000 and a standing requirement of 20,000 per month are completely different propositions, and the second usually earns better pricing and a more reliable supply commitment.
If you are quoting several pack sizes, send them together. Consolidating a full range into one enquiry generally improves pricing across all of it — see our guide on what drives bulk packaging cost for why.
Frequently asked questions
How much safety stock should I hold for packaging?
Enough to cover the lead time plus a realistic allowance for it slipping. If your lead time is four weeks and it occasionally stretches to six, holding four weeks of stock means an occasional stockout. The right buffer depends on how variable your supply has actually been, not on a general rule.
Should I order a year's packaging at once to get a better price?
Rarely. Larger orders lower unit cost but tie up working capital, consume storage space, and lock you into a specification you may want to change. A quarterly schedule with agreed pricing usually captures most of the discount without the downsides.
What wastage allowance should I plan for?
It depends on your line and your process, so measure it rather than assume. If you have never measured, track it across one production run — the number is usually different from what people expect, in both directions.
