Should I Keep More Safety Stock of a Shared Soy Wax Used Across Every SKU than a Fragrance Oil Used in Only One Slow-Selling Candle?

Yes in kilos and rupees, no in days of cover

Measured in kilos and rupees, the shared soy wax should carry far more safety stock than a fragrance oil that goes into one slow candle. In the example on this page — six candle SKUs, every one poured in Luxury Soy Wax CSI 464 at an 8% load into a 200 g jar — the wax needs a buffer of 6.82 kg, worth ₹3,460.60, while the slow Vetiver SKU needs 34.1 g of oil, worth ₹197.97. The wax buffer is roughly 17 times bigger in rupees. That is the answer most makers expect.

Measured in days of cover, the answer flips. The wax buffer covers 4.8 days of average use; the Vetiver buffer covers 10.7 days. Relative to what it uses during a normal lead time, the slow oil needs a bigger cushion (178% of lead-time use against 80% for the wax). The reason is not the oil. It is the demand pattern: six SKUs pulling on one wax smooth each other out, while one slow SKU sells three candles one month and ten the next. Small, lumpy demand needs proportionally more buffer; large, pooled demand needs proportionally less.

So the practical rule is not "more for wax, less for oil" as a reflex. It is: size every buffer with the same formula from your own sales history and your own lead times, then read the result in the unit that matters for the decision — rupees when you are deciding where cash goes, days when you are deciding how exposed a material is, and candles when the quantity is so small that a pack size decides it for you. For a slow oil, that last point usually settles everything: the smallest sensible CSI pack is already larger than the safety stock.

Five material roles, five safety-stock policies

Before any arithmetic, sort every material in your bill of materials by what happens on the day it runs out. A candle line does not stop evenly. If the container wax runs out, every SKU stops at once, because nothing can be poured. If the approved jar or wick runs out, the same thing happens. If one oil runs out, one SKU stops and the rest carry on. If wick stickers run out, you lose an hour finding a workaround. That consequence — how many SKUs stop and for how long — is what the trade calls criticality, and it is the first input to a safety-stock policy.

Two rows deserve emphasis. The first row is where shared materials sit, and it is the reason the question exists: a shared container wax is the textbook line-stopping material. In this example it feeds 233 candles a month, so every day without it costs about 7.8 candles of output. The fourth row is where the slow oil sits. A Vetiver stock-out stops 0.2 candles a day — a fifth of one candle. For many small businesses the sensible response to that is not a large buffer but a short, honest back-order note on the product page. Which of those you choose is a business decision; this page only makes sure the numbers behind it are right. The general version of this sort is in which raw materials to keep as safety stock because running out stops the line.

Start from your own sales history, not a rule of thumb

The cleanest input is six months of candles sold (or poured) per SKU. The table below is the example this page uses — a reader's illustrative history, not CSI data and not a market figure. Replace every number with your own. Notice the last column, the coefficient of variation: the standard deviation of monthly sales divided by the average. It is the simplest measure of how lumpy demand is. The big SKUs wobble by eight to thirteen per cent around their average; Vetiver wobbles by 57%. The whole line, added together, wobbles by only 5.9%.

That last line is the whole story of shared materials in one number. The six SKUs do not peak in the same month, so when Lavender has a strong month and Jasmine a weak one, the wax barely notices. The worst month for the line was 251 candles against an average of 233; the worst month for Vetiver was 10 candles against an average of 6. Pooling demand onto one material is the single biggest reason a shared wax needs proportionally less buffer than any one oil. Combining 20 SKUs' BOMs into one monthly wax requirement shows the requirement side of the same pooling.

The safety-stock formula, worked for wax and for Vetiver

For a small workshop the most useful formula is the one that needs no statistics table: safety stock = (worst daily usage × worst lead time) − (average daily usage × average lead time). It asks a plain question — if the busiest month you have had coincides with the slowest delivery you have had, how much extra would you burn through before the new stock arrives? Lead times here are the reader's labelled examples, 6 days usual and 10 days worst, from placing an order to having it on your bench. CSI does not publish delivery times; measure your own from your order dates and arrival dates, and use the worst one you have actually had.

Read the table down the "buffer ÷ lead-time use" column and the pattern is plain. The wax and the big oils sit between 80% and 92%; Jasmine, at 16 candles a month, needs 108%; Vetiver needs 178%. The smaller and lumpier the demand, the bigger the buffer relative to normal use. But read the "value" column and the order reverses completely: the wax buffer is worth more than all six oil buffers put together (₹3,460.60 against ₹3,171.78).

Criticality: the same formula, read with different eyes

The max-minus-average method already builds criticality in, because it assumes the worst month and the worst delivery together. For the wax, that is the right posture: if the wax runs out during a 10-day delay, you lose up to 78 candles of output across every SKU, and customers waiting on any of them are affected. A line-stopping material deserves the worst-case assumption every time.

For Vetiver the same assumption is optional. If Vetiver runs out for 10 days, you lose about 2 candles of one SKU. Some makers would rather carry the 34.1 g buffer; others would rather mark the SKU "back in a week" and keep the cash. Both are defensible. What is not defensible is holding the slow oil to the same days-of-cover target as the wax — say ten days for everything — because then you would hold 14,291 g of wax and only 32.0 g of Vetiver, under-protecting the oil whose demand is lumpiest and over-thinking the one whose demand is smoothest.

There is one more criticality input that is easy to miss: substitution. A container wax cannot be swapped on the day it runs out, because any wax change voids your burn tests — buffering wax and fragrance differently from fragile glass explains why each material's buffer logic differs. An oil cannot be swapped either, because the customer bought that scent. A wick sticker can be swapped for anything that holds the tab down. The less substitutable the material, the more its buffer should lean towards the worst-case inputs.

When the pack size is the safety stock

Safety stock for a slow oil runs straight into CSI's pack sizes, and this is where the arithmetic becomes a purchasing decision. Vetiver's buffer is 34.1 g — 2.13 candles. Rounded up to whole candles, that is 3 candles or 48 g of oil. CSI lists Vetiver at 15 g (₹187.62), 50 g (₹354.00), 100 g (₹600.00) and 500 g (₹2,900.00); the 1 kg size is out of stock on the 4 October re-check. One 50 g pack covers the buffer with 2 g to spare.

Now look at what Vetiver's normal ordering looks like. At 6 candles a month the SKU uses 96 g of oil a month, so one 100 g pack lasts 1.04 months, and its reorder point (lead-time use plus buffer) is 53.3 g. The simplest physical policy is therefore: keep one sealed 50 g pack as the buffer, and reorder a 100 g pack the day you break the seal on it. You never need to weigh the bottle to know whether you are below the reorder point — the sealed pack tells you. For the slowest SKUs, the smallest economical pack is the safety stock; there is no separate number to manage.

The wax works the other way. At 1,429.1 g a day, even a 10 kg bag of CSI 464 lasts 7.0 days, and the line uses 42.87 kg a month. The pack does not cover the buffer by accident; you have to hold the buffer deliberately, as kilos that stay on the shelf and are only touched during a delay. Because CSI 464 is priced flat at ₹507.40 a kilo from 1 kg to 10 kg, you can hold exactly 1 × 5 kg + 2 × 1 kg without paying a rupee more per kilo than the 10 kg bag. The 500 g bag is the only CSI 464 size that costs more (₹520.00 a kilo). When the requirement is smaller than the minimum pack goes deeper on the slow-oil side of this.

That ₹10,601.86 is a one-off: the cash it takes to put a buffer under the whole line once. After that you only replace what a delay actually consumes. Three rounding details are worth knowing. Lavender's buffer of 220.8 g is cheapest as 2 × 100 g + 2 × 15 g — the two 15 g packs (₹7.00 a gram) cost less than a third 100 g pack would for the last 20.8 g. Jars come in packs of 5 or 10 at the same ₹70.80 each, so 38 jars round to 40. And the Eco Thin C1 10-pack is out of stock, so the smallest wick pack is 20 — 2 × 20-pack covers 38 wicks. Labels and lids are shown as example lines because CSI does not sell them; they are not in the CSI subtotal. At an example holding cost of 1.5% a month (replace it with your own cost of cash), keeping the layer on the shelf costs about ₹159.03 a month — the price of not stopping the line.

Shopping list: the buffer plus one month of cycle stock

A safety stock only works if the normal order sits on top of it. For the example line, one month of average use is 42.87 kg of wax, 3,728 g of oil across six SKUs, 233 jars and 233 wicks. Bought in the cheapest real packs, the month's CSI 464 is 4 × 10 kg + 3 × 1 kg at ₹21,818.20. The order that sets up the policy is therefore the safety-stock layer above plus that month — after which every reorder is just the month.

The split matters because untested materials are the most common source of dead stock. A new oil bought at production size before customers have smelt it, or a wick bought by the hundred before a full-life burn, is inventory that may never be used. The approved, shared materials are the safe place for cash: any SKU will consume them. The same logic, applied across a whole catalogue, is how to identify shared raw materials so bulk buying does not create dead stock, and which inventory to buy frequently and which in bulk turns it into a cash-flow rule.

Keeping the buffers honest every month

A buffer sized once and never revisited drifts out of date within a season. Add the new month to the history, drop the oldest, and recompute. Three changes should trigger an immediate recalculation rather than waiting for the monthly review: a new SKU that uses the shared wax (the line's demand grows and its pattern changes), a delivery slower than your current worst (the lead-time input has moved), and a slow SKU you are considering discontinuing (its buffer should shrink to zero before you stop selling it, not after).

Two signals tell you a buffer is wrong. If you dip into a safety stock more than occasionally, the buffer is too small or your reorder point is too low — the stock is acting as cycle stock. If a safety stock has not been touched in six months, it may be too large, or the material may be slow enough that its pack size is doing the job. For a slow oil, the second signal is the cue to drop the separate buffer and let the pack carry it. The reorder sheet puts both signals in columns so you see them without doing the arithmetic, and the per-wick-size reorder post applies the same logic to a collection with several wick sizes.

This page sits in the procurement cluster that starts with turning a finalised formula into a 30-day purchase order and converting monthly sales into material requirements. The capstone, the complete procurement system from BOM to CSI purchase order, shows where safety stock sits between gross requirement and pack selection. If your buffers are fine but the order still feels too big, combining one jar across eight fragrances and forecasting per SKU are the next places to look.

Frequently asked questions