Material shortage calculation for small batches

The material shortage formula

A material shortage calculation finds the gap between what a planned production run needs and what you can actually use. It works one material at a time, and it turns a vague goal like "make 100 candles on Friday" into an exact purchase list.

Available stock means stock on the shelf minus anything already promised to accepted orders. If the result is zero or negative, the material is covered and needs no purchase. Every positive result becomes a line on your purchase order.

The amount per unit comes straight from your recipe. If your recipe is written per batch, divide each batch amount by the number of units the batch yields first. Our guide to inventory to production planning shows how that conversion feeds every later number.

Why the bill of materials decides your accuracy

A bill of materials, or BOM, lists every input one finished unit needs, with an exact quantity and unit. For a soap bar that means each oil, butter and fragrance by weight, plus the box and label. For a candle it means wax, fragrance oil, wick and jar.

Rounded or guessed quantities produce errors that grow with the batch. Being 5 g out on one candle is 500 g out across 100 candles, which can be the difference between finishing the run and stopping halfway.

Some products use an intermediate you make first, such as a fragrance blend, a dyed wax or a sourdough starter. Treat it as its own recipe with its own materials, so the calculation checks the raw inputs behind it as well as the finished product. Our BOM calculation guide walks through building one with worked examples.

Find the limiting material before you commit labour

Your capacity is set by the scarcest material, not by an average of everything on the shelf. Plenty of wax and fragrance is no help once the jars run out. To find the limit, divide the available stock of each material by the amount one unit needs. The smallest result is the most units you can finish today.

Here is a soy candle maker planning 100 candles:

Jars are the limiting material. The maker can finish 88 candles, not 100, even though wax and fragrance would cover more. Buying more wax changes nothing this week. Buying 12 jars makes the full run possible.

You can check your own numbers in the free material shortage calculator, which shows the shortfall for every material at once.

Add waste from your own batch records

A recipe assumes nothing is lost. Real batches lose material to residue in pots and jugs, spills, trimming, test pours and rejected units. If you ignore that loss, a shortage calculation that looks covered can still leave you short at the end of the run.

Measure your waste rate rather than guessing it. Compare what you actually used with what the recipe called for over your last few batches. If five soap batches called for 15,000 g of oils in total and you used 15,600 g, you lost 600 g, which is a 4% waste rate (600 ÷ 15,000).

Apply the rate before you subtract stock. Using a measured rate keeps purchase orders accurate without the habit of rounding everything up, which ties cash up in excess stock.

Shelf stock is not the same as available stock

A simple count of what is on the shelf can mislead you. Stock already promised to accepted orders is still physically there, but it is no longer free for a new run. If you hold 2,500 g of olive oil and an accepted order will use 900 g, only 1,600 g is available.

Timing matters too. A delivery that arrives after your production day cannot cover that run. Compare each supplier's lead time with the date you plan to make the batch, not just the quantity on order.

This is where the numbers start to move faster than a spreadsheet can follow. Each accepted order changes the available stock for every product that shares those materials. Our guide to production capacity planning explains how shared materials link the capacity of different products.

Worked example: 50 bars of cold process soap

A soap maker plans 50 bars and has measured a 4% waste rate. Each bar needs 60 g olive oil, 30 g coconut oil and 10 g shea butter, plus one box. Available stock, after accepted orders, is 2,500 g olive oil, 2,000 g coconut oil, 450 g shea butter and 40 boxes.

The purchase list is 620 g olive oil, 70 g shea butter and 12 boxes. Coconut oil is covered with 440 g to spare. Always weigh lye and water to your recipe's exact calculation rather than padding them with a waste rate.

When to move beyond manual calculation

Working shortages out by hand is fine for a few products and a few orders a week. It breaks down when products share materials, because every new order changes the numbers for all of them. Ten products that share five materials already create fifty links to keep track of.

Signs it is time for a tool: you find shortages on production day, you recount stock before every promise, or keeping the spreadsheet current takes longer than the production planning itself.

Makeable connects your recipes to live stock, includes your waste in each recipe line, and deducts stock promised to accepted orders, so the shortage and capacity numbers stay current as orders arrive. If you are weighing options, our Inventora alternative comparison sets out the differences. You can test your own recipes on the free plan without a credit card.

Checklist before you confirm a production date

1. Check every recipe line is in exact units, per finished unit, with intermediates listed as their own recipes.

2. Apply a waste rate measured from your last few batches.

3. Subtract stock promised to accepted orders to get available stock.

4. Find the limiting material and the most units you can make today.

5. Order every shortage with enough supplier lead time to arrive before production day.

Common questions