Units that don't lie: box, strip, piece, kg

Keep stock in one base unit and convert boxes, strips, cases and kilograms on top of it. Conversion tables, per-unit pricing and common mistakes.

To manage different units of measure in inventory, pick one base unit per item, which is the smallest quantity you ever sell or count, and hold all stock in that unit. Every other unit (box, strip, case, kilogram) is then just a fixed conversion to the base. Purchases come in as cases, sales go out as pieces, and both turn into the same single stock number. Price each selling unit from the cost per base unit so no pack quietly sells at a loss.

Why do mixed units break stock counts?

Most stock errors are unit errors, not theft or bad counting. A chemist receives 10 boxes, sells 4 strips and 7 loose tablets, and records all three as plain "quantity". After a week the system shows something that matches no shelf.

This happens because the same item is being tracked in three units at once with no rule linking them. Once a "1" can mean a box, a strip or a tablet depending on who typed it, the stock figure can't be trusted, and nor can the reorder level or the stock valuation built on it.

What is a base unit, and how do I choose it?

The base unit is the smallest quantity you will ever do one of these with:

sell to a customer
issue to production or a kitchen
count during stock-take

For a pharmacy that sells loose tablets, the base unit is the tablet. A shop that only ever sells whole strips can use the strip. For loose grains or spices sold by weight, use the gram, or 10 grams if that is the smallest step on your scale. Pick the smallest unit you really deal in. If you pick something larger, you will need fractions later. If you pick something smaller, you are counting things nobody handles.

Once you choose a base unit, don't change it. Changing it later means converting every past transaction.

What does a conversion table look like?

Each item gets a short table: every unit it is bought, sold or counted in, and how many base units that unit holds. The figures below are illustrative. Use your own supplier's pack sizes.

Item	Unit	Contains	In base units
Tablet (base: tablet)	Tablet	—	1
	Strip	10 tablets	10
	Box	10 strips	100
	Carton	24 boxes	2,400
Rice (base: gram)	Gram	—	1
	Kilogram	1,000 g	1,000
	Bag	25 kg	25,000
Soap (base: piece)	Piece	—	1
	Pack	4 pieces	4
	Case	36 packs	144

Always define a unit in base units, not as a multiple of another unit. "Box = 100 tablets" is safer than "box = 10 strips", because if the strip definition changes, the box definition doesn't silently change with it.

How does stock move through the base unit? A worked example

Illustrative figures for one tablet item, base unit tablet:

Opening stock: 3 boxes = 300 tablets
Purchase received: 2 boxes = +200 tablets, giving 500
Sold: 6 strips = −60 tablets, giving 440
Sold loose: 7 tablets = −7, giving 433
Damaged and written off: 1 strip = −10, giving 423

The system holds 423. It can show that as 4 boxes, 2 strips and 3 tablets, because 400 + 20 + 3 = 423. At stock-take, staff can count in whatever unit is on the shelf, and the counts convert back to 423 for comparison. There is only one number underneath, so the different units can't contradict each other.

How do I price each unit correctly?

Start from cost per base unit, then price upward.

Illustrative: a box of 100 tablets costs ₹180, so cost per tablet is ₹1.80.

Selling unit	Base units	Cost	Selling price (illustrative)	Margin per unit
Tablet	1	₹1.80	₹2.50	₹0.70
Strip	10	₹18.00	₹24.00	₹6.00
Box	100	₹180.00	₹225.00	₹45.00

Each larger unit is cheaper per tablet: ₹2.50 loose, ₹2.40 in a strip, ₹2.25 in a box. That is normal and often intended. What you must check is that every unit's price is still above its base cost after any scheme or discount. A bulk-buyer discount on the box can easily end up below ₹180 if nobody checks it against the base cost.

If a pack carries a printed MRP, check how that price applies to loose or part-pack sales of your product category before you set loose prices. Your distributor or CA can point you to the rules that apply.

For tax, GST is charged on the transaction value whatever the unit. The unit you show on the invoice should be one the GST system recognises for reporting. See GST invoice format requirements and confirm the current unit codes on the official GST portal.

How do units affect reorder points and purchasing?

Calculate the reorder point in base units, because daily usage comes from sales, and sales may be in strips, tablets and boxes mixed together. Only the base unit adds them up correctly.

Then convert the order quantity into the purchase unit and round up to whole cases or boxes. Illustrative: if the reorder calculation calls for 1,850 tablets and the supplier sells cartons of 2,400, you order one carton, not 0.77 of one.

For distributors, the purchase unit is often a carton while the selling unit is a box or a dozen to retailers. Our guide to wholesale distributor inventory covers how that split works with party-wise rates and schemes.

Common mistakes

Storing stock in the purchase unit. If stock is held in boxes, a single loose sale leaves you with 9.9 boxes. Fractions creep in and rounding eats stock.
Editing a conversion after use. Changing "case = 12" to "case = 10" recalculates history. Create a new unit for the new pack instead.
Creating a separate item for each unit. "Paracetamol strip" and "Paracetamol box" as two items means you need manual transfers every time you open a box, and they get forgotten.
Variable-weight goods treated as fixed. A "bag" of vegetables that weighs anywhere from 9 to 11 kg isn't a fixed unit. Receive it by actual weight.
Pricing only the main unit. Loose prices get typed in at the counter and drift below cost.

When does this not apply?

The base-unit approach assumes each conversion is fixed. It needs adjusting where:

Weight changes in storage. Grains dry and vegetables lose moisture, so expect a measured shrinkage write-off.
Recipes and yield. A kitchen buys onions in kilograms but the dish uses peeled, chopped weight. That gap is yield loss, not a unit conversion. See restaurant inventory and food costing.
Variants rather than units. A shirt in sizes M and L is two items, not two units of one item. That is a variant matrix, covered in apparel size and colour matrix.

Quick setup checklist

Choose one base unit per item: the smallest you sell, issue or count
List every purchase, sales and counting unit for that item
Define each unit directly in base units
Compute cost per base unit from your latest purchase
Set a price for each selling unit and check each one against cost
Calculate reorder points in base units and round orders to whole purchase units
When a supplier changes pack size, add a new unit and don't edit the old one

FAQ

What is a base unit in inventory? The base unit is the smallest quantity you ever sell, issue or count for an item, such as a tablet, a piece or a gram. Stock is always stored in that unit. Boxes, strips, cases and kilograms are defined as fixed multiples of it, so every movement converts back to one number and the stock figure stays the same whichever way you look at it.

Should the purchase unit and the sales unit be the same? They do not need to be. Many shops buy in cases and sell in pieces, or buy in kilograms and sell in grams. What matters is that both are defined as conversions of the same base unit. Then a purchase of one case and a sale of three pieces reduce and add to one shared stock figure.

How do I price an item sold in more than one unit? Work out the cost per base unit first, then set a selling price for each unit you sell in. A box price does not have to be the strip price multiplied by the strips in the box; many sellers give a pack discount. Check every unit's price against the base cost so no pack ends up selling below cost.

What if a supplier changes the pack size? Treat the new pack size as a new conversion, not an edit to the old one. If you change the old conversion, past purchases get recalculated and your stock history stops matching what was actually received. Close out the old pack, receive the new one under its own conversion, and count the shelf once to confirm.