Unit of Measure Conversion: Buy in Tonnes, Stock in Kilos, Sell in Pieces

Your supplier quotes by the tonne. Your shelf label says kilos. Your bill of materials calls for grams.

Somewhere between those three numbers, a person is doing arithmetic in their head. Usually at 4pm. Usually with a truck waiting at the dock.

That mental arithmetic is where unit of measure conversion either happens properly, inside your system, or happens badly, inside somebody's head. This post is about what changes when the system does it for you.

The Thursday that costs you a week

Here is the version every plant knows.

You buy a structural adhesive powder. The supplier sells it by the metric tonne, because that is how their plant fills sacks. Your stockroom counts it in kilograms, because that is what fits on a shelf label and on a scale. Your product recipe calls for 480 grams per unit, because that is what the process sheet says.

Three units. One material. Nobody thinks about it, because everyone on the floor knows that a tonne is a thousand kilos.

Then Thursday happens.

• A new buyer raises a purchase order for 12, meaning 12 sacks. The supplier reads 12 tonnes.
• Receiving books in 1,250, meaning kilos, into a field that was expecting tonnes.
• The recipe consumes 480, meaning grams, from a balance that is counting kilos.

Each of those is a single keystroke. Each one is defensible in isolation. Together they produce a stock figure that is wrong by three orders of magnitude, and nobody notices until the shelf is bare or the year-end count refuses to reconcile.

The damage is rarely the dramatic kind. It is the slow kind:

• You reorder material you already have, because on paper you have 8 and not 8,000.
• You promise a delivery date against stock that does not exist in the unit you think it does.
• Your product cost is out, because the price per tonne got treated as a price per kilo.
• Every physical count turns into an argument about which number was in which unit.

The workaround most plants land on is two part numbers: one for the box, one for the piece. It works, right up until somebody asks how much of the material you actually have, and the answer requires adding two reports together by hand.

The tell is simple. If anyone in your building keeps a conversion factor in a notebook, a spreadsheet column, or their memory, your system is not doing its job.

What a unit of measure actually is, in plain words

Most people meet units of measure as a dropdown next to a quantity box. That is the symptom, not the idea.

The idea is this: units that measure the same kind of thing belong together, and inside that group, one unit is the yardstick.

In Qualis, that group is a category. Weight is a category. Volume is a category. Length is a category. Countable things (piece, dozen, pair, hundred) are a category too.

Inside each category, exactly one unit is the reference. It has a ratio of 1, and every other unit in that category is described in terms of it.

That is the whole model. A tonne is 1000 kilos. A gram is 0.001 kilos. Say it once, and the system can convert any weight to any other weight forever, without a single person retyping a factor.

The categories ship ready to use, so most teams never build one from scratch. You open the module and find Length, Weight, Volume, Time, Units, and a long list of others already populated.

Open a category and you get its units side by side, ordered by size, with the reference marked by a star. There is a converter built into the page: type a number and every unit shows what that quantity equals.

Five dozen is sixty pieces. You knew that. The point is that your system knows it too, in exactly the same place, for every unit you use.

How it works in Qualis

Step one: tell each product the three units it lives in

A product in Qualis carries three units, and they answer three different questions:

• Inventory UoM. The unit your stock is counted in. This is the truth on the shelf.
• Purchase UoM. The unit you order in, because it is the unit your supplier sells in.
• Sales UoM. The unit you sell in, because it is the unit your customer buys in.

Set the inventory unit to Kilogram and something useful happens immediately. The purchase and sales unit pickers stop offering you anything that is not a weight.

This is deliberate. All three units must belong to one category, because units from different categories cannot convert. If you try to save a product whose units disagree, the save is refused with a plain instruction: pick units from a single category.

The form shows you the category it has locked onto, with a Reset Category link when you genuinely need to change direction. You cannot quietly end up with a product that is bought in metres and stocked in kilos.

Step two: record how each supplier actually sells it

The same material rarely comes packed the same way from every source. One supplier sells tonnes, another sells 25 kilo sacks, a third quotes per pound.

So the purchasing terms live per supplier, on the product, not as one global guess:

Each supplier line carries the price, the unit that price is quoted per, the minimum order, an order multiple, and the lead time in days.

Now watch what that does to a purchase order. Pick the product and the supplier on a line, and the line fills itself in:

• the price, together with the unit it is quoted in, because a price with no unit basis is not a price
• a quantity that already respects the supplier's minimum and their order multiple
• the order's expected delivery date, calculated from the longest lead time on the order, since a shipment is only complete when its slowest line arrives

There is one thing it deliberately refuses to do. If the supplier quotes in a different currency from the one the order is written in, the price is left blank with the reason shown, rather than carried across at some invented rate. Converting money is a buyer's decision, not a prefill's.

Step three: let the stock ledger keep one honest unit

This is the part that matters most, and the part you never see.

Every movement of stock in Qualis carries its own quantity and its own unit. Before that movement touches an inventory balance, it is converted into the product's inventory unit. The balance is then labelled with that unit, so the number and its unit never drift apart.

Read that table for a second. Titanium bar in kilograms, hex bolts in pieces, hydraulic fluid in litres, all in one list, each number carrying the unit it is actually in. Nobody had to normalise anything by hand.

The same conversion runs everywhere stock is committed or consumed: reserving material when a sales order is confirmed, delivering it, receiving finished goods off a manufacturing order, issuing parts to a maintenance job. One conversion rule, applied at every door into the stock ledger, rather than a different rule per screen.

Because reservations are converted the same way as receipts, the availability number you promise against and the number you actually receive speak the same language. That is what makes available to promise trustworthy rather than optimistic.

The rule that keeps unit of measure conversion honest

Here is the design decision worth pausing on.

When a conversion cannot be done, Qualis stops. It does not shrug and write the raw number down.

A converter that cannot convert throws, with a message that tells you what to fix: the unit is missing, or the two units are in different categories, or a ratio is set to zero. The stock movement fails and the balance is left untouched.

That sounds harsh until you consider the alternative. The alternative is a system that quietly writes 12 into a field meaning kilos when the 12 meant tonnes, and lets you find out six weeks later during a physical count. A loud failure at the moment of the mistake is far cheaper than a silent one that ages.

There is a boundary here, and it is worth stating plainly: conversions work inside a category. Weight converts to weight, volume to volume, count to count. Turning 12 pieces into 3 kilograms is not a unit conversion at all, it is a weight per piece, which is a different fact about your product and belongs in a different field. Qualis draws that line on purpose, and tells you when you have crossed it, instead of guessing.

The end to end picture

Put together, one material moves through four stages and four different units, and no human converts anything:

You buy in the supplier's unit. You stock in your unit. You build in the recipe's unit. You ship in the customer's unit. The conversion happens at the boundary each time, from a ratio you defined once.

Why this is worth caring about

Multi unit handling is one of those features that is easy to tick on a comparison sheet and hard to live with, so it pays to look at the shape of it rather than the checkbox.

Some systems keep it switched off until someone finds the setting. Odoo's Inventory app ships with units of measure as a configuration option somebody has to activate before any of it applies.

Some define the conversion per item, and only for buying. Katana lets you flag an item as purchased in a different unit and enter a conversion rate on that item's card. It is a real capability, and it is scoped to purchase orders. The recipe still works in stock units, and the rate lives on the item rather than in shared unit data, so the same "one roll is 100 square feet" fact gets re-entered on every item it applies to.

Some lock the decision the moment you save. In NetSuite, purchase and sale units are set when you create the item record and cannot be changed after it is saved, which turns a data entry choice on day one into a permanent property of the item.

Qualis takes a different route on all three counts. Units are shared master data, defined once per category and reused by every product. Purchasing terms are recorded per supplier, so two vendors can sell the same material in two different units without cloning the product. And the conversion is enforced at the stock ledger, not bolted onto one document type.

The practical payoff is small and constant rather than dramatic. Buyers stop converting in their heads. Receiving books what the delivery note says. Counts stop needing a translator. And your costs, your reorder points and your promises are all computed from numbers that mean what they say.

If your costs are built on unit quantities, this is upstream of everything. A standard cost rollup is only as good as the units the quantities are expressed in.

Frequently Asked Questions

What is unit of measure conversion in an ERP?

It is the system's ability to hold one product's quantity in different units for different purposes, and to convert between them automatically. You might buy a material in tonnes, hold stock in kilograms, and consume it in grams. Conversion means every one of those numbers refers to the same physical quantity without anyone doing the arithmetic.

Can I buy in one unit and stock in another?

Yes. A product in Qualis has a purchase unit, an inventory unit and a sales unit. Purchase orders are written in the purchasing unit, and the quantity is converted into the inventory unit before it reaches your stock balance, so the shelf figure stays in the unit your team counts in.

Why do units have to be in the same category?

Because a conversion is only meaningful between units that measure the same kind of thing. Kilograms convert to tonnes; kilograms do not convert to metres. Qualis groups units into categories with one reference unit each, and refuses to save a product whose purchase, inventory and sales units come from different categories.

What happens if a conversion cannot be done?

The operation fails and says why, and the stock balance is left untouched. This is intentional. Writing an unconverted number into a balance labelled with a different unit is the mistake that costs you a physical count later, so Qualis treats it as an error rather than a fallback.

Can two suppliers sell me the same material in different units?

Yes. Purchasing terms are recorded per supplier on the product, each with its own price, the unit that price is quoted per, minimum order quantity, order multiple and lead time. Choosing the supplier on a purchase order line pulls that supplier's terms, not a global average.

Do I have to set up all my units before I start?

No. The common categories and their units are there from the start, including weight, volume, length, time and countable units. You add your own only when your business measures something the standard set does not cover, and the system computes the conversions from the single ratio you give it.

The bottom line

Units of measure look like a settings screen. In practice they are the grammar of your inventory. Get them right once and every downstream number, your stock figures, your costs, your reorder points and your delivery promises, inherits that correctness for free.

Get them wrong, and you pay for it one keystroke at a time, in a currency of write offs and arguments about whose number was right.

If your team is still converting units in their heads, see how Qualis handles it across purchasing, stock and production, and give that arithmetic back to the software.