A grocery sells what it buys: a packet bought is a packet sold. A hardware shop does not. It buys a box of a hundred screws and sells six. It buys a reel of cable and sells three metres.
That difference is why ordinary stock-management methods do not hold in this trade. Stock is not a quantity, it is a quantity and a unit — and forgetting the second is the mistake that throws everything else off.
Pick the reference unit, and never change it
The rule is simple and admits no exception: you count in the smallest unit you sell. If you sell screws one at a time, your stock is in screws, never in boxes.
A box of a hundred then becomes an entry of a hundred units, not an entry of one. It feels wrong at goods-in — you put one box on the shelf, not a hundred screws — and it is nonetheless the only way to have a figure that stays right after the first retail sale.
The conversion factor, written down once
For every product bought in bulk and sold loose there is exactly one number to record: how many units in a pack. One box = 100 screws. One case = 12 boxes = 1200 screws. One reel = 50 metres.
Write it on the shelf, in marker, beside the item. It looks crude and it is exactly what most shops are missing: the factor lives in the owner's head, so it disappears the day the nephew is on the counter.
Never count the same thing twice
The most common error is not converting badly, it is double counting: the loose screws in the bin plus the sealed boxes in the back, counted as boxes.
The cure fits in one sentence: an unopened box counts as its contents in units, not as one. Two sealed boxes and 43 screws in the bin is not "2 boxes + 43", it is 243 screws.
A stock that mixes units cannot be corrected. It has to be recounted.
The unit price is not the case price divided
If the box of a hundred costs you 80 dirhams, each screw costs you 0.80. Selling it at 0.80 would be selling at cost; and selling at the wholesale price divided by a hundred gives away for free the very service the customer came in for.
Retail costs more than wholesale, and rightly so: you are the one carrying the whole box, the space in the back store, and the risk on the last forty screws nobody will ever ask for. A screw at 1.50 against a cost of 0.80 leaves 0.70 a unit — 70 dirhams on the box, against the 20 you would have made reselling it sealed.
What software does here that a notebook cannot
Keeping a conversion factor by hand is perfectly possible. What is not, is applying it to every sale: each time six screws go out, someone has to take six units off and know that the sealed box in the back is worth a hundred.
This case and unit stock management is exactly the arithmetic Gestio does at the moment of sale: stock is held in units, purchases are entered by pack, and the conversion happens on its own. It is also what makes the real margin knowable on an item sold both ways. See the features.