# inventory.reorder-point
The stock level at which to place a replenishment order:
reorder point = lead-time demand + safety stock
lead-time demand = demand x leadTimeDays / demandPeriodDays, rounded up
Demand is given per period (units a week, a month, a year) with the period's
length in days, so a daily rate of 33.3 units (1000 a month) needs no float:
the division happens once, exactly, with `math.round-div`.
**Lead-time demand is rounded up**, never to nearest. 1000 a month over a 7-day
lead time is 233.33 units; ordering at 233 plans to run out a third of a unit
early. Safety stock is already whole units and is added unchanged.
Compare the result with stock on hand **plus stock already on order**, or an
order placed yesterday triggers another today.
Zero lead time or zero demand leaves just the safety stock. Negative values and
a demand period of zero days are errors.
Source: the standard textbook definition, e.g. APICS Dictionary ("order point")
and Silver, Pyke and Thomas, *Inventory and Production Management in Supply
Chains*, 4th ed., ch. 6.