Decay rate is a model parameter representing a fractional loss of usable inventory over a stated period. In replenishment planning it can estimate a daily loss allowance, but it is not a product shelf-life rule, a physical count or a direct measurement of every source of shrinkage.
A simple fractional-loss model
If no consumption or replenishment occurs and the same fraction d is lost each day:
I(t) = I0 × (1 − d)^t
At an illustrative 5% daily loss, 100 units become 100 × 0.95^7 = 69.83 modeled usable units after seven days. This does not mean a food, medicine or other dated product is safe for seven days. Follow the actual product requirements and approved release procedure.
Constant fractional loss is a simplifying assumption. A whole lot reaching expiry on one date behaves differently from a small fraction being lost every day. Damage, theft, unrecorded transfers, recipe yield and a counting mistake also need separate explanation.
Add consumption without counting loss twice
The current LineNow planning recurrence applies the configured loss before each day's non-decay usage:
I(t + 1) = (1 − d) × I(t) − c
Opening stock for T days = (c / d) × ((1 − d)^(-T) − 1)
Use c for consumption that excludes the loss already represented by d. For d = 0, use c × T instead of dividing by zero.
With c = 18 units/day, T = 7 days and d = 0.05, the model requires 155.51 opening units, versus 126 without loss. That is a 23.42% increase before a separate safety buffer or software cap. It is a scenario calculation, not a recommended spoilage allowance.
The ordering of events matters. Applying usage before daily loss produces a different formula. State the recurrence when comparing calculators so a difference in outputs is not mistaken for an arithmetic error.