Last-time buy calculator

How it works

When a supplier announces that a part will be discontinued, you can buy the parts you expect to need for the rest of the product's support life in one last order. This tool sizes that order from the demand forecast, the repair spares the installed base will draw, a safety buffer and the stock you already hold, then prices it with holding cost and tests what a too-small or too-large buy would cost against the price of a redesign.

1. Forecast production demand. Time zero is the last-time-buy date. The profile gives the rate at each moment, and each year of the support period takes the quantity that falls inside it. A compound trend steps the rate each year; the other profiles are continuous curves that start at today's run rate. The last year is shortened if the support period is not a whole number of years.trend: year k = D x (1 + g)^(k − 1) x length; flat: D x length; linear to zero: D x (length − (b^2 − a^2) / (2 S)); exponential: D x (r^a − r^b) / (−ln r), with r = 1 − decline rate
2. Add parts lost in your own process. A scrap or yield loss in your own build means you must consume more parts than you ship.parts consumed in production = units shipped / (1 − scrap)
3. Add repair spares. Units in the field fail and some are repaired with this part. The field base is the installed base plus, if you choose, the units you ship during support, less a yearly retirement of old units. Each year uses the average base for that year.repair demand in a year = average field base x failure rate x repair yield x length of the year; field base = (installed base + units shipped so far + half this year's shipments) x (1 − retirement)^(mid-year)
4. Size the safety stock. With a service level, the buffer is a normal quantile times the standard deviation of demand over the period. Each year's demand is taken to vary independently by the coefficient of variation you enter, so the period standard deviation grows with the square root of the sum of squares. With a fixed buffer, the stock is a share of the total demand.sigma = CV x sqrt(sum of (demand in year k)^2); safety stock = z x sigma, with z = the normal quantile of the service level; or safety stock = buffer % x total demand
5. Allow for storage loss. Parts that sit in store lose some usable stock each year to moisture, solderability or handling. A part used at time t must have been bought as 1 / (1 − loss)^t parts. Safety stock is held to the end of the period.gross requirement = sum over years of (demand in year k) / (1 − loss)^(mid-year k) + safety stock / (1 − loss)^S
6. Net, round and price the buy. Stock on hand and open orders come off the gross requirement. What is left is rounded up to the minimum order quantity and then to a multiple of the pack size.net buy = max(0, gross requirement − stock on hand − open orders); order = ceiling(max(net buy, MOQ) / pack) x pack; purchase cost = order x unit price
7. Holding cost and present value. Existing stock is used first, so the new buy sits on the shelf until the old stock runs out. Holding cost is the yearly rate on the average value of the buy on the shelf, discounted to the buy date at the cost of capital. The purchase is paid on day one.holding cost = sum over years of (holding rate x unit price x average buy on hand x length) / (1 + discount)^(mid-year); total cost of the buy = purchase cost + present value of holding cost
8. Test the alternative quantities. For quantities from nothing to double the recommendation the tool runs the same demand against the stock and counts what is left over and what is missing. Leftover units are written off at a salvage rate. Units that cannot be covered are bought later at a multiple of the unit price, discounted from the year they are needed.total cost at a quantity = purchase + holding PV + shortfall units x price x shortfall multiple (discounted) − leftover x price x salvage rate (discounted)
9. Compare with a redesign. A redesign or second source costs a one-off sum and then buys the same demand, beyond the stock you hold, at the replacement price. The break-even redesign cost is the figure at which both routes cost the same.redesign route = redesign cost + PV of replacement price x units not covered by existing stock; break-even redesign cost = total cost of the buy − PV of replacement parts

Sources

The methods and default figures rest on these public sources. The wording here is our own; read the source for its full text.

• DMSMS Lifetime Buy Characterization Via Data Mining of Historical Buys (opens in a new tab), CALCE, University of Maryland (Sandborn, Prabhakar and Feng), DMSMS Conference, Orlando, November 2007. Frames a lifetime buy as stocking enough parts for current and expected demand, and the difficulty of choosing the quantity.
• IEC 62402:2019, Obsolescence management (opens in a new tab), International Electrotechnical Commission, 29 May 2019. The framework for an obsolescence management plan and the choice of resolution (the standard itself is paid; this tool follows only its general approach).
• J-STD-048, Notification Standard for Product Discontinuance (replaces JESD48) (opens in a new tab), IPC, JEDEC and ECIA, Revision A, December 2025. How suppliers must warn customers of a planned discontinuance, which sets the window for a last-time buy. Earlier JESD48 revisions carried the same purpose.