Downtime cost and critical-spares calculator
Is it worth keeping a spare on the shelf? Put in what an hour of stoppage costs, what the part costs and how long it takes to get, and see the break-even.
Include lost output, idle labour, scrap and penalties
Count only days the line would normally run
Hours the line would normally run
Defaults are examples. Replace them with your own figures.
Cost of waiting for the part
R 4 000 000
160 production hours lost over 10 days, if the whole line stops.
Spare pays for itself if it saves
0.7hours
That is about 43 minutes of stoppage at R 25 000 an hour.
Worth holding if failure chance exceeds
0.4%
Over the period you would otherwise rely on a long lead time. Ignores storage and capital cost.
Price your critical spares
Send these details to an engineer and get availability, lead time and price.
How it works
Exposure is the cost of the downtime while you wait: hours stopped × cost per hour. The break-even probability is the spare's price divided by that exposure. If you think the part is more likely than that to fail during the period, holding it pays for itself.
The calculator ignores holding costs such as capital tied up and storage, so include them yourself if the spare is expensive or will be held for many years. It also assumes the whole line stops while you wait.
Use your own cost per hour. Include lost production, labour standing idle, penalties and scrap, not only maintenance cost.
Common questions
How do I work out my cost per hour of downtime?
Take the gross margin the line earns per hour, then add idle labour, scrap or spoilage on restart, and any contractual penalties. Production and finance usually have a figure already.
Which spares should I stock first?
Start with parts that would stop the whole line, that fail without warning and that have a long lead time. PLC CPUs, main drives, power supplies and key I/O modules are common candidates.
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