How Many Spare Units a Fleet Should Keep

Spare units are the line most often cut first when budgets tighten, and the one most quickly regretted when something breaks. Sizing them is not guesswork but a matter of weighing two figures: how often units fail, and how quickly replacements can arrive.

What Spares Are Actually For

Spares are not insurance against mass failure but cover for a time gap. When one unit fails, the user need not wait for the replacement process to complete — they pick up a spare and carry on working.

Their value is measured in working hours not lost rather than hardware owned. So the calculation is always weighed against the cost of one person stopping work for several days.

The Two Figures That Decide

The first is fault frequency. On a normally used fleet, failures rendering a unit unusable are relatively rare, but issues requiring temporary replacement are more common.

The second is replacement time. If a replacement unit can arrive within hours, the internal spare requirement is far smaller. If it takes days, spares become important.

For fleets rented with fast replacement terms in a served area, much of the spare function is already covered by the provider. What still needs preparing internally is usually for sudden needs rather than for failures.

Adjusting for Fleet Size

Small fleets behave differently from large ones. Below ten units, a single spare already provides meaningful protection, and adding more is rarely worthwhile.

On mid-sized fleets, spares are typically calculated as a small proportion of the total, with location as an added consideration. Fleets spread across several offices need spares at each, because moving units between offices consumes the very time you are trying to save.

On large fleets, spares should be grouped by configuration. A spare whose specification differs sharply from the failed unit does not solve the problem for its user.

Needs That Are Often Forgotten

Beyond failures, several situations demand units available quickly.

New hires starting earlier than planned. This happens routinely and is almost always sudden.

Training or examinations requiring several units at once at short notice.

Consultants or auditors arriving for short periods who need devices meeting company security standards.

Units undergoing data wiping ahead of return while their users still need to work.

Keeping Spares Actually Ready

A spare that is not ready to use is equivalent to no spare at all. This problem occurs more often than expected.

Store spares with the standard company configuration already installed, enrolled in device management, and connected to the office network. Preparing all that at the moment of need means the gap you wanted to avoid happens anyway.

Power them on periodically so updates install and batteries do not sit empty for long stretches.

Record spares in the asset list. Units kept in a cupboard are the most likely to disappear from records, and the most likely to be forgotten at end-of-contract collection. How to maintain those records sits in our IT asset audit article.

Calculating from Downtime Cost, Not Unit Price

The clearest way to judge whether spares are worthwhile is comparing them against the cost of one person unable to work.

Take one role, estimate the value of a working day, then multiply by how many days a replacement typically takes to arrive without spares. Compare that figure against the rental cost of one spare unit across the same period.

For roles whose work can wait a few days without consequence, spares are often not worthwhile. For roles where stopping immediately affects customers or production, one spare is almost always cheaper than one day of downtime.

This calculation also explains why spare counts do not scale linearly with fleet size. What determines it is not how many units you hold but how quickly replacements arrive and how expensive downtime is for particular roles.

For fleets with fast provider replacement terms, much of the spare function is already covered and the internal requirement shrinks considerably.

Spares for Scheduled Needs, Not Only Failures

Most spare unit usage turns out to serve not failures but needs that were actually foreseeable.

New hires whose start date moves forward. This happens routinely and almost always feels sudden from the IT team's perspective.

Training, examinations or workshops requiring several units across a few days.

Consultants, auditors or vendors arriving for short periods who need devices meeting company security standards.

Units undergoing data wiping ahead of return while their users still need to work until the final day.

Because most of these are scheduled, mapping the company activity calendar often reveals when spares will be used, which helps size them.

Keeping Spares Genuinely Ready

A spare needing half a day of preparation before use is not a spare but an idle unit.

Store them with the standard system image installed, enrolled in device management, and office network profiles active. Preparing all of this at the moment of need means the gap you wanted to avoid happens anyway.

Power them on periodically so updates install and batteries do not sit empty too long. Units stored for months without powering on typically spend a long time installing updates when first needed.

Store them somewhere more than one person knows about. A spare known only to one person becomes useless while that person is on leave.

Record spares in the asset list. Units kept in cupboards are the most likely to disappear from records and the most likely to be forgotten at end-of-contract collection. The method sits in our IT asset audit article.

Frequently Asked Questions

Do spares need the highest specification?

Not the highest, but they should follow the configuration most common across the fleet, since that is what will most likely be needed.

Are spares charged at full rental rates?

Generally yes, because the unit remains allocated to you. Discuss with your provider whether their fast replacement terms can reduce your internal spare requirement.

What if the fleet spans several cities?

Calculate spares per location rather than in total. Travel time between cities strips a centralised spare pool of its purpose.

In Short

The right number of spares is enough to cover the replacement gap rather than enough to face a worst case. Start by checking your provider's replacement terms, because that figure most determines how many you need to hold yourself.

References & Sources

How to assess the impact of a device outage on work is covered in NIST SP 800-34 Rev. 1 — Contingency Planning Guide (accessed 30 September 2026), which includes a business impact analysis template. Recording units, spares included, follows CIS Control 1 (accessed 30 September 2026).