Total Cost of Ownership

Definition

TCO is the sum of every cost a decision creates over its life. In technology decisions the infrastructure line is visible and frequently the smallest; the people line is invisible and usually dominant.

What belongs in the calculation

• Infrastructure — compute, storage, network, and the transfer charges nobody models.
• Licences and subscriptions, including how they scale (per core, per user, per request).
• Build effort — engineering time to first production use.
• Run effort — the recurring one. On-call, patching, upgrades, incident response, capacity management, security review. This is the term that separates "we can build this ourselves cheaper" from reality.
• Opportunity cost — what those engineers would otherwise have built.
• Exit cost — migration away, contract termination, data extraction.
• Risk-adjusted cost — expected cost of the outages, breaches or compliance failures the option makes more or less likely.

Where the analysis usually goes wrong

"We can self-host this for a tenth of the licence cost." True on the infrastructure line, and it omits an engineer's ongoing attention. Half an engineer per year is a substantial sum, and it is a sum that does not appear on any invoice, which is precisely why it is omitted.

Ignoring the growth curve. A licence that is cheap at current scale and priced per unit of something that will grow tenfold is a future problem, and it should be modelled at the projected scale rather than the current one.

Comparing build cost with buy cost. The right comparison is build plus run over N years against buy over the same period. Three to five years is the usual horizon.

Omitting exit. A choice that is cheap to enter and expensive to leave has a cost that only appears when you most need flexibility.

The framing that makes TCO useful in an argument

Convert everything to cost per unit of business value — per order, per active user, per transaction. That makes options comparable and connects the technical decision to something the business already measures. It also reveals when a cost that looks large in absolute terms is trivially small per unit, and when a small absolute cost is unsustainable per unit at scale.

Interview question

"A team proposes self-hosting a managed service to save £80,000 a year in licence fees. What do you add to their calculation?"