What it means
Uptime measures availability, which is simply the proportion of a period during which something is operating as promised. A website, a payment system, a factory line and a delivery fleet can all be measured this way.
The opposite is downtime, the time when the asset is not available. Contracts for software and hosting services often include a service level agreement, or SLA, which is a written promise about uptime.
If the supplier misses the target, the customer usually receives a credit against future fees rather than a cash refund. Reading the small print on what counts as downtime matters, because planned maintenance and some outages are often excluded.
Small differences in the percentage hide large differences in real time. A target of 99% allows about seven hours of downtime in a 30-day month, while 99.9% allows only around 43 minutes.
Each extra nine usually costs the supplier much more to deliver, which is reflected in the price. For a business, the key question is what an hour of downtime costs.
An online retailer loses sales and may lose customers who do not return, while a factory loses output and may pay idle staff. Comparing that cost with the price of a more reliable service shows whether a higher uptime guarantee is worth buying.
Uptime is also used for equipment in operations, where it can be measured as the share of scheduled time a machine runs. In that setting it is closely tied to maintenance planning and overall equipment effectiveness, so finance teams use it to justify spending on repairs or replacement.
It helps to separate uptime that a supplier promises from uptime that a business actually experiences. Monitoring tools that record real outages give finance teams the evidence needed to claim credits and to challenge a supplier at renewal time.
In practice
Real-world examples
Example 1
A software company sells a booking platform with a promise of 99.9% uptime. In one month the platform is down for 90 minutes, which is more than the 43.2 minutes allowed. The customer claims a service credit under the agreement.
Example 2
A packaging plant tracks the uptime of its main filling machine. The machine is scheduled for 400 hours a month and runs for 360 of them, so uptime is 90%. The operations manager uses the figure to argue for a new set of spare parts.
Example 3
A hospital pays for a hosted patient records system and negotiates a stricter uptime commitment than the standard offer. The supplier prices the higher guarantee at an extra $24,000 a year. The hospital judges that cost to be lower than the cost of delayed care and staff time during outages.
Think of it
Uptime is like a delivery van. If the van spends all day on the road delivering parcels, it has 100% uptime. If it breaks down in the driveway for two hours, its uptime drops, and you miss out on deliveries.
Formula
Calculation
Uptime % = ((Total time - Downtime) / Total time) x 100
Cost of downtime = Downtime hours x Loss per hour
A 30-day month has 30 x 24 x 60 = 43,200 minutes. An online store's payment system is down for a total of 216 minutes during the month. Uptime = (43,200 - 216) / 43,200 x 100 = 42,984 / 43,200 x 100 = 99.5%. The store earns an average of $9,000 of sales an hour, so 216 minutes is 3.6 hours and the lost sales = 3.6 x 9,000 = $32,400. A 99.9% service would have allowed only 43.2 minutes of downtime, which would have limited the loss to 0.72 x 9,000 = $6,480.Case study
Seen in the real world
Northgate Ticketing is an illustrative, fictional company that sells event tickets online. Its hosting contract promised 99.5% uptime, and the finance director assumed that was good enough because it sounded close to perfect.
During a busy ticket release, the site was down for 4 hours. Sales run at about $25,000 an hour at that time of year, so the outage cost 4 x 25,000 = $100,000 in lost sales, while the contract credit was only $2,000.
In the illustrative follow-up, the company compared the extra $30,000 annual cost of a 99.95% plan with its expected downtime losses and chose to upgrade. The lesson is that the percentage on the contract matters less than the cost of the minutes it allows.
Watch out
Common mistakes
- Treating 99% and 99.9% as nearly the same, when the first allows about ten times as much downtime as the second.
- Assuming the service credit will cover losses, when credits are usually capped at a small share of the monthly fee.
- Ignoring what the contract excludes, such as planned maintenance windows, which can make real availability lower than the headline figure.
Questions
People also ask
What is the difference between uptime and availability?
They are usually used interchangeably, although availability sometimes includes only unplanned outages while uptime counts everything.
How do I convert an uptime percentage into minutes?
Multiply the total minutes in the period by the downtime share, so 99.9% over 43,200 minutes allows 43,200 x 0.1% = 43.2 minutes.
Who pays when uptime is missed?
The supplier normally owes a credit under the agreement, but the customer carries any losses beyond that credit unless the contract says otherwise.