# How to Calculate a MTTF

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Most of the equipment your business relies on will need to be replaced at some point. Calculating how likely failures are and when they'll occur is a crucial part of forward planning. You'll need those projections to plan for future capital expenditures, to schedule maintenance or preventive upgrades, and -- often -- simply to purchase equipment that's appropriate to your needs. One statistical tool that's used to assess a product's reliability is its mean time to failure, or MTTF.

## The Calculation

Time isn't always the determining factor in an MTTF calculation. Instead, it's a measure of use that's appropriate to the product. Actual hours in operation is suitable for a computer chip or one of the hard drives in a server, while for firearms it might be shots fired and for tires, it's mileage. To arrive at an MTTF, you'd simply test a set number of units for a predetermined time. Multiply the number of units by the time -- or whichever measure you're testing to assess reliability -- to arrive at a number of unit-hours. Divide the unit-hours by the number of failures, and that's your MTTF. If you tested 100 units for 100 hours and saw two failures, you'd have an MTTF of 5,000 hours.

## Use and Limitations

That doesn't mean you'd automatically get 5,000 hours' use -- roughly six months -- out of those components. For one thing, the numbers might vary if you tested a second batch of components. More importantly, the MTTF is a figure that might be skewed sharply by factors such as a high failure rate within the first several hours of operation. The MTTF is a useful quick calculation, but more powerful and flexible statistical tools such as the Weibull failure curve provide a better guide to a product's reliability.