An estimated MTBF value of 100,000 hours (roughly, 140 months) at 25 °C and under full load is fairly common. Such a rating expects that, under the described conditions, 77% of the PSUs will be operating failure-free over three years (36 months); equivalently, 23% of the units are expected to fail within three years of operation. For the same example, only 37% of the units (fewer than a half) are expected to last 100,000 hours without failing. The formula for calculating predicted reliability, , is